Physical Geology

Front 1

aa flow

Back 1

a lava flow that develops a partially solidified surface as it moves forward, which breaks the surface into a rough, rubbly mass.

Front 2

ablation

Back 2

the loss of ice and snow from a glacier, generally through melting or evaporation.

Front 3

abrasion

Back 3

of a stream, the process by which a stream's rocky bed is eroded away by the constant friction and impact of tumbling rock fragments, gravel, and sediment.

Front 4

abyssal fan

Back 4

a fan-shaped accumulation of sediment that forms at the mouth of submarine canyons.

Front 5

abyssal plain

Back 5

a very flat expanse of horizontally deposited sediment that accumulates on the ocean floor at the base of a continental rise.

Front 6

accreted terrane

Back 6

a terrane that appears to have formed in place along a continent's margin through accumulation and orogeny.

Front 7

accretionary wedge

Back 7

an accumulation of marine sediment, derived from the subducting plate, that builds up at the edge of a subdiction zone.

Front 8

acid rain

Back 8

an environmentally harmful acidic rain that results from rain mixing with chemical pollutants in the atmosphere.

Front 9

active continental margin

Back 9

marked by a landward continental shelf, a continental slope that forms a sidewall of an oceanic trench and is much steeper than that of a passive continental margin, and an area of earthquake and volcano activity.

Front 10

advancing glacier

Back 10

a glacier that exhibits outward or downslope movement

Front 11

aftershock

Back 11

one of the small earthquakes that may follow the main earthquake

Front 12

A horizon

Back 12

the uppermost soil horizon; characterized by the downward movement of water

Front 13

alkali soil

Back 13

a pedocal type of soil toxic to plant growth because of its high salt content

Front 14

alluvial fan

Back 14

a feature similar to a delta; a large, fanlike accumulaton of sediment dropped where a stream emerges from rugged terrain, such as the edge between a mountain canyon and a flat plain

Front 15

alpine

Back 15

referring to high mountain regions

Front 16

alpine glaciation

Back 16

glaciation usually restricted to deep balleys in high mountainous terrain.

Front 17

angle of repose

Back 17

the steepest angle at which loose material will remain in place

Front 18

angular unconformity

Back 18

the contact that separates a younger, gently dipping rock unit from older underlying rocks that are tilted or deformed layered rock

Front 19

anticline

Back 19

a fold that is arched upward to form a ridge

Front 20

aphanitic

Back 20

fine grained

Front 21

aquiclude

Back 21

an impermeably layer such as clay that slows the flow of groundwater

Front 22

aquifer

Back 22

a porous, permeable, saturated formation of rock or soil through which groundwater flows easily

Front 23

aquitard

Back 23

a formation such as shale, clay or unfractured igneous rocks that slows water flow

Front 24

arc-continental convergence

Back 24

the result when intervening ocean is destroyed by subduction, welding an island arc to the continental edge

Front 25

arete

Back 25

a sharp ridge that commonly extends downward from a horn to separate two adjacent glacial valleys

Front 26

arroyo

Back 26

a narrow gorge with steep walls and a gravel bottom; produced over time by flash floods

Front 27

artesian well

Back 27

a well that taps water from a confined aquifer

Front 28

aseismic ridge

Back 28

a chain of seamounts and guyots (not associated with earthquakes)

Front 29

assimilation

Back 29

the process by which pieces of the country rock melt and mix within a body of magma

Front 30

asthenosphere

Back 30

an area composed of the flexible mantle beneath the lithosphere (lower mantle)

Front 31

atoll

Back 31

a circular reef in deep water that shelters a lagoon; the result of reef development around the flank of a volcano that has since subsided but to which the corals are still anchored

Front 32

axial plane

Back 32

of a fold, the plane that separates rocks on one side of a fold from those that dip in the opposite direction on the other side

Front 33

backarc basin

Back 33

the area on the continental side of an island arc or magmatic arc

Front 34

backarc thrust belt

Back 34

the belt of rocks that has been thrust toward the continental interior from the magmatic arc area along low-angle faults

Front 35

backswamp

Back 35

a poorly drained and marshy area behind a natural levee

Front 36

backwash

Back 36

the water that flows back down the beach into the surf zone

Front 37

bajada

Back 37

the joining of alluvial fans at the front of a mountain range in a rolling surface of sediment and gravel

