Physical Geography

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Adiabatic

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A process in which heat does not enter or leave a system. In the atmospheric sciences, adiabatic processes are often used to model internal energy changes in rising and descending parcels of air in the atmosphere. When a parcel of air rises in expands because of a reduction in pressure. If no other non-adiabatic processes occur (like condensation, evaporation and radiation), expansion causes the parcel of air to cool at a set rate of 0.98° Celsius per 100 meters. The opposite occurs when a parcel of air descends in the atmosphere. The air in a descending parcel becomes compressed. Compression causes the temperature within the parcel to increase at a rate of 0.98° Celsius per 100 meters.

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Adiabatic Cooling

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The cooling of a rising parcel of air due to adiabatic processes.

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Advection

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Advection involves the transfer of heat energy by means of horizontal mass motions through a medium.

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Ablation

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Surface removal of ice or snow from a glacier or snowfield by melting, sublimation, and/or calving.

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Ablation Zone

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Region in a glacier where there is a surface net removal of snow and/or ice by melting, sublimation, and/or calving.

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Absolute Humidity

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Measurement of atmospheric humidity. Absolute humidity is the mass of water vapor in a given volume of air (this measurement is not influenced by the mass of the air). Normally expressed in grams of water vapor per cubic meter of atmosphere at a specific temperature.

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Absorption (Atmospheric)

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Atmospheric absorption is defined as a process in which solar radiation is retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. In general, the absorption of solar radiation by substances in the Earth's atmosphere results in temperatures that get no higher than 1800° Celsius. According to Wien's Law, bodies with temperatures at this level or lower would emit their radiation in the longwave band.

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Abyssal Fan

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Fan shaped accumulation of sediment from rivers that is deposited at the base of a submarine canyon within a ocean basin.

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Accretion

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The growth of the continental masses over geologic time via the addition of marine sediments. These sediments are added on to the edges of the continents through tectonic collision with other oceanic or continental plates.

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Accumulation

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Surface addition of snow to a glacier or snowfield.

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Accumulation Zone

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(1) Region in a glacier where there is a surface net addition of snow.
(2) Part of a hillslope that has a net gain of material leading to a progressive raising of the slope's surface.

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Active Layer

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Upper zone of soil in higher latitude locations that experiences daily and seasonal freeze-thaw cycles.

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Actual Evapotranspiration

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Is the amount of water that is actually removed from a surface due to the processes of evaporation and transpiration.

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Adaptive Radiation

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The evolution of a number of new species from one or a few ancestor species over many thousands or millions of years. Normally occurs after a mass extinction creates a number of vacant ecological niches or when a radical change in the environment produces new ecological niches.

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Advection Fog

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Fog generated when winds flow over a surface with a different temperature. Two types of advection fog exist. When warm air flows over a cold surface it can produce fog through contact cooling. Cold air blowing over a warm moist surface produces a form of advection fog know as evaporation fog.

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Aeolian

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Geomorphic process involving wind. Alternative spelling eolian.

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Aeolian Landform

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Is a landform formed from the erosion or deposition of weathered surface materials by wind. This includes landforms with some of the following geomorphic features: sand dunes, deflation hollows, and desert pavement. Alternative spelling eolian landform.

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Aftershock

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Smaller earth tremors that occur seconds to weeks after a major earthquake event.

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Aggradation

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Readjustment of the stream profile where the stream channel is raised by the deposition of bed load.

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Air Mass

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A body of air whose temperature and humidity characteristics remain relatively constant over a horizontal distance of hundreds to thousands of kilometers. Air masses develop their climatic characteristics by remaining stationary over a source region for a number of days. Air masses are classified according to their temperature and humidity characteristics.

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Aeolian

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Geomorphic process involving wind. Alternative spelling eolian.

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Aeolian Landform

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Is a landform formed from the erosion or deposition of weathered surface materials by wind. This includes landforms with some of the following geomorphic features: sand dunes, deflation hollows, and desert pavement. Alternative spelling eolian landform.

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Aftershock

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Smaller earth tremors that occur seconds to weeks after a major earthquake event.

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Aggradation

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Readjustment of the stream profile where the stream channel is raised by the deposition of bed load.

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Air Mass

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A body of air whose temperature and humidity characteristics remain relatively constant over a horizontal distance of hundreds to thousands of kilometers. Air masses develop their climatic characteristics by remaining stationary over a source region for a number of days. Air masses are classified according to their temperature and humidity characteristics.

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Albedo

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Is the reflectivity of a surface.

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Aleutian Low

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Subpolar low pressure system found near the Aleutian Islands. Most developed during the winter season. This large-scale pressure system spawns mid-latitude cyclones.

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Allophane

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Hydrated aluminosilicate substance ordinarily found associated with clay minerals.

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Alluvial Fan

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Large fan shaped terrestrial deposit of alluvial sediment on which a braided stream flows over. Form as stream load is deposited because of a reduction in the velocity of stream flow.

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Alluvial Terraces

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Flat elevated benches composed of unconsolidated alluvium found either side of a stream channel. Formed when a stream down cuts into its floodplain.

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Alluvium

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Sediment that originates from a stream.

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Alpine Glacier

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Small glacier that occupies a U-shaped valley on a mountain. Also called a mountain glacier.

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Alpine Permafrost

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Form of permafrost that exists at high altitudes in mountainous environments.

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Alpine Tundra

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High altitude biome dominated by a few species of dwarf shrubs, a few grasses, sedges, lichens, and mosses. Productivity is low in this biome because of the extremes of climate. Quite similar to tundra.

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Altitude

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Vertical distance above sea-level.

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Altocumulus Clouds

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Middle altitude cloud that is colored from white to gray. This cloud is composed of a mixture of water droplets and ice crystals. It appears in the atmosphere as layers or patches that are well rounded and commonly wavelike. Found in an altitude range from 2,000 to 8,000 meters.

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Altostratus Clouds

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Gray-looking middle altitude cloud that is composed of water droplets and ice crystals. Appears in the atmosphere as dense sheet like layer. Can be recognized from stratus clouds by the fact that you can see the Sun through it. Found in an altitude range from 2,000 to 8,000 meters.

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Amphibole

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A group of double chained inosilicate minerals whose basic chemical unit is the tetrahedron (SiO 4 ). They are common rock forming minerals and are found in most igneous and metamorphic rocks. They form at low temperatures with the presence of water in the crystallization environment. There are about 60 recognized mineral types in this group.

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Andesite

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An extrusive igneous rock that develops from a magma that is chemically between felsic and mafic and whose mineral crystals are fine.

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Andisols

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These soils develop from parent materials that are volcanic in origin.

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Anemometer

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Mechanical instrument used to measure wind speed. These instruments commonly employee three methods to measure this phenomenon: 1) A device with three or four open cups attached to a rotating spinal. The speed of rotation is then converted into a measurement of wind speed; 2) A pressure plate that measures the force exerted by the moving wind at right angles; 3) An instrument consisting of a heated-wire where electrical resistance (temperature of the wire) is adjusted to account for heat lost by air flow. The faster the wind the greater the heat loss and thus the more energy that is required to keep the wire at a constant temperature. As a result, wind speed is measured through the drain of electrical current.

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Aneroid Barometer

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Barometer that measures atmospheric pressure via the expansion and contraction of a sealed hollow cell which is partially depleted of air.

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Angle of Incidence

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Angle at which the Sun's rays or insolation strike the Earth's surface. If the Sun is positioned directly over head or 90° from the horizon, the incoming insolation strikes the surface of the Earth at right angles and is most intense.

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Angle of Repose

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Measurement commonly used in civil engineering. It is the maximum angle at which a material can be inclined without failing. Geomorpologist use this measurement for determining the stability of slope to mass movements.

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Antarctic Circle

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Latitude of 66.5° South. The northern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year.

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Antarctic High

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A region of high pressure that occupies central Antarctic throughout the year. This pressure system is responsible for very cold temperatures and extremely low humidity.

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Anticline

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A fold in rock layers that forms an arch.

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Anticyclone

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An atmospheric pressure system consisting of an area of high pressure and outward circular surface wind flow. In the Northern Hemisphere winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise.

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Aphelion

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It is the point in the Earth's orbit when it is farthest from the Sun (152.5 million kilometers). Aphelion occurs on the 3rd or 4th of July.

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Aquiclude

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Rock formations that are impermeable to groundwater water.

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Aquifer

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Rock formations that store groundwater water.

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Aquifer Recharge Area

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Surface area that provides water for an aquifer.

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Archean

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Geologic eon that occurred from 2500 to 3800 million years ago. During this time period, the first single-celled prokaryote organisms evolved and developed.

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Archipelago

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A group of islands that have an arc shaped distribution. These islands are usually of volcanic origin and are associated with subduction zones.

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Arête

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Sharp topographic ridge that separates cirques on a mountain that is or has been glaciated.

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Arkose

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A type of sedimentary sandstone that contains a large quantity of weathered feldspar grains. This type of sedimentary rock forms in arid conditions.

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Artesian Water

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Groundwater that is confined by two impermeable layers beneath the Earth's surface.

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Artesian Well

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A well where the water rises and flows out to the surface because of hydrostatic pressure.

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Asthenosphere

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Zone in the Earth's mantle that exhibits plastic properties. Located below the lithosphere at between 100 and 200 kilometers.

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Atmospheric Pressure

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Weight of the atmosphere on a surface.

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Atmospheric Stability

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Relative stability of parcels of air relative to the atmosphere that surrounds them.

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Aurora

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Multicolored lights that appear in the upper atmosphere (ionosphere) over the polar regions and visible from locations in the middle and high latitudes. Caused by the interaction of solar wind with oxygen and nitrogen gas in the atmosphere. Aurora in the Northern Hemisphere are called aurora borelis and aurora australis in the Southern Hemisphere.

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Autumnal Equinox

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One of two days during the year when the declination of the Sun is at the equator. The autumnal equinox denotes the first day of the fall season. For the Northern Hemisphere, the date of autumnal equinox on either September 22 or 23 (changes yearly). March 20 or 21 is the date of the autumnal equinox in the Southern Hemisphere. During the autumnal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night.

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Available Water

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Portion of the capillary water that is available for plant root uptake.

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Backshore slope

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Sloping bank landward of the shore. This coastal feature is composed of relatively non-mobile sediments.

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Backswamp

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Marshy low lying area in a stream's floodplain. Commonly found behind levees.

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Backwash

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The return water flow of swash. This sheet of water flows back to ocean because of gravity.

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Badlands

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(1) Term used to describe a part of South Dakota.
(2) Term used to describe a semi-arid landscape that has been influenced by heavy fluvial erosion. Characterized by deep ravines and gullies, shape ridges, and a generally barren surface.

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Bajada

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Consecutive series of alluvial fans forming along the edge of a linear mountain range. Surface of this feature undulates in a rolling fashion as one moves from the center of one alluvial fan to another. Normally occurs in arid climates.

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Bank-Caving

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Collapse of stream bank material into a stream channel.

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Barchan Dune

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Crescent shaped sand dune that has its long axis transverse to the wind and its crescent tips pointed downwind.

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Barometer

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Instrument that measures atmospheric pressure.

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Basal Sliding

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The sliding of a glacier over the surface it rests on. Caused by the gradient of the slope and the weight of the glacier's mass.

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Basalt

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A dark colored fine grained igneous rock formed from mafic magma.

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Basalt Plateau

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Extensive continental deposits of basaltic volcanic rock.

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Basaltic Magma

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Mafic magma that forms basaltic igneous rocks.

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Base Flow

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Rate of discharge in a stream where only the throughflow and groundwater flow from subsurface aquifers contribute to the overall flow.

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Base Level

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The subterranean elevation below which a stream cannot vertically erode sediment. For many streams this hypothetical elevation is sea-level.

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Basement Rock

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Very old granite and metamorphic rocks found in continental crust. These rocks make up the continental shield.

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Basin

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A topographic rock structure whose shape is concave downwards.

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Batholith

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A large mass of subsurface intrusive igneous rock that has its origins from mantle magma.