Front 38

balanced budget

Back 38

of a glacier, a situation in thich there is neither advancement nor recession

Front 39

bar

Back 39

an elongate sedimentary accumulation of sand or gravel deposited by current action in a stream or other body of water

Front 40

barchan dune

Back 40

a solitary, crescent-shaped dune that forms in areas of sparse sand

Front 41

barchanoid dune

Back 41

a variety of dune intermediate between barchan and transverse dunes; barchanoid dunes form scalloped rows of sand perpendicular to the wind

Front 42

barrier island

Back 42

a large, elongate mass of sand that parallels a coast and forms an island

Front 43

barrier reef

Back 43

an elongate reef that develops offshore parallel to a coastline and is separated from the coastline by a deep lagoon

Front 44

basal sliding

Back 44

the movement of a glacier generated by its sliding on a thin film of water resulting from the pressure of the glacier's weight

Front 45

Basin and Range topography

Back 45

a series of steep mountain ranges separated by broad valley floors

Front 46

batholith

Back 46

a pluton larger that 100 km at the earth's surface; usually granitic and made up of diapirs

Front 47

beach

Back 47

the strip of sand or gravel (more rarely silt) that covers a shoreline from the low-water edge to a well-defined point of higher elevation

Front 48

beach drift

Back 48

the zig-zag pattern by which sediment is moved across a beach face by breaking waves

Front 49

beach face

Back 49

the side of a beach facing the ocean

Front 50

bedding

Back 50

the pattern (usually horizontal) of layering in which sediments are deposited

Front 51

bedding planes

Back 51

demarcations of different layers of sediment

Front 52

bed load

Back 52

of a stream, the heavier, coarser-grained earth material that travels on or near the bed of a stream; of wind, the heavier grains (usually sand) that hop and skip along the ground by saltation

Front 53

Benioff zone

Back 53

a zone that slopes downward from an oceanic trench and underneath the overlying crustal plate at 30 to 60 degrees; an area of earthquake origination

Front 54

bergshcrund

Back 54

a crevasse, commonly filled with rock fall debris, that forms where a glacier separates from a cirque wall

Front 55

berm

Back 55

the landward edge of a beach

Front 56

B horizon

Back 56

the middle soil horizon into which the leached materials from the A horizon often precipitate

Front 57

bituminous coal

Back 57

a common form of coal that is soft and black

Front 58

black smoker

Back 58

a submarine hot spring that results from high heat flows and convection currents at divergent plate boundaries and that deposits solid masses of metallic minerals

Front 59

blowout

Back 59

a bowl-like depression caused by deflation

Front 60

bottomset bed

Back 60

the finest silt and clay particles that are carried out from a delta into deeper water or slide down a delta front into deeper water

Front 61

Bowen's reaction series

Back 61

a description of the progression of mineral formation as magmas cool and crystallize

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braided stream

Back 62

a stream in which the water has lost its main channel and flows in an interconnectin network of rivulets around numerous bars

Front 63

breaker

Back 63

a high wave in which the crest falls forward in front of the main body of the wave

Front 64

breakwater

Back 64

a wall built parallel to the shoreline to provide quiet water

Front 65

breccia

Back 65

rock composed of coarse-grained, angular fragments of broken rocks that have been cemented together and lithified

Front 66

brittle strain

Back 66

strain that occurs when a stess is great enough to break or fracture a rock

Front 67

budget

Back 67

of a glacier, the ratio between ice gained and ice lost

Front 68

butte

Back 68

a landform resulting from the erosion of a mesa

Front 69

caldera

Back 69

a depression larger than a crater, at least 1 km in diameter, that forms at the top of a volcano when the summit is destroyed during an eruption or when the crater floor collapses into the magma chamber below

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caliche

Back 70

a hardpan formed by the precipitation of salt by evaporation

Front 71

calving

Back 71

the breaking off of large blocks of ice from a glacier

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capillary action

Back 72

the process by which surface tension causes water to rise up into unfilled pore spaces

Front 73

capillary fringe

Back 73

the lower part of the unsaturated zone that draws water upward from the saturated zone

Front 74

cast

Back 74

a fossil formed when the organic remains dissolved, leaving an opening (mold) shaped like the oraganism and later filled with calcite or silica

Front 75

cementation

Back 75

the step in lithification in which minerals fill some or all of the pore space and adhere to the sediment fragments, thus producing a sedimentary rock

Front 76

chemical sedimentary rock

Back 76

a sedimentary rock resulting from biological or chemical prcesses, generally underwater, that crystallizes minerals that accumulate on the sea floor

Front 77

chemical weathering

Back 77

the process by which rain, water, and atmospheric gases decompose minerals, destroy chemical and mineralogical bonds, and form new minerals

Front 78

chill zone

Back 78

the fine-grained edge of a rock intrusion

Front 79

C horizon

Back 79

the lowest soil horizon, which lies directly above the bedrock; composed partly of soil and partly of decomposing bedrock fragments

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cinder cone

Back 80

a feature composed of pyroclastic material (not lavas) ejected from a volcanic vent

Front 81

circum-Pacific belt

Back 81

an earthquake belt that folors the rim of the Pacific Ocean; the Ring of Fire

Front 82

cirque

Back 82

a steep-sided, cirular hollow carved in the top of a mountain from an alpine glacier

Front 83

clastic sedimentary rock

Back 83

a sedimentary rock formed from the consolidation of material such as gravel, sand, or clay (sediment) derived from the weathering and breakdown or preexisting rocks

Front 84

cleavage

Back 84

the ability of a mineral to break along preferred directions, usually along the faces of layered crystals

Front 85

coal

Back 85

a dark-colored sedimentary rock that contains a high percentage of organic plant material

Front 86

coast

Back 86

the strip of land near the ocean that includes the beach and the immediate inland area beside it

Front 87

coastal straightening

Back 87

the process of the headlands being cut back and the flanking beaches being widened

Front 88

collision boundary

Back 88

a convergent boundary that separates two continental plates that are pusshes into contact

Front 89

columnar structures

Back 89

cooled and contracted flood basalt in vertical, parallel, generally 6-sided structures (columnal jointing)

Front 90

compaction

Back 90

the step in lithification in which the grains of sediment are packed more tightly together

Front 91

composite volcano

Back 91

a volcano that consists of alternating layers of lava and pyroclastic debris; built up over millions of years, such volcanos are characterized by long periods of dormancy

Front 92

compressive stress

Back 92

stess applied to a rock from opposite directions, compressing and flattening the rock mass

Front 93

cone of depression

Back 93

the area and shape of a drawdown around a well

Front 94

confined aquifer

Back 94

an aquifer overlain by a less permeable bed that keeps the water in the aquifer under pressure

Front 95

connate water

Back 95

water trapped in the original sediments during depositino and lithification

Front 96

contact

Back 96

the plane of separation between any two different kinds of rock

Front 97

contact metamorphic deposit

Back 97

a hydrothermal deposit that results from hot solutions that leave a cooling intrusion and deposit minerals in cracks in country rock

Front 98

contact metamorphism

Back 98

the process by which country rock surrounding a hot magma intrusion is metamorphosed by the high heat flow coming from the intrusion

Front 99

contamination plume

Back 99

the elongate area of contaminated groundwater that is downgradient from the point source of leakage

Front 100

continental divide

Back 100

the topographic ridgeline that separates the streams that flow in opposite directions and empty into different oceans

Front 101

continental drift

Back 101

the theory that the continents were once joined together and somehow then split and moved apart

Front 102

continental glaciation

Back 102

glaciation that affects a broader, flatter part of a continental land mass than does alpine glaciation

Front 103

continental rise

Back 103

a very low-angle ridge of sediment that forms between the lower part of the continental slope and the abyssal plain

Front 104

continental shelf

Back 104

a shallow, very gently sloping platform that extends seaward from the edge of a continent

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continental slope

Back 105

an area that extends from the seaward edge of a continental shelf into the deep ocean at an average angle of 4 to 5 degrees

Front 106

continent-continent convergence

Back 106

the result when two continents collide

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continuous branch

Back 107

the type of magmatic differentiation in which minerals form continuously during cooling

Front 108

contour current

Back 108

a current that flows parallel to the edge of a continental slope

Front 109

convection currents

Back 109

currents within a material that are driven mostly by changing temperature gradients

Front 110

convergent boundary

Back 110

a fault boundary marked by plates that come together

Front 111

core

Back 111

the zone of the earth that includes the inner and outer core

Front 112

country rock

Back 112

the surrounding rock that invades in the formation of intrusive rocks

Front 113

crater

Back 113

the circular depression at the top of a volcano

Front 114

craton

Back 114

a continental interior that has been structurally inactive for a prolonged time, usually hundreds of millions of years or longer