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Bay

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A body of sheltered water found in a crescent shaped coastal configuration of land.

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Beach

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The terrestrial interface area in between land and a water body where there are accumulations of unconsolidated sediments like sand and gravel. These deposits are laid down by the action of breaking waves.

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Beaufort Wind Scale

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Descriptive system that determines wind speed by noting the effect of the wind on the environment. Originally developed for use at sea by Admiral Beaufort of the British Navy in 1806.

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Bed Load

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Portion of the stream load that is carried along the stream bed without being permanently suspend in the flowing water.

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Bergschrund

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A deep crevasse commonly found at the head of an alpine glacier. Forms when the glacial ice pulls away from the mountain side.

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Berm

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Low hill of sand that forms along coastal beaches.

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Bermuda High

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High pressure system that develops over the western subtropical North Atlantic. Also called Azores High.

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Big Bang

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Theory that suggests that about 15 billion years ago all of the matter and energy in the Universe was concentrated into an area smaller than a atom. At this instant, matter, energy, space and time did not exist. Then suddenly, the Universe began to expand at an incredible rate and matter, energy, space and time came into being. As the Universe expanded, matter began to coalesce into gas clouds, and then stars and planets. Some scientists believe that this expansion is finite and will one day cease. After this point in time, the Universe will begin to collapse until a Big Crunch occurs.

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Black Body

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Is a body that emits electromagnetic radiation, at any temperature, at the maximum possible rate per unit surface area. This body also absorbs all electromagnetic radiation that is intercepted by it.

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Blowout Depression

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Saucer shaped depressions created by wind erosion. At the leeward end of the feature there usually is a deposit of sand. Blowouts are found in coastal beach areas and in arid and semiarid regions of the world. These features are smaller than a deflation hollow.

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Body Wave

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Type of seismic wave that travels through the interior of Earth.

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Bottomset Bed

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Horizontal deltaic deposit of alluvial sediment composed of fine silt and clay.

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Bowen Reaction Series

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Model that explains the origin of the various types of igneous rocks. It suggests that the presence or absence of particular minerals in igneous rocks depends on the temperature of crystallization and on the magma's original chemical composition.

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Braided Stream

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Shallow stream channel that is subdivided into a number of continually shifting smaller channels that are separated by bar deposits.

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Calcification

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A dry environment soil-forming process that results in the accumulation of calcium carbonate in surface soil layers.

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Calcite

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Mineral formed from calcium carbonate. Common mineral found in limestone.

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Caldera

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A large circular depression in a volcano.

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Caldera Volcano

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Explosive type of volcano that leaves a large circular depression. Some of these depressions can be as large as 40 kilometers in diameter. These volcanoes form when wet granitic magma quickly rises to the surface of the Earth.

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Calving

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The loss of glacier mass when ice breaks off into a large water body like an ocean or a lake.

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Cambrian

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Geologic period that occurred from 570 to 505 million years ago. During this period, invertebrates become common in the oceans and the Burgess Shale was formed.

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Canadian Shield

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Very old igneous and metamorphic shield rock that covers much of northern Canada. Created more than two to three billion years ago.

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Canyon

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Steep-sided valley where depth is considerably greater than width. These features are the result of stream erosion.

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Carbonation

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Is a form of chemical weathering where carbonate and bicarbonate ions react with minerals that contain calcium, magnesium, potassium, and sodium.

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Catastrophism

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General theory that suggests that certain phenomena on the Earth are the result of catastrophic events. For example, the Biblical Flood is responsible for sedimentary rock formations and the extinction of the dinosaurs.

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Cave

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A natural cavity or recess that is roughly positioned horizontally to the surface of the Earth.

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Cenozoic

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Geologic era that occurred from 65 million years ago to today.

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Central Vent

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The main passage way by which volcanic magma travels to the Earth's surface.

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Centripetal Force

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Force required to keep an object moving in a circular pattern around a center of rotation. This force is directed towards the center of rotation. Common in meteorological phenomena like tornadoes and hurricanes.

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Chalk

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Form of limestone. This sedimentary rock is composed of the shells and skeletons of marine microorganisms.

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Chemical Weathering

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Breakdown of rock and minerals into small sized particles through chemical decomposition.

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Chinook Wind

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The name of a North American wind that occurs on the leeward side of mountains. This wind is warm and has a low humidity.

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Cinder Cone Volcano

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A small volcano, between 100 and 400 meters tall, made up of exploded rock blasted out of a central vent
at a high velocity. These volcanoes develop from magma of basaltic to intermediate composition.

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Circum-Pacific Belt

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A zone circling the edge of the Pacific Ocean basin where tectonic subduction causes the formation of volcanoes and trenches. Also called the ring of fire.

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Cirque

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Glacially eroded rock basin found on mountains.

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Cirrocumulus Clouds

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Patchy white high altitude cloud composed of ice crystals. Found in an altitude range from 5,000 to 18,000 meters.

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Cirrostratus Clouds

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High altitude sheet like clouds composed of ice crystals. These thin clouds often cover the entire sky. Found in an altitude range from 5,000 to 18,000 meters.

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Cirrus Clouds

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High altitude cloud composed of ice crystals. The appearance of these clouds is white feather like patches, filaments or thin bands. Found in an altitude range from 5,000 to 18,000 meters.

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Clastic Sedimentary Rock

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Sedimentary rocks that are formed by the lithification of weathered rock debris that has been physically transported and deposited.

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Clay

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Mineral particle with a size less than 0.004 millimeters in diameter. Also see silt and sand.

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Cleavage

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The tendency of some minerals or rocks to break along planes of weakness. This weakness occurs because of the nature of the bonds between mineral grains.

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Climate

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General pattern of weather conditions for a region over a long period time (at least 30 years).

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Climograph

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Two dimensional graph that plots a location's air temperature and precipitation on times scales that range from a 24 hour period to a year.

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Closed Talik

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Is a form of localized unfrozen ground (talik) in an area of permafrost. It is completely enclosed by permafrost in all directions.

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Coal

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Sedimentary rock composed of the compacted, lithified and altered remains of plants. Coal is a solid, combustible mixture of organic compounds, hydrocarbons, with 30% to 98% carbon by weight, mixed with various amounts of water and small amounts of sulfur and nitrogen compounds. It is formed in several stages as the remains of plants are subjected to heat and pressure over millions of years.

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Coastal Zone

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Relatively nutrient-rich, shallow part of the ocean that extends from the high-tide mark on land to the edge of the continental shelf.

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Col

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Saddle like depression found between two mountain peaks. Formed when two opposing cirque glaciers back erode an arête.

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Cold Front

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A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass.

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Cold Glacier

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Glacier in which the ice found from the its surface to base has a temperature as cold as -30° Celsius throughout the year. This is well below the pressure melting point. Pressure melting can cause the melting of ice at the base of these glaciers.

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Composite Volcano

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Volcano created from alternate layers of flows and exploded rock. Their height ranges from 100 to 3,500 meters tall. The chemistry of the magma of these volcanoes is quite variable ranging from basalt to granite.

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Condensation

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The change in state of matter from vapor to liquid that occurs with cooling. Usually used in meteorology when discussing the formation of liquid water from vapor. This process releases latent heat energy to the environment.

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Cone of Depression

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Cone shaped depression occurring horizontally across a water table. Causes by excessive removal of groundwater by a surface well.

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Confined Aquifer

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Aquifer between two layers of relatively impermeable earth materials, such as clay or shale.

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Confined Groundwater

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Groundwater trapped between two impervious layers of rock.

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Continental Arctic Air Mass (A)

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Air mass that forms over extensive landmass areas of the high latitudes. In the Northern Hemisphere, these system form only in winter over Greenland, northern Canada, northern Siberia, and the Arctic Basin. Continental Arctic air masses are very cold and extremely dry. These air masses are also very stable.

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Continental Crust

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Granitic portion of the Earth's crust that makes up the continents. Thickness of the continental crust varies between 20 to 75 kilometers. See sial layer.

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Continental Divide

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The elevated area that occurs on a continent that divides continental scale drainage basins.

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Continental Drift

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Theory that suggests that the Earth's crust is composed of several continental plates that have the ability to move. First proposed by A. Snider in 1858 and developed by F.B. Taylor (1908) and Alfred Wegener (1915).

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Continental Glacier

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Largest type of glacier with a surface coverage in the order of 5 million square kilometers.

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Continental Plate

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A rigid, independent segment of the lithosphere composed of mainly granite that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's continental plates are an average 125 kilometers thick and were formed more than 3 billion years ago.

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Continental Polar Air Mass (cP)

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Air mass that forms over extensive landmass areas of middle to high latitudes. In North America, these system form over northern Canada. Continental Polar air masses are cold and very dry in the winter and cool and dry in the summer. These air masses are also atmospherically stable in both seasons.

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Continental Tropical Air Mass (cT)

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Air mass that forms over extensive landmasses areas of the low latitudes. In North America, these system form over southwestern United States and northern Mexico. Continental Tropical air masses are warm and dry in the winter and hot and dry in the summer. These air masses are also generally unstable in the winter and stable in the summer.

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Contour (Line)

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Line on a topographic map that connects all points with the same elevation.

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Continuous Permafrost

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Form of permafrost that exists across a landscape as an unbroken layer.

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Convectional Lifting

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The vertical lifting of parcels of air through convective heating of the atmosphere. This process can initiate adiabatic processes inside the air parcel.

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Convectional Precipitation

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Is the formation of precipitation due to surface heating of the air at the ground surface. If enough heating occurs, the mass of air becomes warmer and lighter than the air in the surrounding environment, and just like a hot air balloon it begins to rise, expand and cool. When sufficient cooling has taken place saturation occurs forming precipitation. This process is active in the interior of continents and near the equator forming cumulus clouds and possible later thunderstorms. Rain is usually the precipitation type that is formed, and in most cases this moisture is delivered in large amounts over short periods of time in extremely localized areas.

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Convergence

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Horizontal inflow of wind into an area. Once at the area, the wind then travels vertically.

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Convergence Precipitation

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The formation of precipitation due to the convergence of two air masses. In most cases, the two air masses have different climatological characteristics. One is usually warm and moist, while the other is cold and dry. The leading edge of the latter air mass acts as an inclined wall or front causing the moist warm air to be lifted. Of course the lifting causes the warm moist air mass to cool due to expansion resulting in saturation. This precipitation type is common at the mid-latitudes where cyclones form along the polar front. Also called frontal precipitation.

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Convergent Lifting

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The vertical lifting of parcels of air through the convergence of opposing air masses in the atmosphere. This process can initiate adiabatic processes inside the air parcel.

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Core

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The core is a layer rich in iron and nickel found in the interior of the Earth. It is composed of two sub-layers: the inner core and outer core. The core is about 7,000 kilometers in diameter.

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Coriolis Force

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An apparent force due to the Earth's rotation. Causes moving objects to be deflected to the right in the Northern Hemisphere and to the left in the Southern hemisphere. Coriolis force does not exist on the equator. This force is responsible for the direction of flow in meteorological phenomena like mid-latitude cyclones, hurricanes, and anticyclones.

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Counter-Radiation

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Redirection of the Earth's longwave

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Creep

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Slow mass movement of soil downslope. Occurs where the stresses on the slope material are too small to create a rapid failure.

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Cretaceous

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Geologic period that occurred roughly 65 to 144 million years ago. During this period, the first flowering plant species appear and dinosaurs are at their greatest diversity. Dinosaurs die out at the end of this period.

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Crevasse

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(1) Opening on a levee that allows for the drainage of water from the floodplain to the stream channel.
(2) Fracture on the brittle surface of a glacier.

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Crust

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Earth's outer most layer of solid rock. Between 7 to 70 kilometers thick.

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Cryosol Soil

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This soil is common to high latitude tundra environments. The main identifying feature of this soil is a layer of permafrost within one meter of the soil surface.

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Cryotic

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Something that is frozen.