Front 115

creep

Back 115

a mass-wasting event slow enough that it cannot be detected as it is occuring

Front 116

crevasse

Back 116

a deep crack of fissure in a glacier

Front 117

cross-bedding

Back 117

a sedimentary structure in which the bedding planes of a particular unit are inclined compared to the bedding of the enclosing rocks

Front 118

crude oil

Back 118

a liquid containing hydrocrabons that forms in organic-rich or fossil-rich seiments and rocks

Front 119

crust

Back 119

the outermost zone of the earth, its exterior layer

Front 120

crustal rebound

Back 120

the process by which crustal rocks that were downwarped by a glacier's wieght slowly return to normal elevation after the glacier's retreat

Front 121

daughter products

Back 121

products created as an element undergoes radio-active decay

Front 122

debris avanlanche

Back 122

a rapidly churning mass of rock debris, soil, water, and air that reaces down very sttep slopes

Front 123

debris flow

Back 123

a mass-wasting event in which movements and turbulence occur throughout the mass

Front 124

debris slide

Back 124

the rapid movement of a mass of debris as a single unit

Front 125

deflation

Back 125

the removal of sediment from a land surface by wind

Front 126

delta

Back 126

a thick, roughly wedge-shaped accumulation of sediment deposited at the mouth of a stream

Front 127

dendritic drainage pattern

Back 127

a veinlike drainage pattern that develops in a rock type that erodes uniformly, such as granite

Front 128

depositional coast

Back 128

a gently sloped coast that has been built up by sediments deposeted from longshore drift

Front 129

depth of focus

Back 129

of an earthquake, the distance between the epicenter and the focus

Front 130

desert

Back 130

an area that receives less that 25 cm (10 inches) of precipitation annually

Front 131

desert pavement

Back 131

a large surface of the desert floor that is covered by pebbles and stones that resemble rounded paving stones; caused by deflation or temperature changes

Front 132

desiccation crack

Back 132

a crack the develops when a muddy sediment is exposed to air and begins to dry out; these cracks combine to form a polygonal pattern

Front 133

detachment fault

Back 133

a low-angle fault above which is often a series of thrust faults and below which is undeformed bedrock

Front 134

dewatering

Back 134

the step in lithificatino in which increasing pressure squeezes out some of the water between sediment particles

Front 135

diapir

Back 135

a small magma blob resulting from localized melting of the crust; a component of batholiths

Front 136

differential stress

Back 136

stress usually caused by tectonic forces applied to a body of rock from different, but not opposite, directions, stretching the rock mass into an elongate shape

Front 137

differential weathering

Back 137

the result of the resistance of some rocks more than other rocks to weathering, creating uneven rates of erosion and sometimes spectacular formations

Front 138

differentiation

Back 138

the process by which a magma forms different minerals according to changes in temperature and pressure

Front 139

dike

Back 139

an intrusive rock that generally occupies a discordant, or cross-cutting, crack or fracture that crosses the trend of layering in the country rock

Front 140

dip angle

Back 140

the angle between the horizontal plane and a tilted bedding plane

Front 141

dip-slip fault

Back 141

a fault in which movement is parallel to the dip of the fault plane in an up or down direction between two blocks

Front 142

disconformity

Back 142

an erosional contact usually parallel to the bedding planes of the upper and lower rock units

Front 143

discontinuous branch

Back 143

the type of magmatic differentiation in which minerals form at discrete temperatures and not continuously during cooling

Front 144

disseminated deposit

Back 144

a hydrothermal deposit in which teh metal ore is evenly distributed in generally low concentrations throughout large masses of rock

Front 145

dissolved load

Back 145

earth material in a stream that has been dissolved into ions and carried in solution

Front 146

distributary

Back 146

a small, shifting channel that spreads out across a delta from the main river channel and disperses the sediment load

Front 147

divergent boundary

Back 147

a fault boundary marked by plates that move away from one another

Front 148

downcutting

Back 148

the erosion directly downward by a stream channel

Front 149

drainage basin

Back 149

the area drained by a stream and its tributaries

Front 150

drainage divide

Back 150

a ridge that separates one drainage basin from another

Front 151

drawdown

Back 151

a local lowering of the water table around a well

Front 152

drumlin

Back 152

a long, narrow, rounded ridge of till whose long axes parallel the direction a glacier traveled