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Cumulus Cloud

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Puffy clouds with relatively flat bases. Cumulus clouds form when moist warm air bubbles vertically escape from the Earth's surface. Found in an altitude range from 300 to 2,000 meters.

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Cumulonimbus Cloud

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A well developed vertical cloud that often has top shaped like an anvil. These clouds are very dense with condensed and deposited water. Weather associated with this cloud includes: strong winds; hail; lightning; tornadoes; thunder; and heavy rain. When this weather occurs these clouds are then thunderstorms. Can extend in altitude from a few hundred meters above the surface to more than 12,000 meters.

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Cyclone

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Area of low pressure in the atmosphere that displays circular inward movement of air. In the Northern Hemisphere circulation is counterclockwise, while Southern Hemisphere cyclones have clockwise wind patterns.

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Debris Flow

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A type of mass movement where there is a downslope flow of a saturated mass of soil, sediment, and rock debris.

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Deflation

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Process where wind erosion creates blowout depressions or deflation hollows by removing and transporting sediment and soil.

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Deflation Hollow

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A surface depression or hollow commonly found in arid and semiarid regions caused by wind erosion. Also see the related blowout depression.

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Degradation

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Readjustment of the stream profile where the stream channel is lowered by the erosion of the stream bed. Usually associated with high discharges.

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Delta

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Large deposit of alluvial sediment located at the mouth of a stream where it enters a body of standing water.

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Dendritic

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Term used to describe the stream channel pattern that is completely random. Resembles the branching pattern of blood vessels or tree branches.

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Deposition

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(1) The change in state of matter from gas to solid that occurs with cooling. Usually used in meteorology when discussing the formation of ice from water vapor. This process releases latent heat energy to the environment.
(2) Laying down of sediment transported by wind, water, or ice.

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Depositional Landform

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Is a landform formed from the deposition of weathered and eroded surface materials. On occasion, these deposits can be compressed, altered by pressure, heat and chemical processes to become sedimentary rocks. This includes landforms with some of the following geomorphic features: beaches, deltas, floodplains, and glacial moraines.

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Depression

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(1) Concave hollow found on the Earth's surface.
(2) Term used to describe a cyclone or an atmospheric low pressure system.

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Deranged Drainage

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Drainage pattern that is highly irregular. Areas that have experienced continental glaciation may have this type of drainage pattern.

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Desertification

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Conversion of marginal rangeland or cropland to a more desert like land type.

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Detrital Rock

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Sedimentary rock that is composed of particles transported to their place of deposition by erosional processes. Examples of such rock include sandstone and shale.

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Devonian

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Geologic period that occurred roughly 360 to 408 million years ago. During this period, the first amphibians and trees appear.

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Dew Point

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Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%.

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Diorite

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A coarse grained igneous rock of intrusive origin that is darker and chemically more mafic than granite.

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Discontinuous Permafrost

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Form of permafrost that contains numerous scattered pockets of unfrozen ground.

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Dissolved Load

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Portion of the stream load that is in solution in the flowing water.

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Distributary

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A smaller branching stream channel that flows away from a main stream channel. Common on deltas. Opposite of tributary.

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Divergence

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Horizontal outflow of wind from an area. In a surface divergence, outflow originates from the upper atmosphere.

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Divide

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The topographic ridge that separates drainage basins.

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Doldrums

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Area of low atmospheric pressure and calm westerly winds located at the equator. Similar to Intertropical Convergence Zone.

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Dune

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(1) Stream bed deposit found streams whose channel is composed mainly of sand and silt. Dunes are about 10 or more centimeters in height and are spaced a meter or more apart and are common in streams with high velocities.
(2) Terrestrial deposit of sand that resembles a mound or ridge that was formed from aeolian processes.

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Drainage Basin

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Land surface region drained by a length of stream channel.

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Drainage Divide

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Topographic border between adjacent drainage basins or watersheds.

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Drainage Pattern

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Geometric pattern that a stream's channels take in the landscape. These patterns are controlled by factors such as slope, climate, vegetation, and bedrock resistance to erosion.

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Drift

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Any material deposited by a glacier.

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Drought

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Climatic condition where water loss due to evapotranspiration is greater than water inputs through precipitation.

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Drumlin

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A hill shaped deposit of till. The shape of these features resembles an elongated teaspoon laying bowl down. The tapered end of the drumlin points to the direction of glacier advance. Drumlins come in assorted sizes. Lengths can range from 100 to 5,000 meters and heights can be as great as 200 meters.

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Dry Adiabatic Lapse Rate (DALR)

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The rate of decline in the temperature of a rising parcel of air before it has reached saturation. This rate of temperature decline is 9.8° Celsius per 1000 meters because of adiabatic cooling.

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Dry Line

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A boundary the separates dry and moist air in the warm sector of a mid-latitude cyclone wave. Found ahead of the cold front.

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Dyke

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Thin vertical veins of igneous rock that form when magma enters and cools in fractures found within the crust. Also see intrusive igneous rock.

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Earthquake

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Is a sudden motion or trembling in the Earth. The motion is caused by the quick release of slowly accumulated energy in the form of seismic waves. Most earthquakes are produced along faults, tectonic plate boundaries, or along the mid-oceanic ridges.

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Earthquake Focus

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Point of stress release in an earthquake.

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Easterly Wave

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Atmospheric disturbance in the tropical trade winds. Occasionally these systems intensify into hurricanes.

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Effusive Eruption

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Volcanic eruption where low-viscosity basaltic magma is released. This type of eruption is not explosive and tends to form shield volcanoes.

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Elastic Rebound Theory

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Theory that describes how earthquakes arise from the horizontal movement of adjacent tectonic plates along a linear strike-slip fault. This theory suggests that the two plates moving in opposite directions become locked for some period of time because of friction. However, the accumulating stress overcomes the friction and causes the plate to suddenly move over a short time period which generates an earthquake.

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El Nino

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Name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru. When this warming occurs the tropical Pacific trade winds weaken and the usual upwelling of cold, nutrient rich deep ocean water off the coast of Ecuador and Peru is reduced. The El Nino normally occurs around Christmas and lasts usually for a few weeks to a few months. Sometimes an extremely warm event can develop that lasts for much longer time periods.

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Endogenic

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Refers to a system that is internal to the Earth.

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Entrainment

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One of three distinct processes involved in erosion. More specifically, it is the process of particle lifting by an agent of erosion.

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Environmental Lapse Rate (ELR

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The rate of air temperature increase or decrease with altitude. The average ELR in the troposphere is an air temperature decrease of 6.5° Celsius per 1000 meters rise in elevation.

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Eon

Back 202

Longest geologic time unit.

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Epicenter

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Surface location of an earthquake's focus.

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Epoch

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Geologic time unit that is shorter than a period.

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Equinox

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Two days during the year when the declination of the Sun is at the equator. The September equinox occurs on September 22 or 23. The March equinox occurs on March 20 or 21. On these days, all locations on our planet (except the poles) experience equal (12 hour) day and night.

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Era

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Geologic time unit that is shorter than an eon but longer than a period.

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Erosion

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The removal of weathered sediment or rocks by the forces of wind, water, and ice.

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Erratic

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A large rock boulder that has been transported by glaciers away from its origin and deposited in a region of dissimilar rock.

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Esker

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Long twisting ridges of sand and gravel found on the Earth's surface. Created when the deposits of subsurface glacial streams are placed on the ground after glacial melting.

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Estuary

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Somewhat enclosed coastal area at the mouth of a river where nutrient rich fresh water meets with salty ocean water.

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Eutrophication

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Physical, chemical and biological changes in a water body as a result of the input nitrogen and phosphorus.

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Eutrophic Lake

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Lake that has an excessive supply of nutrients, mostly in the form of nitrates and phosphates.

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Evaporation

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the process by which liquid water is converted into a gaseous state. Evaporation can only occur when water is available. It also requires that the humidity of the atmosphere be less than the evaporating surface (at 100% relative humidity there is no more evaporation). The evaporation process requires large amounts of energy. For example, the evaporation of one gram of water at a temperature of 100° Celsius requires 540 calories of heat energy (600 calories at 0° Celsius).

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Evaporation Fog

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A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called steam fog or sea smoke.

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Evapotranspiration

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Combined loss of water to the atmosphere via the processes of evaporation and transpiration.

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Exogenic

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Refers to a system that is external to the Earth.

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Exotic Stream

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A stream that has a course that begins in a humid climate and end in an arid climate. Because of reductions in precipitation and and increases in evaporation, the discharge of these streams deceases downslope. Examples of such streams are the Nile and Colorado Rivers.

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Explosive Eruption

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Volcanic eruption where high-viscosity granite-rich magma causes an explosion of ash and pyroclastic material. This type of eruption is common to composite and caldera volcanoes.

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Extrusive Igneous Rock

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Igneous rock that forms on the surface of the Earth.

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Eye

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Area in the center of a hurricane that is devoid of clouds.

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Fault

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A fracture in rock caused by stress.

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Fault Plane

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The plane that represents the fracture surface of a fault.

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Feldspar

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A group of common aluminum silicate minerals that contains potassium, sodium, or calcium.

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Felsic Magma

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Magma that is relatively rich in silica, sodium, aluminum, and potassium. This type of magma solidifies to form rocks relatively rich in silica, sodium, aluminum, and potassium.

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Ferrel Cell

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Three-dimensional atmospheric circulation cell located at roughly 30 to 60° North and South of the equator.

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Ferricretes

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Sedimentary rock created by the chemical precipitation of iron.

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Field Capacity

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The water remaining in a soil after the complete draining of the soil's gravitational water.

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Firn

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Névé on a glacier that survives the year's ablation season.

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Firn Limit

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The lower boundary of the zone of accumulation on a glacier where snow accumulates on an annual basis.

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Fissure

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Opening or crack in the Earth's crust.

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Fjord

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A glacial valley or glacial trough found along the coast that is now filled with a mixture of fresh water and seawater.

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Flash Flood

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A rapid and short-lived increase in the amount of runoff water entering a stream resulting in a flood.

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Floodplain

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Relatively flat area found alongside the stream channel that is prone to flooding and receives alluvium deposits from these inundation events.

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Föhn Wind

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European equivalent of chinook wind.

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Fog

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Fog exists if the atmospheric visibility near the Earth's surface is reduced to 1 kilometer or less. Fog can be composed of water droplets, ice crystals or smoke particles. Fogs composed primarily of water droplets are classified according to the process that causes the air to cool to saturation. Common types of this type of fog include: radiation fog; upslope fog; advection fog; evaporation fog; ice fog; and frontal fog.

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Fold

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Wavelike layers in rock strata that are the result of compression.

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Folding

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The deformation of rock layers because of compressive forces to form folds.

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Foliation

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Process where once randomly distributed platy minerals in a rock become reoriented, because of metamorphism, in a parallel manner.

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Foreset Bed

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Deltaic deposit of alluvial sediment that is angled 5 to 25° from horizontal. Most of the delta is made up of these deposits.

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Foreshock

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Small earth tremors that occur seconds to weeks before a significant earthquake event.

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Freezing Rain

Back 241

A type of precipitation. Occurs when liquid rain hits a cold surface and then immediately freezes into ice. For this to occur, a surface temperature inversion is usually required. In such an inversion, the surface must have a temperature below freezing, while the temperature of the atmosphere where the precipitation forms is above freezing.

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Fresh Water

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Water that is relatively free of salts.

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Frictional Force

Back 243

Force acting on wind near the Earth's surface due to frictional roughness. Causes the deceleration of wind.

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Front

Back 244

Transition zone between air masses with different weather characteristics.

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Frontal Fog

Back 245

Is a type of fog that is associated with weather fronts, particularly warm fronts. This type of fog develops when frontal precipitation falling into the colder air ahead of the warm front causes the air to become saturated through evaporation.

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Frontal Lifting

Back 246

Lifting of a warmer or less dense air mass by a colder or more dense air mass at a frontal transitional zone.

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Frost

Back 247

Deposition of ice at the Earth's surface because of atmospheric cooling.

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Frost Point

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Is the temperature at which water vapor saturates from an air mass into solid usually forming snow or frost.

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Frost Wedging

Back 249

A process of physical weathering in which water freezes in a crack and exerts force on the rock causing further rupture.

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Fujita Tornado Intensity Scale

Back 250

Tornado classification system developed by T. Theodore Fujita. This system six levels from F0 to F5. These levels are based on the estimated speed of the tornado's winds from proxy information like property damage.

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Funnel Cloud

Back 251

A tornado which is beginning its descent from the base of a cumulonimbus cloud. This severe weather event may or may not reach the ground surface.

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Gabbro

Back 252

An intrusive igneous rock that develops from mafic magma and whose mineral crystals are coarse. Mineralogically this rock is identical to basalt.

Front 253

Galaxy

Back 253

An assemblage of millions to hundreds of billions of stars.

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Gelifluction

Back 254

Form of mass movement in periglacial environment where a permafrost layer exists. It is characterized by the movement of soil material over the permafrost layer and the formation of lobe-shaped features. Also see solifluction.

Front 255

General Circulation Model (GCM)

Back 255

Computer-based climate model that produces future forecast of weather and climate conditions for regions of the Earth or the complete planet. Uses complex mathematical equations and physical relationships to determine a variety of climate variables in a three-dimensional grid.

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Geology

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The field of knowledge that studies the origin, structure, chemical composition, and history of the Earth and other planets.

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Geomorphic Threshold

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The amount of slow accumulated change a landform can take before it suddenly moves into an accelerated rate of change that takes it to a new system state.

Front 258

Geostrophic Wind

Back 258

Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.

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Glacial (glaciation)

Back 259

1) Period of time during an ice age when glaciers advance because of colder temperatures.
(2) Involving glaciers and moving ice. Usually pertaining to processes associated with glaciers.

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Glacial Drift

Back 260

A generic term applied to all glacial and glaciofluvial deposits.

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Glacial Ice

Back 261

A very dense form frozen water that is much harder than snow, névé, or firn.

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Glacial Lake

Back 262

A natural impoundment of meltwater at the front of a glacier.

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Glacial Retreat

Back 263

The backwards movement of the snout of a glacier.

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Glacial Polish

Back 264

The abrasion of bedrock surfaces by materials carried on the bottom of a glacier. This process leaves these surfaces smooth and shiny.

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Glacial Surge

Back 265

A rapid forward movement of the snout of a glacier.

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Glacial Trough

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A deep U-shaped valley with steep valley walls that was formed from glacial erosion. At the base of many of these valleys are cirques.

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Glacial Uplift

Back 267

Upward movement of the Earth's crust following isostatic depression from the weight of the continental glaciers.

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Glacier

Back 268

A large long lasting accumulation of snow and ice that develops on land. Most glaciers flow along topographic gradients because of their weight and gravity.

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Glaciofluvial

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Geomorphic feature whose origin is related to the processes associated with glacial meltwater.

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Gneiss

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A metamorphosed coarse grained igneous rock. In this rock you get the recrystallization of quartz, feldspars, micas and amphiboles into bands.

Front 271

Graben Fault

Back 271

This fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys.

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Graded Stream

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A stream that has a long profile that is in equilibrium with the general slope of the landscape. A graded profile is concave and smooth. Stream's maintain their grade through a balance between erosion, transportation, and deposition. Erosion removes material from bumps in the profile and deposition fills in dips.

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Gradient

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The steepness of a slope as measured in degrees, percentage, or as a distance ratio (rise/run).

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Gradient Wind

Back 274

Horizontal wind in the upper atmosphere that moves parallel to curved isobars. Results from a balance between pressure gradient force, Coriolis force, and centripetal force.

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Granite

Back 275

Medium to coarse grained igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.

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Granitic Magma

Back 276

Felsic magma that generates mainly granitic rocks.

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Graupel

Back 277

A type of precipitation that consists of a snow crystal and a raindrop frozen together. Also called snow pellets.

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Gravel

Back 278

A term used to describe unconsolidated sediments composed of rock fragments. These rock fragments have a size that is greater than 2 millimeters.

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Gravitational Water

Back 279

Water that moves through soil due to gravitational forces. Soil water in excess of hygroscopic water and capillary water.

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Ground Frost

Back 280

Frost that penetrates the soil surface in response to freezing temperatures.

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Ground Ice

Back 281

General term used to describe all bodies of ice in the ground surface of the permafrost layer.

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Ground Moraine

Back 282

A thick layer of till deposited by a melting glacier.

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Groundwater

Back 283

Water that occupies the pore spaces found in some types of bedrock.

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Groundwater Recharge

Back 284

The replenishment of groundwater with surface water.

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Gulf Stream

Back 285

Warm ocean current that originates in and around the Caribbean and flows across the North Atlantic to northwest Europe.

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Gust Front

Back 286

A boundary found ahead of a thunderstorm that separates cold storm downdrafts from warm humid surface air. Winds in this phenomenon are strong and fast.

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Gypsum

Back 287

Sedimentary rock created by the chemical precipitation of calcium, sulfur, and oxygen.

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Hadean

Back 288

Geologic eon that occurred from 3800 to 4600 million years ago. The Earth's oldest rocks date to the end of this time period.

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Hadley Cell

Back 289

Three-dimensional atmospheric circulation cell located at roughly 0 to 30° North and South of the equator. The Hadley cell consists of rising air (intertropical convergence zone) at the equator and descending air (subtropical highs) at 30° North and South.

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Hail

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Hail is a solid form of precipitation that has a diameter greater than 5 millimeters. Occasionally, hailstones can be the size of golf balls or larger. Hailstones of this size can be quite destructive. The intense updrafts in mature thunderstorm clouds are a necessary requirement for hail formation.

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Halite

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Sedimentary rock created by the chemical precipitation of sodium and chlorine.

Front 292

Hanging Valley

Back 292

A secondary valley that enters a main valley at an elevation well above the main valley's floor. These features are result of past erosion caused by alpine glaciers. Hanging valleys are often the site of spectacular waterfalls.

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Hardpan

Back 293

Impervious layer found within the soil. It can result from the precipitation of iron, illuviation of clay or the cementing of sand and gravel by calcium carbonate precipitates.

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Headwaters

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Upper portion of stream's drainage system.

Front 295

High Pressure

Back 295

An area of atmospheric pressure within the Earth's atmosphere that is above average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars it is called an anticyclone.

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Holocene Epoch

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Period of time from about 10,000 years ago to today. During this period glaciers retreated because of a warmer global climate. Time of modern humans.

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Horn

Back 297

Pyramidal peak that forms when several cirques erode a mountain from three or more sides.

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Horst Fault

Back 298

A fault that is produced when two reverse faults cause a block of rock to be push up.

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Hot Spot

Back 299

A volcanic area on the surface of the Earth created by a rising plume of magma.

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Hurricane

Back 300

An intense cyclonic storm consisting of an organized mass of thunderstorms that develops over the warm oceans of the tropics. To be classified as a hurricane, winds speeds in the storm must be greater than 118 kilometers per hour.

Front 301

Hydraulic Gradient

Back 301

The slope of the water table or aquifer. The hydraulic gradient influences the direction and rate of groundwater flow.

Front 302

Hydrograph

Back 302

A graph describing stream discharge over time.

Front 303

Hydrostatic Pressure

Back 303

Force caused by water under pressure.

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Hygrometer

Back 304

An instrument for measuring atmospheric humidity.

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Hygroscopic

Back 305

Substances that have the ability to absorb water and therefore accelerate the condensation process.

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Hygroscopic Water

Back 306

Water held within 0.0002 millimeters of the surface of a soil particle. This water is essentially non-mobile and can only be removed from the soil through heating.

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Ice Age

Back 307

Period of time when glaciers dominate the landscape of the Earth. The last major Ice Age was during the Pleistocene epoch.

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Iceberg

Back 308

A mass of ice found floating in the ocean or a lake. Often icebergs form when ice calves from land-based glaciers into the water body. Icebergs can be dangerous to shipping in high and mid-latitude regions of the ocean because 90 percent of their mass lies below the ocean surface.

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Ice Cap

Back 309

Large dome-shaped glacier found covering a large expanse of land. Smaller than an ice sheet.

Front 310

Ice Fall

Back 310

An area of crevassed ice on a glacier. Caused when the base of the glacier flows over steep topography.

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Ice Fall

Back 311

An area of crevassed ice on a glacier. Caused when the base of the glacier flows over steep topography.

Front 312

Ice Field

Back 312

Large level area of glacial ice found covering a large expanse of land. Similar in size to an ice cap but does not have a dome-shape.

Front 313

Ice Jam

Back 313

The accumulation of ice at a specific location along a stream channel. Can cause the reduction of stream flow down stream of the obstruction and flooding upstream.

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Icelandic Low

Back 314

Subpolar low pressure system found near Iceland. Most developed during the winter season. This large-scale pressure system spawns mid-latitude cyclones.

Front 315

Ice Pellets

Back 315

A type of precipitation. Ice pellets or sleet are transparent or translucent spheres of frozen water that fall from clouds. Ice pellets have a diameter less than 5 millimeters. To form, this type of precipitation requires an environment where raindrops develop in an atmosphere where air temperature is above freezing. These raindrops then fall into a lower layer of air with freezing temperatures. In this lower layer of cold air, the raindrops freeze into small ice pellets. Like freezing rain, an air temperature inversion is required for development of ice pellets.

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Ice Sheet

Back 316

A dome-shaped glacier covering an area greater than 50,000 square kilometers. Greenland and Antarctica are considered ice sheets. During the glacial advances of the Pleistocene ice sheets covered large areas of North America, Europe, and Asia. Larger than an ice cap.

Front 317

Ice Shelf

Back 317

Large flat layer of ice that extends from the edge of the Antarctic ice cap into the Antarctic Ocean. Source of icebergs.

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Ice Wedge

Back 318

Wedge-shaped, ice body composed of vertically oriented ground ice that extends into the top of a permafrost layer. These features are approximately 2 to 3 meters wide at their top and extend into the soil about 8 to 10 meters. Form in cracks that develop in the soil during winter because of thermal contraction. In the spring, these cracks fill with liquid water from melting snow which subsequently re-freezes. The freezing process causes the water to expand in volume increasing the size and depth of the crack. The now large crack fills with more liquid water and again it freezes causing the crack to enlarge.This process continues for many cycles until the ice wedge reaches its maximum size.

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Igneous Rock

Back 319

Rocks formed by solidification of molten magma either beneath (intrusive igneous rock) or at (extrusive igneous rocks) the Earth's surface.

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Infiltration

Back 320

The absorption and downward movement of water into the soil layer.

Front 321

Infiltration Capacity

Back 321

The ability of a soil to absorb surface water.

Front 322

Inner Core

Back 322

Inner region of the Earth's core. It is thought to be solid iron and nickel with a density of about 13 grams per cubic centimeter. It also has a diameter of about 1220 kilometers.

Front 323

Insolation Weathering

Back 323

Form of physical weathering. Involves the physical breakdown of minerals and rock due to thermal expansion and contraction.

Front 324

Instability

Back 324

Atmospheric condition where a parcel of air is warmer that the surrounding air in the immediate environment. This condition causes the parcel to rise in the atmosphere. Also see unstable atmosphere.

Front 325

Interception

Back 325

Is the capture of precipitation by the plant canopy and its subsequent return to the atmosphere through evaporation or sublimation. The amount of precipitation intercepted by plants varies with leaf type, canopy architecture, wind speed, available radiation, temperature, and the humidity of the atmosphere.

Front 326

Interglacial

Back 326

Period of time during an ice age when glaciers retreated because of milder temperatures.

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Intertropical Convergence Zone (ITCZ)

Back 327

Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating. Location of the thermal equator.

Front 328

Intrusive Igneous Rock

Back 328

A mass of igneous rock that forms when magma from the mantle migrates upward and cools and crystallizes near, but not at, the Earth's surface. Also called plutonic igneous rock. Also see dyke, sill, and batholith.