Front 153

ductile

Back 153

of a rock, flowing plastically in response to stress

Front 154

dust storm

Back 154

a windstorm that carries large amounts of sand or sediment through the air

Front 155

earthflow

Back 155

the movement of earth material down a hillside as a viscous fluid; earthflows typically occur on steep slopes with thick soil cover that becomes saturated by heavy rains

Front 156

earthquake

Back 156

the ground shaking caused by rock that suddenly move or jolt in response to tectonic stress

Front 157

ebb currents

Back 157

tidal currents preceding low tide

Front 158

elastic rebound theory

Back 158

the theory that suggests that in some cases energy is stored in rock that is being bent (deformed) by tectonic forces until the energy in the rock exceeds the rock's chemical bonds and it breaks, realeasing energy and causing motion

Front 159

elastic strain

Back 159

strain after which the body of a rock returns to its previous shape when stress has been removed

Front 160

end moraine

Back 160

an extensive and typically crascent shaped pile of till built up at the front ot a glacier

Front 161

ephermal stream

Back 161

a stream the flows intermittently as a result of periods of sudden rainfall

Front 162

epicenter

Back 162

the point on the surface directly above the focus of an earthquake

Front 163

erosion

Back 163

the picking up of sediment and soil particles by an agent such as wind or water

Front 164

erratic

Back 164

a boulder that has been deposited by a glacier and is not derived from the local bedrock

Front 165

esker

Back 165

a long, winding ridge or outwash deposited in streams flowing through ice caves and tunnels at the base of a glacier

Front 166

estuary

Back 166

a drowned river mouth from an older coastline; appears as a long arm of ocean water extending inland from the coast

Front 167

evaporitic rock

Back 167

a rock formed from minerals that chemically precipitated from water

Front 168

exfoliation

Back 168

the process by which curved sheets of rock loosen and fall from a weathered rock surface

Front 169

exfoliation dome

Back 169

a large rounded landform (usually composed of intrusive rocks) that results from exfoliation

Front 170

exotic terrane

Back 170

a terrane that did not form naturally through accretion and has likely collided with the continental margin

Front 171

extrusive igneous rock

Back 171

igneous rock that crystallized from liquid magmas that reache the surface and were generally vented as volcanic lavas

Front 172

failed rift

Back 172

rifting that ceases before the crustal mass has been separated into parts

Front 173

fall

Back 173

a mass-wasting movement in which earth material free-falls from a steep face or cliff and generally collects at the base as talus

Front 174

fault

Back 174

an area in which rock has been displaced along a fracture, such as having one side that is moved up or down

Front 175

fault block mountain

Back 175

a mountain that is bordered on both sides by steeply dipping faults, such as a horst

Front 176

fault gouge

Back 176

the broken material within a fault

Front 177

fault plane

Back 177

a plane of fracture in a rock along which movement has occured

Front 178

felsic rock

Back 178

a rock the is rich in silica, potassium, sodium, and aluminum and that contains only small amounts of iron, magnesium, and calcium

Front 179

fetch

Back 179

the distance that wind travels over a surface of water

Front 180

fiord

Back 180

a steep-walled, fingerlike coastal inlet that was carved by glacial action and later flooded by the rising sea (fjord)

Front 181

firn

Back 181

rounded granules formed by the compaction of snow by pressure from overlying snow and cemented by ice

Front 182

first-motion studies

Back 182

studeis that indicate whether the first rock motion in an earthquake was a push (the rock moved toward the seismograph station) or a pull (away from seismograph station)

Front 183

fissile

Back 183

splitting naturally along layers

Front 184

flash flood

Back 184

flood resulting from very heavy rainfall over short periods

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flood currents

Back 185

tidal currents preceding high tide

Front 186

floodplain

Back 186

the area created on both sides of a stream when periodic flooding deposits mud and silt over extensive, low-lying areas

Front 187

flow

Back 187

a mass-wasting movement in which the mass moves downslope like a viscous fluid

Front 188

flowing artesian well

Back 188

a well that taps an aquifer under confining pressure that is sufficient to force the water to rise naturally to the surface through the well

Front 189

focus

Back 189

the point of origin of an earthquake

Front 190

fold

Back 190

a bend in a layered rock

Front 191

fold and thrust belt

Back 191

a mountain-building event in which rocks are folded during tectonic stress and detached as thin layers along thrust zones, which vertically stack the layers; typically occurs on the continental side of a magmatic arc