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Island Arc

Back 329

A line of volcanic islands found of the ocean that have been created by the convergence of two tectonic plates and the subsequent subduction of one of the plates beneath the other. Subduction cause magma plumes to rise to the Earth's surface creating the volcanic islands.

Front 330

Isobar

Back 330

Lines on a map joining points of equal atmospheric pressure.

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Isostacy

Back 331

The buoyant condition of the Earth's crust floating in the asthenosphere. The greater the weight of the crust the deeper it floats into the asthenosphere. When weight is remove the crust rises higher.

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Isostatic Depression

Back 332

Large scale sinking of the crust into the asthenosphere because of an increase in weight on the crustal surface. Common in areas of continental glaciation where the crust was depressed by the weight of the ice.

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Isostatic Rebound

Back 333

The upward movement of the Earth's crust following isostatic depression.

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Isothermal Layer

Back 334

Vertical layer in the atmosphere where temperature remains unchanged. In the Earth's atmosphere, three isothermal layers are found in the lower regions of the stratosphere, mesosphere, and the thermosphere.

Front 335

Isotopic Dating

Back 335

Dating technique used to determine the age of rock and mineral through the decay of radioactive elements.

Front 336

Jet Stream

Back 336

Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. A number of jet streams have been identified in the atmosphere. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers. This jet stream flows from west to east at average speeds, depending on the time of year, between 110 to 185 kilometers per hour. Another strong jet stream occurs above the sub-tropical highs at an altitude of 13 kilometers. This jet stream is commonly called the subtropical jet stream. The subtropical jet stream's winds are not as strong as the polar jet stream.

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Joint

Back 337

A fracture in a rock where no movement has taken place or where no movement has taken place perpendicular to the surface of the fracture. Important in rock weathering because it increases the exposed surface area.

Front 338

Jurassic

Back 338

Geologic period that occurred roughly 144 to 208 million years ago. During this period, the first birds and mammals appear and large areas of the continents are covered by shallow seas.

Front 339

Kame

Back 339

A steep conical hill composed of glaciofluvial sediments. This feature develops when glacial crevasses and depressions in stagnant glacial ice are filled with sand and gravel deposits from sediment loaded meltwater.

Front 340

Kame Terrace

Back 340

A long flat ridge composed of glaciofluvial sediment. This feature forms along the margin of a valley glacier where the glacial ice meets the valley's slope. Sediment is deposited by laterally flowing meltwater streams.

Front 341

Kaolinite

Back 341

A type of clay that is widespread in tropical and subtropical regions. It is produced from the weathering of granite.

Front 342

Karst

Back 342

Landform type with limestone bedrock and dominated by geomorphic features created from solution chemical weathering.

Front 343

Katabatic Wind

Back 343

Any wind blowing down the slope of a mountain.

Front 344

Kettle Hole

Back 344

Depression found in glacial deposits. Created when a piece of ice from a retreating glacier becomes embedded in soft glacial till or glacial drift deposits. Many are filled with water to form a small lake or pond.

Front 345

Kettle Moraine

Back 345

An area of glaciofluvial influenced moraine deposits pitted with kames and kettle holes.

Front 346

Köppen Climate Classification

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System that uses monthly precipitation and temperature data and total annual precipitation data to classify a location's climate into one of five main categories: Tropical Moist Climates; Dry Climates; Moist Mid-latitude Climates with Mild Winters; Moist Mid-Latitude Climates with Cold Winters; and Polar Climates. These categories are further divided into number of subcategories. First developed in 1918 by German biologist W. Köppen, this system has undergone a number of modifications.

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Lahar

Back 347

A very rapid type of downslope mass movement that involving mudflows from volcanic ash.

Front 348

Landfall

Back 348

The coastline location where a tropical storm or hurricane moves from ocean onto land.

Front 349

Landslide

Back 349

Term used to describe the downslope movement of soil, rock, and other weathered materials because of gravity.

Front 350

La Nina

Back 350

Condition opposite of an El Nino. In a La Nina, the tropical Pacific trade winds become very strong and an abnormal accumulation of cold water occurs in the central and eastern Pacific Ocean.

Front 351

Laminar Flow

Back 351

Movement of water within a stream that occurs as uninterrupted parallel flows. Laminar flow generally occurs in areas where friction is low.

Front 352

Latent Heat

Back 352

Is the energy required to change a substance to a higher state of matter (solid > liquid > gas). This same energy is released from the substance when the change of state is reversed (gas > liquid > solid).

Front 353

Lateral Moraine

Back 353

Moraine that is found along the sides of a glacier. Commonly found on glaciers that occupy a valley.

Front 354

Laurasia

Back 354

Northern section of Pangaea.

Front 355

Lava

Back 355

Molten magma released from a volcanic vent or fissure.

Front 356

Leaf Drip

Back 356

The rain water that fall to the ground surface from plant leaves after it has been intercepted by these structures.

Front 357

Lee

Back 357

Side of a slope that is opposite to the direction of flow of ice, wind, or water. Opposite of stoss.

Front 358

Levee

Back 358

Ridge of coarse deposits found alongside the stream channels and elevated above the floodplain. Forms from the deposition of sediment during floods.

Front 359

Limestone

Back 359

Sedimentary rock composed of carbonate minerals, especially calcium carbonate. Limestone can be created by clastic and non-clastic processes. Clastic limestones are formed from the break up and deposition of shells, coral and other marine organisms by wave-action and ocean currents. Non-clastic limestones can be formed either as a precipitate or by the lithification of coral reefs, marine organism shells, or marine organism skeletons.

Front 360

Liquefaction

Back 360

Temporary transformation of a soil mass of soil or sediment into a fluid mass. Occurs when the cohesion of particles in the soil or sediment is lost. Often triggered by seismic waves from an earthquake. For this condition to take place the pore spaces between soil particles must be at or near saturation.

Front 361

Lithification

Back 361

Process by which sediments are consolidated into sedimentary rock.

Front 362

Loam

Back 362

A soil that contains a roughly equal mixture of clay, sand, and silt. Good for growing most crops.

Front 363

Loess

Back 363

Deposits of silt laid down by aeolian processes over extensive areas of the mid-latitudes during glacial and postglacial times.

Front 364

Longshore Transport

Back 364

The transport of sediment in water parallel to a shoreline.

Front 365

Long Wave

Back 365

A large wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with short waves. Also called Rossby waves.

Front 366

Low Pressure

Back 366

An area of atmospheric pressure within the Earth's atmosphere that is below average. If this system is on the Earth's surface and contains circular wind flow and enclosed isobars it is called a cyclone.

Front 367

Lower Mantle

Back 367

Layer of the Earth's interior extending from 670 to 2,900 kilometers below the surface crust. Composed of ultramafic rock. This layer is hot and plastic and part of the mantle layer.

Front 368

Lysimeter

Back 368

Meteorological instrument used to measure potential and actual evapotranspiration.

Front 369

Mafic Magma

Back 369

Magma that is relative poor in silica but rich in calcium, magnesium, and iron content. This type of magma solidifies to form rocks relatively rich in calcium, magnesium, and iron but poor in silica.

Front 370

Magma

Back 370

Molten rock originating from the Earth's interior.

Front 371

Magma Plume

Back 371

A rising vertical mass of magma originating from the mantle.

Front 372

Magnetic Field

Back 372

The space influence by magnetic force. The Earth's magnetic field is believed to be generated by the planet's core.

Front 373

Magnetic Reversal

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A change in the polarity of the Earth's magnetic field. In the past 4 million years there have been nine reversals.

Front 374

Magnetosphere

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Zone that surrounds the Earth that is influenced by the Earth's magnetic field.

Front 375

Mantle

Back 375

Layer of the Earth's interior composed of mostly solid rock that extends from the base of crust to a depth of about 2,900 kilometers.

Front 376

Marble

Back 376

Metamorphic rock created by the recrystallization of calcite and/or dolomite.

Front 377

Maritime Effect

Back 377

The effect that large ocean bodies have on the climate of locations or regions. This effect results in a lower range in surface air temperature at both daily and annual scales. Also see Continental Effect.

Front 378

Maritime Polar Air Mass (mP)

Back 378

Air mass that forms over extensive ocean areas of the middle to high latitudes. Around North America, these air mass system form over the Atlantic and Pacific oceans at the middle latitudes. Maritime Polar air masses are mild and humid in summer and cool and humid in winter. In the Northern Hemisphere, maritime polar air masses are normally unstable during the winter. In the summer, atmospheric stability depends on the position of the air mass relative to a continent. Around North America, Maritime Polar air masses found over the Atlantic are stable in summer, while Pacific systems tend to be unstable.

Front 379

Maritime Tropical Air Mass (mT)

Back 379

Air mass that forms over extensive ocean areas of the low latitudes. Around North America, these system form over the Gulf of Mexico and the eastern tropical Pacific. Maritime Tropical air masses are warm and humid in both winter and summer. In the Northern Hemisphere, maritime tropical air masses can normally stable during the whole year if they have form just west of a continent. If they form just east of a continent, these air masses will be unstable in both winter and summer.

Front 380

Meander

Back 380

Sinuous shaped stream channel. Usually found in streams flowing over a very shallow elevation grade.

Front 381

Medial Moraine

Back 381

Deposit of material found down the center of a glacier. Created when two glacier and their lateral moraines merge.

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Meltwater

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Water produced from the melting of snow and/or glacial ice.

Front 383

Mesa

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A flat topped hill that rises sharply above the surrounding landscape. The top of this hill is usually capped by a rock formation that is more resistant to weathering and erosion.

Front 384

Mesocyclone

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A cylinder of cyclonically flowing air that form vertically in a severe thunderstorm. They measure about 3 to 10 kilometers across. About 50% of them spawn tornadoes.

Front 385

Mesosphere

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Atmospheric layer found between the stratosphere and the thermosphere. Usually located at an average altitude of 50 to 80 kilometers above the Earth's surface. Air temperature within the mesosphere decreases with increasing altitude.

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Mesozoic

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Geologic era that occurred from 245 to 65 million years ago.

Front 387

Metamorphic Rock

Back 387

A rock that forms from the recrystallization of igneous, sedimentary or other metamorphic rocks through pressure increase, temperature rise, or chemical alteration.

Front 388

Mid-Latitude Cyclone

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Cyclonic storm that forms primarily in the middle latitudes. Its formation is triggered by the development of troughs in the polar jet stream. These storms also contain warm, cold and occluded fronts. Atmospheric pressure in their center can get as low as 970 millibars. Also called wave cyclones or frontal cyclones.

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Mid-Oceanic Ridge

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Chain of submarine mountains where oceanic crust is created from rising magma plumes and volcanic activity. Also associated with this feature is plate divergence which creates a rift zone.

Front 390

Milky Way Galaxy

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Aggregation of about 400 billion stars in a flattened, disk-shaped structure in space. Our solar system is found in this structure.

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Mineral

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Component of rocks. A naturally occurring inorganic solid with a crystalline structure and a specific chemical composition.

Front 392

Mississippian

Back 392

Geologic period that occurred roughly 320 to 360 million years ago. During this period, insects undergo major speciation and ferns first appear. Trees become a dominant plant form on continents.

Front 393

Mixing Ratio

Back 393

The ratio between the weight (mass) of water vapor (or some other gas) held in the atmosphere compared to the weight of the dry air in a given volume of air. Usually measured in grams water vapor (or gas) per kilogram of dry air.

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Moho Discontinuity

Back 394

The lower boundary of the crust. At this boundary seismic wave velocities show an increase in speed as they enter the upper mantle.

Front 395

Monocline

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A fold in layered rock that creates a slight bend.

Front 396

Monsoon

Back 396

A regional scale wind system that predictably change direction with the passing of the seasons. Monsoon winds blow from land to sea in the winter, and from sea to land in the summer. Summer monsoons are often accompanied with precipitation.

Front 397

Moraine

Back 397

A hill of glacial till deposited directly by a glacier.

Front 398

Mountain Breeze

Back 398

Local thermal circulation pattern found in areas of topographic relief. In this circulation system, surface winds blow from areas of higher elevation to valley bottoms during the night.

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Mouth

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End of a stream. Point at which a stream enters a lake, sea, or ocean.

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Mudflow

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Form of mass movement where fine textured sediments and soil mix with water to create a liquid flow.

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Mudstone

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Fine grained sedimentary rock composed of lithified silt and clay particles.

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Muskeg

Back 402

Poorly drained marshes or swamps found overlying permafrost.

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Needle Ice

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A form of periglacial ground ice that consists of groups ice slivers at or immediately below the ground surface. Needle ice is about a few centimeters long.

Front 404

Neutral Atmosphere

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Condition in the atmosphere where isolated air parcels do not have a tendency to rise or sink. The parcels of air tend to be same temperature as the air that surrounds them.

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Névé

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Partially melted and compacted snow that has a density of at least 500 kilograms per cubic meter.

Front 406

Nickpoint

Back 406

Point on the long profile of a stream where the gradient is broken sudden drop in elevation. Nickpoints are the locations of rapids and waterfalls.

Front 407

Nimbostratus Clouds

Back 407

Dark, gray low altitude cloud that produces continuous precipitation in the form of rain or snow. Found in an altitude range from the surface to 3,000 meters.

Front 408

Nivation

Back 408

Process where snow patches initiate erosion through physical weathering, meltwater flow, and gelifluction.

Front 409

Noctilucent Clouds

Back 409

High altitude clouds composed of ice crystals that appear to glow silver or bright blue shortly after sunset.

Front 410

Non-Clastic Sedimentary Rock

Back 410

Sedimentary rocks that are created either from chemical precipitation and crystallization, or by the lithification once living organic matter.

Front 411

Normal Fault

Back 411

Vertical fault where one slab of the rock is displaced up and the other slab down. It is created by tensional forces acting in opposite directions.

Front 412

Normal Lapse Rate

Back 412

Average rate of air temperature change with altitude in the troposphere. This value is approximately a decrease of 6.5° Celsius per 1000 meters rise in elevation.

Front 413

Nuee Ardente

Back 413

A glowing cloud of dense hot volcanic gas and ash that moves downslope at high speeds, incinerating the landscape.

Front 414

Obsidian

Back 414

Glassy dark colored volcanic rock. Usually composed of rhyolite.

Front 415

Occluded Front

Back 415

A transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.

Front 416

Ocean Current

Back 416

Large scale horizontal flow of ocean water that is persistent and driven by atmospheric circulation.

Front 417

Oceanic Crust

Back 417

Basaltic portion of the Earth's crust that makes up the ocean basins. Approximately 5 to 10 kilometers thick

Front 418

Oceanic Plate

Back 418

A rigid, independent segment of the lithosphere composed of mainly basalt that floats on the viscous plastic asthenosphere and moves over the surface of the Earth. The Earth's oceanic plates are an average 75 kilometers thick and were formed less than several hundred million years ago at one of the Earth's mid-oceanic ridges.

Front 419

Ocean Trench

Back 419

Deep depression found at the edge of the ocean floor. Represents area of tectonic plate subduction.

Front 420

Oligotrophic Lake

Back 420

Lake with a low supply of nutrients in its waters. Also see eutrophic lake and mesotrophic lake.

Front 421

Olivine

Back 421

Common silicate mineral found in rocks formed from mafic magma. Its chemical composition varies between Mg2SiO4 and Fe2SiO4.

Front 422

Open Talik

Back 422

Is a form of localized unfrozen ground (talik) in an area of permafrost. It is open to the ground surface but enclosed to permafrost below and at its sides.

Front 423

Orogenesis

Back 423

The process of mountain building through tectonic forces of compression and volcanism.

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Orogenic Belt

Back 424

A major range of mountains on the continents.

Front 425

Orographic Uplift

Back 425

Uplift of an air mass because of a topographic obstruction. Uplift also causes the cooling of the air mass. If enough cooling occurs condensation can occur and form into orographic precipitation.

Front 426

Orographic Precipitation

Back 426

Is precipitation that forms when air is forced to rise because of the physical presence of elevated land. As the parcel rises it cools as a result of adiabatic expansion at a rate of approximately 10° Celsius per 1,000 meters until saturation. The large amounts of precipitation along the west coast of Canada are due mainly to this process.

Front 427

Outer Core

Back 427

Outer region of the Earth's core. It is believed to be liquid nickel and iron and has a density of about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of about 2,250 kilometers.

Front 428

Outgassing

Back 428

The release of gas from cooling molten rock or the interior of the Earth. Much of the atmosphere's gaseous constituents, like water vapor, nitrogen, and argon, came from outgassing.

Front 429

Outwash

Back 429

Glaciofluvial sediments deposited by meltwater streams at the edge of a glacier.

Front 430

Outwash Plain

Back 430

A flat or gentle sloping surface of glaciofluvial sediments deposited by meltwater streams at the edge of a glacier. Usually found in close spatial association with moraines.

Front 431

Overbank Flow

Back 431

Movement of flood waters outside a stream channel during period of high discharge.

Front 432

Overland Flow

Back 432

The topographic movement of a thin film of water from precipitation to lower elevations. With time, this water will begin to organizing its flow into small channels called rills. The rills converge to form progressively larger channels until stream channels are formed. Occurs when the infiltration capacity of an area's soil has been exceeded. Also called sheet flow or runoff.

Front 433

Oxbow Lake

Back 433

Is portion of abandoned stream channel filled with stagnant water and cut off from the rest of the stream. Oxbow lakes are created when meanders are cut off from the rest of the channel because of lateral stream erosion.

Front 434

Ozone

Back 434

Tri-atomic oxygen that exists in the Earth's atmosphere as a gas.

Front 435

Pacific High

Back 435

High pressure system that develops over the central Pacific Ocean near the Hawaiian Islands. Also called the Hawaiian High.

Front 436

Paleozoic

Back 436

Geologic era that occurred from 570 to 245 million years ago.

Front 437

Pan

Back 437

(2) Compact soil horizon that has a high clay content.
(3) Large natural basin or depression found in desert climates.

Front 438

Pangaea

Back 438

Hypothetical super continent that existed in the geological past. Its break-up created the current configuration of landmasses found on the Earth.

Front 439

Paternoster Lakes

Back 439

A linear series of mountain valley lakes that are formed from glacial erosion. They form behind glacial moraines or in glacially carved out rock basins. The name of this feature is related to the series of lakes looking like a string of beads.

Front 440

Patterned Ground

Back 440

Term used to describe a number of surface features found in periglacial environments. These features can resemble circles, polygons, nets, steps, and stripes. The development of some of these shapes is thought to be the result of freeze-thaw action.

Front 441

Peak Annual Flow

Back 441

The largest discharge produced by a stream during a one year period.

Front 442

Pediment

Back 442

A gradually sloping bedrock surface located at the base of fluvial-eroded mountain range. Found in arid locations and normally covered by fluvial deposits.

Front 443

Pediplain

Back 443

An arid landscape of little relief that is occasionally interrupted by the presence of scattered inselbergs. Formed by the coalescence of several pediments.

Front 444

Perched Water Table

Back 444

Water table that is positioned above the normal water table for an area because of the presence of a impermeable rock layer.

Front 445

Percolation

Back 445

Vertical movement or infiltration of water from the Earth's surface to its subsurface. Movement usually stops when the flowing water reaches the water table.

Front 446

Peridotite

Back 446

Coarse grained ultramafic igneous rock composed mainly of olivine and pyroxene. The mantle is though to be composed primarily of this rock type.

Front 447

Periglacial

Back 447

Landforms created by processes associated with intense freeze-thaw action in an area high latitude areas or near an alpine or continental glacier.

Front 448

Perihelion

Back 448

It is the point in the Earth's orbit when it is closest to the Sun (147.5 million km). Perihelion occurs on the 3rd or 4th of January.

Front 449

Period

Back 449

Geologic time unit that is shorter than an era but longer than a epoch.

Front 450

Permafrost

Back 450

Zone of permanently frozen water found in high latitude soils and sediments.

Front 451

Permeability

Back 451

A measure of the ability of soil, sediments, and rock to transport water horizontally and vertically. Permeability is dependent on the porosity of the medium the water is flowing through. Some rocks like granite have very poor permeability, while rocks like shale are actually quite pervious. As for soils, sand is the most pervious, while clay has the lowest permeability

Front 452

Phanerozoic

Back 452

Geologic eon that occurs from 2500 million years ago to today. During this time period, life becomes more diversified and complex.

Front 453

Physical Weathering

Back 453

Breakdown of rock and minerals into small sized particles through mechanical stress.

Front 454

Piedmont Glacier

Back 454

A large glacier formed from the merger of several alpine glaciers.

Front 455

Pingo

Back 455

A large conical mound that contains an ice core. This feature can be up to 60 to 70 meters in height. Form in regions of permafrost. Common in the Mackenzie Delta region of Canada. Also see the related palsa.

Front 456

Pitted Topography

Back 456

Landscape characterized by numerous kettle holes on a glacial outwash plain.

Front 457

Plane of the Ecliptic

Back 457

Hypothetical two-dimensional surface in which the Earth's orbit around the Sun occurs.

Front 458

Plateau Basalt

Back 458

An accumulation of horizontal flows of basaltic lava.

Front 459

Plate Tectonics

Back 459

Theory suggesting that the Earth's surface is composed of a number of oceanic and continental plates. Driven by convection currents in the mantle, these plates have the ability to slowly move across the Earth's plastic asthenosphere. This theory is very important to geology and geomorphology because it helps to explain the occurrence and formation of mountains, folds, faults, volcanoes, earthquakes, ocean trenches, and the mid-oceanic ridges.

Front 460

Platform

Back 460

Horizontal sedimentary deposits found on top of continental shield deposits.

Front 461

Pleistocene Epoch (Ice Age)

Back 461

Period of time from about 2 million years ago to 10,000 years ago. During this period areas of land at higher and middle latitudes where covered with glacial ice.

Front 462

Plucking

Back 462

Erosive process of particle detachment by moving glacial ice. In this process, basal ice freezes in rock surface cracks. As the main body of the glacial ice moves material around the ice in the cracks is pulled and plucked out. Also called quarrying.

Front 463

Pluton

Back 463

Any mass of intrusive igneous rock.

Front 464

Point Bar

Back 464

Stream bar deposit that is normally located on the inside of a channel bend.

Front 465

Polar Cell

Back 465

Three-dimensional atmospheric circulation cell located at roughly 60 to 90° North and South of the equator. Vertical air flow in the Polar cell consists of rising air at the polar font and descending air at the polar vortex.

Front 466

Polar Front

Back 466

Weather front located typically in the mid-latitudes that separates arctic and polar air masses from tropical air masses. Along the polar front we get the development of the mid-latitude cyclone. Above the polar front exists the polar jet stream.

Front 467

Polar High

Back 467

Surface area of atmospheric high pressure located at about 90° North and South latitude. These high pressure systems produced by vertically descending air currents from the polar vortex.

Front 468

Polar Jet Stream

Back 468

Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. The polar jet stream exists in the mid-latitudes at an altitude of approximately 10 kilometers. This jet stream flows from west to east at speeds between 110 to 185 kilometers per hour. Also see jet stream and subtropical jet stream.

Front 469

Polar Stratospheric Clouds

Back 469

High altitude clouds found in the stratosphere where the temperature is less than -85° Celsius. Commonly found over Antarctica. Have a role in the creation of the ozone hole over Antarctica.

Front 470

Pore Ice

Back 470

A form of periglacial ground ice that is found in the spaces that exist between particles of soil.

Front 471

Porosity

Back 471

The void spaces found in rock, sediment, or soil. Commonly measured as the percentage of void space in a volume of substance.

Front 472

Potential Evapotranspiration

Back 472

Is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no limitation on water supply.

Front 473

Precambrian

Back 473

Span of geologic time that dates from 4.6 billion to 570 million years ago. Made up of three geologic eras: Hadean, Archean, and Proterozoic.

Front 474

Precession of the Equinox

Back 474

Wobble in the Earth's polar axis. This motion influences the timing aphelion and perihelion over a cyclical period of 23,000 years.

Front 475

Precipitation

Back 475

(1) Is any aqueous deposit, in liquid or solid form, that develops in a saturated atmosphere (relative humidity equals 100%) and falls to the ground generally from clouds. Most clouds, however, do not produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome natural updrafts found in the atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere as clouds.
(2) The state of being precipitated from a solution.

Front 476

Pressure Gradient Force

Back 476

Force due to spatial differences in atmospheric pressure. Usually expressed in millibars or kilopascals per unit distance (meters or kilometers). This force is primarily responsible for the formation of wind.

Front 477

Pressure Melting Point

Back 477

Temperature at which minerals deep within the Earth and ice below the surface of a glacier are caused to melt because of the introduction of pressure.

Front 478

Proterozoic

Back 478

Geologic eon that occurred from 570 to 2500 million years ago. During this time period, the first single-celled and multi-celled eukaryotic organisms evolved and developed.

Front 479

Psychrometer

Back 479

Instrument used to measure atmospheric humidity. It consists of two thermometers (wet-bulb and a dry-bulb) one of which has its bulb covered by a moistened wick. Humidity is determined by the difference in readings between the two thermometers after air has passed over both of them for a specific time period.

Front 480

P-Wave

Back 480

A seismic wave that moves material in push-pull fashion in the direction of its travel. This type of seismic wave can travel through solids, liquids, and gases. Also called a primary wave.

Front 481

Pyroclastic Material

Back 481

Pieces of volcanic rock thrown out in a volcanic explosion.

Front 482

Quartz

Back 482

Mineral with the chemical formula SiO2. Quartz is common in continental crust but rare in oceanic crust.

Front 483

Quartzite

Back 483

Metamorphic rock rich in quartz created by the recrystallization of sandstone.

Front 484

Quaternary

Back 484

Geologic period that occurred roughly 1.6 million years ago to today. During much of this period continental glaciers in the Northern Hemisphere covered large regions of land surface in the high and mid-latitudes. Homo sapiens appear about 200,000 years BP (before present) and become the first species to alter the Earth's environment on a large-scale.

Front 485

Radiation Fog

Back 485

A type of fog that is also called ground fog. Radiation fog is generated by near surface cooling by radiation loss during the evening hours. For the fog to develop, the overnight cooling must cause saturation occur. This type of fog is normally quite shallow.

Front 486

Rainshadow Effect

Back 486

Reduction of precipitation commonly found on the leeward side of a mountain. The reduction in precipitation is the result of compression warming of descending air.

Front 487

Recessional Moraine

Back 487

Moraine that is created during a pause in the retreat of a glacier. Also called a stadial moraine.

Front 488

Recharge Area

Back 488

The area on the Earth's surface that receives water for storage into a particular aquifer.

Front 489

Recurrence Interval

Back 489

The average time period that separates natural events of a specific magnitude. For example, floods of a specific stream discharge level.

Front 490

Reflection (Atmospheric

Back 490

Process where insolation is redirect by 180° after striking a particle. This redirection causes 100% loss. Most of the reflection in the Earth's atmosphere occurs in clouds because of light's interception with particles of liquid and frozen water. The reflectivity of a cloud can range from 40-90%.

Front 491

Regolith

Back 491

Loose layer of rocky material overlying bedrock.

Front 492

Relative Humidity

Back 492

The ratio between the actual amount of water vapor held in the atmosphere compared to the amount required for saturation. Relative humidity is influenced by temperature and atmospheric pressure.

Front 493

Reverse Fault

Back 493

This vertical fault develops when compressional force causes the displacement of one block of rock over another.

Front 494

Rhyolite

Back 494

A fine grained extrusive igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.

Front 495

Ribbon Falls

Back 495

Spectacular narrow waterfalls that occur at the edge of a hanging valley.

Front 496

Richter Scale

Back 496

A logarithmic measurement scale of earthquake magnitude. This scale measures the energy released by the largest seismic wave associated with the earthquake.

Front 497

Rift

Back 497

Zone between two diverging tectonic plates. The mid-oceanic ridge is an area where such plate divergence is occurring.

Front 498

Rill

Back 498

A very small steep sided channel carrying water. This landscape feature is intermittent and forms for only a short period of time after a rainfall.

Front 499

Rime

Back 499

Deposit of ice crystals that occurs when fog or super cooled water droplets comes in contact with an object with a temperature below freezing (0° Celsius). This deposit develops outward on the windward side of the object.

Front 500

Roche Moutonnee

Back 500

A feature of glacial erosion that resembles an asymmetrical rock mound. It is smooth and gently sloping on the side of ice advance. The lee-side of this feature is steep and jagged.

Front 501

Rock Cycle

Back 501

General model describing the geomorphic and geologic processes involved in the creation, modification and recycling of rocks.

Front 502

Rockfall

Back 502

Type of mass movement that involves the detachment and movement of a small block of rock from a cliff face to its base. Normally occurs when the rock has well defined bedding planes that are exaggerated by freeze-thaw action or thermal expansion and contraction.

Front 503

Rock Slide

Back 503

Large scale mass movement of rock materials downslope.

Front 504

Roll Cloud

Back 504

A dense, cigar shaped cloud found above the gust front of a thunderstorm. Air within the cloud rotates around the long axis.

Front 505

Runoff

Back 505

The topographic flow of water from precipitation to stream channels located at lower elevations. Occurs when the infiltration capacity of an area's soil has been exceeded. It also refers to the water leaving an area of drainage. Also called overland flow.

Front 506

Ultramafic

Back 506

Rock that is rich in magnesium and iron content.

Front 507

Unconfined Aquifer

Back 507

Aquifer that is not restricted by impervious layers of rock.

Front 508

Unconfined Groundwater

Back 508

Groundwater that is not restricted by impervious layers of rock.

Front 509

Undercut Bank

Back 509

Steep bank found on the inside of stream meanders. Formed by the erosion that occurs when a stream channel moves horizontally.

Front 510

Uniformitarianism

Back 510

Is a theory that rejects the idea that catastrophic forces were responsible for the current conditions on the Earth. The theory suggested instead, that continuing uniformity of existing processes were responsible for the present and past conditions of this planet.

Front 511

Universe

Back 511

All of the observable phenomena in the celestial cosmos.

Front 512

Unloading

Back 512

The releasing of downward pressure on rocks because of removal of overlying material by erosion.

Front 513

Unstable Atmosphere

Back 513

Condition in the atmosphere where isolated air parcels have a tendency to rise. The parcels of air tend to be warmer than the air that surrounds them.

Front 514

Upper Mantle

Back 514

Layer of the Earth's interior extending from the base of the crust to 670 kilometers below the surface. Part of the Earth's mantle layer. The upper mantle is composed of peridotite, an ultramafic magma primarily made up of the minerals olivine and pyroxene. The top layer of the upper mantle, 100-350 km below surface, is called the asthenosphere.

Front 515

Upslope Fog

Back 515

Fog produced by air flowing over topographic barriers. As the air is forced to rise, it is cooled by adiabatic expansion. Upslope fog is most common on the windward slopes of hills or mountains.

Front 516

Valley

Back 516

A linear depression in the landscape that slopes down to a stream, lake or the ocean. Formed by water and/or ice erosion.

Front 517

Vapor Pressure

Back 517

Pressure exerted by water vapor molecules in a given quantity of atmosphere.

Front 518

Varve

Back 518

A thin yearly deposit of sediment found on the bottom of a lake. Within each yearly varve, there are variations in the color and the texture of the material deposited. The thickness of the varve and its associated layers can be used to reconstruct past environmental conditions influencing the lake.

Front 519

Ventifact

Back 519

A loose piece of rock that has been polished smooth by wind transported particles. Common in arid environments.

Front 520

Volcanic Ash

Back 520

Small sized particles ejected from explosive volcanoes.

Front 521

Volcanic Pipe

Back 521

A dyke reaches the surface of the Earth. Also called volcanic neck

Front 522

Volcanic Vent

Back 522

An opening on a volcano through which lava is released and rock fragments and ash are ejected.

Front 523

Volcano

Back 523

An elevated area of land created from the release of lava and ejection of ash and rock fragments from and volcanic vent.

Front 524

Warm Front

Back 524

A transition zone in the atmosphere where an advancing warm air mass displaces a cold air mass.

Front 525

Wash

Back 525

(1) Coarse alluvial sediments.
(2) The downslope movement of small particles of soil by overland flow. Also called sheetwash.
(3) A term used in the United States for a shallow intermittent stream channel found in arid and semi-arid regions.

Front 526

Water Consumption

Back 526

The complete removal of water from some type of source, like groundwater, for some use by humans. This water is not returned to the source. Compare with water withdrawal.

Front 527

Waterfall

Back 527

(1) A location in the long profile of a stream where water flows vertically. A nickpoint.
(2) Verical drop in elevation that causes a stream's dischange to flow vertically.

Front 528

Watershed

Back 528

Catchment area of a drainage basin.

Front 529

Waterspout

Back 529

A vortex of rapidly moving air over water that is associated with some thunderstorms.

Front 530

Water Table

Back 530

Top surface of groundwater.

Front 531

Weather

Back 531

The state of the atmosphere at a specific time and place.

Front 532

Weathering

Back 532

Physical, chemical or biological breakdown of rocks and minerals into smaller sized particles.

Front 533

Westerlies

Back 533

Dominant winds of the mid-latitudes. These winds move from the subtropical highs to the subpolar lows from west to east.

Front 534

Wet Deposition

Back 534

The transport of gases and minute liquid and solid particles from the atmosphere to the ground surface with the aid of precipitation or fog.

Front 535

Wetland

Back 535

Natural land-use type that is covered by salt water or fresh water for some time period. This land type can be identified by the presence of particular plant species or characteristic conditions.

Front 536

Wien's Law

Back 536

This radiation law suggests that the wavelength of maximum emission of any body is inversely proportional to its absolute temperature. The following equation mathematically describes this law:
lmax = C/T
where lmax is the body's maximum emitted wavelength of radiation in micrometers (µm),
C is a constant equal to 0.2897,
and T is the temperature of the body in Kelvins.

Front 537

Wilting Point

Back 537

The point at which the rate of water leaving a plant's leaves is greater than the water uptake by the roots. At this point the plant will fail to recover its turgidity.

Front 538

Wind Ripples

Back 538

Wind ripples are miniature sand dunes between 5 centimeters and 2 meters in length and 0.1 to 5 centimeters in height. They are created by saltation when the sand grains are of similar size and the wind has a constant speed. Also called sand ripples.

Front 539

Windward

Back 539

Upwind side or side directly influenced to the direction that the wind blows from.

Front 540

Yazoo Tributary

Back 540

Small tributary channel that is prevented from joining the main stream channel by the presence of levees. Yazoo tributaries tend to flow on the floodplain parallel to the main stream channel

Front 541

Zonal

Back 541

Movement of wind or ocean waters in a direction that is roughly parallel to the lines of latitude.

Front 542

Zone of Accumulation

Back 542

Area of a glacier where additions of snow exceed losses of ice from melting, evaporation, and sublimation.

Front 543

Zone of Ablation

Back 543

Area of a glacier where losses of ice from melting, evaporation, and sublimation exceed additions of snow annually.

Front 544

Zone of Aeration

Back 544

Horizontal zone that extends from the top of the water table to the ground surface. Soil and rock pore spaces in this zone may and may not have water.

Front 545

Zone of Saturation

Back 545

Groundwater zone within the Earth's bedrock where all available pores spaces are filled by water. Found beneath the water table.

Front 546

Salinization

Back 546

Pedogenic process that concentrates salts at or near the soil surface because evapotranspiration greatly exceeds water inputs from precipitation.

Front 547

Saltation

Back 547

Transport of sediment initiated by moving air or water where particles move from a resting surface to the transport medium in quick continuous repeated cycles.

Front 548

Saltwater Intrusion

Back 548

The invasion of saltwater into freshwater aquifers in coastal and inland areas. This condition can be cause when groundwater, which charges the aquifer, is withdrawn faster than it is recharged by precipitation.

Front 549

Sand Dune

Back 549

A hill or ridge of aeolian sand deposits with a minimum height of less than one meter and a maximum height of about 50 meters. Found in hot deserts and along sandy coastlines.

Front 550

Sandstone

Back 550

A type of sedimentary rock that contains a large quantity of weathered quartz grains.

Front 551

Sand Wedge

Back 551

A form of ice wedge that contains accumulations of wind blown sand in long vertical layers. A form of periglacial ground ice.

Front 552

Saturated Adiabatic Lapse Rate (SALR)

Back 552

The rate of decline in the temperature of a rising parcel of air after it has reached saturation. This rate is less than the dry adiabatic lapse rate (9.8° Celsius per 1000 meters) because of the heat energy added to the ascending air parcel from condensation and deposition processes.

Front 553

Saturation Mixing Ratio

Back 553

Mass of water vapor that a kilogram of dry air can hold at saturation. Measured in grams.

Front 554

Schist

Back 554

A medium to coarse grained metamorphic rock with well developed bedding planes derived from the foliated recrystrallization of platy like minerals like mica.

Front 555

Scour

Back 555

(1) Refers to the erosive power of water.
(2) Abrasive effects of rocks and sediments incorporated in the ice base of a glacier.

Front 556

Sea-Floor Spreading

Back 556

The process of oceanic crust creation and sea-floor movement that occurs at the mid-oceanic ridge.

Front 557

Seamount

Back 557

A volcanic mountain found on an ocean basin that has an origin not related to a mid-oceanic ridge or a tectonic subduction zone.

Front 558

Sediment

Back 558

Solid material that has been or is being eroded, transported, and deposited. Transport can be due to fluvial, marine, glacial or aeolian agents.

Front 559

Sedimentary Rock

Back 559

Rocks formed by the deposition, alteration and/or compression, and lithification of weathered rock debris, chemical precipitates, or organic sediments. Also see clastic vs non-clastic sedimentary rocks.

Front 560

Seepage

Back 560

(1) The gradual movement of water into the soil layer.
(2) Slow movement of sub-surface water to the surface. This flow is not great enough to call it a spring.

Front 561

Segregated Ice

Back 561

A form of periglacial ground ice that consists of almost pure ice that often exists as an extensive horizontal layer. The ice layer grows because of the active migration of water from around the feature. These features are found just below the active layer.

Front 562

Seif

Back 562

(1) A large sand dune that is elongated in the general direction of the dominant winds.
(2) A sand dune formed by winds from multiple directions.

Front 563

Seismic

Back 563

Shaking displacement usually caused by an earthquake.

Front 564

Shale

Back 564

Fine grained sedimentary rock composed of lithified clay particles.

Front 565

Shear Stress

Back 565

Stress caused by forces operating parallel to each other but in opposite directions.

Front 566

Shear Wave

Back 566

A seismic wave that creates wave-like motion perpendicular to the direction of seismic energy propagation. Also called S-wave.

Front 567

Sheeting

Back 567

A form of physical weathering of rock where surface sheets of material fracture and exfoliate because of pressure release. Also see exfoliation dome.

Front 568

Sheetwash

Back 568

The removal of loose surface materials by overland flow. Process of erosion.

Front 569

Shield

Back 569

A large stable area of exposed very old (more than 600 million years) igneous and metamorphic rock found on continents. This rock forms the nucleus of the continents.

Front 570

Shield Volcano

Back 570

Volcano created from alternate layers of lava flows. Shield volcanoes are slightly sloping having a gradient between 6 and 12°. Their height can be as high as 9000 meters. The chemistry of the magma of these volcanoes is basaltic.

Front 571

Short Wave

Back 571

A small wave in the polar jet stream and the westerlies that extends from the middle to the upper troposphere. Often associated with the formation of a mid-latitude cyclone at the ground surface. Contrasts with long waves.

Front 572

Sial Layer

Back 572

The part of the crust that forms the continents and is composed of relatively light, granitic rocks.

Front 573

Siberian High

Back 573

High pressure system that develops in winter over northern central Asia.

Front 574

Silicate Magma

Back 574

Magma that is felsic in composition.

Front 575

Sill

Back 575

Horizontal planes of igneous rock that run parallel to the grain of the original rock deposits.They form when magma enters and cools in bedding planes found within the crust. Also see intrusive igneous rock.

Front 576

Siltstone

Back 576

Fine grained sedimentary rock composed of lithified silt particles.

Front 577

Sima Layer

Back 577

The part of the crust that forms the ocean basins and lower layers in the crust and is composed of relatively heavy, basaltic rocks.

Front 578

Sinkhole

Back 578

A pit like hole in found in areas of karst. These features are caused by the weathering of limestone or dolomite by subsurface drainage. Also called a sink or doline.

Front 579

Slate

Back 579

A fine grained metamorphic rock with well developed bedding planes derived from the slight recrystrallization of shale.

Front 580

Snout

Back 580

Front end of a glacier. Also called the terminus.

Front 581

Snow

Back 581

A type of solid precipitation that forms in clouds with an air temperature below freezing. Snow forms when water vapor deposits directly as a solid on a deposition nuclei. Snowflakes begin their life as very tiny crystals developing on a six-sided hexagonal deposition nuclei. The developing snowflak, then grows fastest at the six points of the nuclei as these surfaces are more exposed to atmosphere's water vapor. Snowfall is most common with the frontal lifting associated with mid-latitude cyclones during fall, winter, and spring months when air temperatures are below freezing.

Front 582

Snow Line

Back 582

Altitudinal or latitudinal limit separating zones where snow does not melt during the summer season from areas in which it does. Similar to the concept of firm limit except that it is not limited to glaciers.

Front 583

Snowfield

Back 583

An area of permanent snow accumulation. Usually at high altitudes or latitudes.

Front 584

Snow Pellets

Back 584

A form of precipitation also known as graupel. Snow pellets are white, spherical bits of ice with a maximum diameter of 5 millimeters. Snow pellets develop when supercooled droplets freeze on snowflakes. Snow pellets often fall for a brief time period when precipitation transforms from ice pellets to snow. Snow pellets can be easily distinguished from packed snowflakes as they tend to bounce when they strike the ground. Packed snowflakes are not dense enough to cause them to bounce.

Front 585

Soil

Back 585

Layer of unconsolidated material found at the Earth's surface that has been influenced by the soil forming factors: climate, relief, parent material, time, and organisms. Soil normally consists of weathered mineral particles, dead and living organic matter, air space, and the soil solution.

Front 586

Soil Moisture Recharge

Back 586

The process of water filling the pore space found in a soil (storage).

Front 587

Solifluction

Back 587

Form of mass movement in environments that experience freeze-thaw action. It is characterized by the slow movement of soil material downslope and the formation of lobe-shaped features. Also see gelifluction.

Front 588

Southern Oscillation

Back 588

Reversal of atmospheric circulation in tropical Pacific Ocean that triggers the development of an El Nino.

Front 589

Spheroidal Weathering

Back 589

A type of below ground chemical weathering where the corners of jointed rocks become rounded over time. Rock changes from a rectangular to more round shape.

Front 590

Sporadic Permafrost

Back 590

Form of permafrost that exists as small islands of frozen ground in otherwise unfrozen soil and sediments.

Front 591

Squall Line

Back 591

A band of thunderstorm development found ahead of a cold front.

Front 592

Stable Atmosphere

Back 592

Condition in the atmosphere where isolated air parcels have a tendency to sink. The parcels of air tend to be cooler than the air that surrounds them.

Front 593

Star

Back 593

A large and very massive, self-luminous celestial body of gas that illuminates via the radiation derived from its internal source of energy.

Front 594

Stationary Front

Back 594

A transition zone in the atmosphere where there is little movement of opposing air masses and winds blow towards the front from opposite directions.

Front 595

Stephan-Boltzmann Law

Back 595

This radiation law suggests the amount of radiation given off by a body is proportional to the 4th power of its temperature as measured in Kelvin units. This law can be expressed by the following simple equation:
E* = sT 4
where E* is the amount of radiation emitted by the body in Watts per square meter,
s is a constant equal to 0.0000000567,
and T is the temperature of the body in Kelvins.

Front 596

Stoma (pl. Stomata)

Back 596

Small opening on the surface of a plant that is used for gas exchange.

Front 597

Stoss

Back 597

Side of a slope that faces the direction of flow of ice, wind, or water. Opposite of lee.

Front 598

Stratified Drift

Back 598

A type of glacial drift that has been partially sorted by glaciofluvial meltwater.

Front 599

Strata

Back 599

The layers or beds found in sedimentary rock.

Front 600

Stratocumulus Clouds

Back 600

Low altitude gray colored cloud composed of water droplets that has a patchy appearance. Each cloud patch consists of a rounded mass. This cloud has a somewhat uniform base and normally covers the entire sky. Between the patches blue sky can be seen. Found in an altitude range from the surface to 3,000 meters.

Front 601

Stratosphere

Back 601

Atmospheric layer found at an average altitude of 11 to 50 kilometers above the Earth's surface. Within the stratosphere exists the ozone layer. Ozone's absorption of ultraviolet sunlight causes air temperature within the stratosphere to increase with altitude.

Front 602

Stratus Clouds

Back 602

Low altitude gray colored cloud composed of water droplets. This cloud has a uniform base and normally covers the entire sky. It is also quite thick and can obscure the Sun. Light precipitation is often found falling from it. Found in an altitude range from the surface to 3,000 meters.

Front 603

Stream

Back 603

A long narrow channel of water that flows as a function of gravity and elevation across the Earth's surface. Many streams empty into lakes, seas or oceans.

Front 604

Stream Discharge

Back 604

A river or stream's rate of flow over a particular period of time. Usually measured by a current meter and expressed in cubic meters per second. Stream discharge depends on the volume and velocity of the flow.

Front 605

Stream Gradient

Back 605

The change in elevation from a stream's headwaters to its mouth expressed in degrees, percentage, or as a distance ratio (rise/run).

Front 606

Stream Load

Back 606

Refers to the material or sediment carried by a stream. In normally consists of three components: bed load (pebbles and sand which move along the stream bed without being permanently suspend in the flowing water), suspended load (silts and clays in suspension) and dissolved load (material in solution).

Front 607

Striations

Back 607

Grooves of scratches found in surface rock that are the result of glacial abrasion.

Front 608

Structural Landform

Back 608

Is a landform created by massive Earth movements due to plate tectonics. This includes landforms with some of the following geomorphic features: fold mountains, rift valleys, and volcanoes.

Front 609

Subduction (Tectonic)

Back 609

Process of plate tectonics where one lithospheric plate is pushed below another into the asthenosphere.

Front 610

Subduction Zone

Back 610

Linear area where tectonic subduction takes place.

Front 611

Sublimation

Back 611

Process where ice changes into water vapor without first becoming liquid. This process requires approximately 680 calories of heat energy for each gram of water converted.

Front 612

Subpolar Glacier

Back 612

Glacier in which the ice found from the its surface to base has a temperature as cold as -30° Celsius throughout the year. This is well below the pressure melting point. However, melting does occur in the accumulation zone in the summer. One of the three types of glaciers: cold glacier; temperate glacier; and subpolar glacier.

Front 613

Subtropical High Pressure Zone

Back 613

Surface zone of atmospheric high pressure located at about 30° North and South latitude. These high pressure systems produced by vertically descending air currents from the Hadley cell.

Front 614

Subtropical Jet Stream

Back 614

Relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. The subtropical jet stream exists in the subtropics at an altitude of approximately 13 kilometers. This jet stream flows from west to east and has a speed that is somewhat slower that the polar jet stream. Also see jet stream.

Front 615

Suspended Load

Back 615

Portion of the stream load that is carried almost permanently suspended in flowing water.

Front 616

Syncline

Back 616

A fold in rock layers that forms a trough-like bend.