Front 192

folded mountain

Back 192

a mountain created by intense compressional forces that fold, fault, and metamorphose the rocks, a process that resulted in many of the world's biggest mountain belts

Front 193

foliation

Back 193

the alignment of parallel layers or bands of mineral grains in a rock subjectes to prolonged defferential stress and/or shearing

Front 194

footwall

Back 194

the block that underlies an inclined dip-slip fault

Front 195

forearc basin

Back 195

the relativily undistubed expanse of ocean floor between an accretionary wedge and an island arc

Front 196

foreland basin

Back 196

a shallow continental basin behind a magmatic arc, a result of subsidence.

Front 197

foreset bed

Back 197

a sandy bed that composes the main body of a delta

Front 198

foreshock

Back 198

one of the small earthquakes that may precede the main earthquake

Front 199

fossil

Back 199

the trace of a plant of animal in a sedimentary rock

Front 200

fossil assemblage

Back 200

a group of different kinds of fossils that coexisted; more useful than single types of fossils in determining the age of a formation

Front 201

fossil fuel

Back 201

coal, oil, or gas derived from organic-rich rocks

Front 202

fracture

Back 202

a crack in a rock along which no motion has taken place

Front 203

fringing reef

Back 203

a flat expanse of reef that is attached directly to shore

Front 204

frost heaving

Back 204

the process by which rock and soil are lifted berically by the formation of ice and repeated freezing and thawing

Front 205

frost wedging

Back 205

the widening and deepening of cracks by ice, breaking off pieces, and slabs of rock

Front 206

fumerole

Back 206

a vent in or near a volcano from which steam and other gases escape from molten rock below

Front 207

gaining stream

Back 207

a stream into which groundwater flows from the saturated zone

Front 208

geologic cross section

Back 208

a vertical "slice" across a map area; depicts the spatial relationships of rock units and structures beneath the surface

Front 209

geophysics

Back 209

a field concerning the application of the laws of physics to the study of earth

Front 210

geostatic pressure

Back 210

pressure that is equally applied to all sides of a deeply bured mass of rock

Front 211

geothermal energy

Back 211

the energy produced when exceptionally hot water underground is tapped by wells and used to generate electricity

Front 212

geothermal gradient

Back 212

the rate at which temperature increases with depth

Front 213

geyser

Back 213

an explosive hot spring that periodically erupts scalding water and steam; water temperatures in a geyser are generally near boiling

Front 214

geyserite

Back 214

a build-up of ledgelike layers, generally of calcite or silica, around a geyser

Front 215

glaciation

Back 215

the movement of an ice mass over a land surface

Front 216

glacier

Back 216

a large mass of ice that forms on land during cooler climatic periods

Front 217

Gondwanaland

Back 217

a paleocontinent that consisted of what is now Africa, India, S. America, Australia, and Antarctica

Front 218

gossan

Back 218

a rusty, iron-bearing cap; a remnant of a weathered metallic ore deposit at teh surface

Front 219

graben

Back 219

a feature formed when a block that is bounded by normal faults slips downward, usually because of a tensional force, creating a valleylike depression

Front 220

graded bed

Back 220

a bed in which the base consists of coarser material and subsequent upward through sand and silt to the finest clay sizes at the top

Front 221

graded stream

Back 221

a stream that has smoothed out its longitudinal profile to resemble a smooth, concave-upward curve

Front 222

gravity meter

Back 222

a device that measures the force of gravity between a mass inside the instrument and the earth

Front 223

groin

Back 223

one of a series of walls built perpendicular to the coast to widen beaches that are losing sand to longshore drift

Front 224

ground moraine

Back 224

a thin, widespread layer of till deposited across the surface as an ice sheet melts

Front 225

groudwater

Back 225

water derived from rain and melting snow that percolates downward from the surface and collects in the open pore spaces between soil particles or in cracks and fissures in bedrock

Front 226

guyot

Back 226

a submerged, flat-topped seamount that was once above sea level and was eroded flat by continual wave action

Front 227

half-life

Back 227

the time it takes for half of a known quantity of radioactive material to convert to daughter products

Front 228

halo

Back 228

the zone of metamorphism surrounding an intrusion in contact metamorphism

Front 229

hanging valley

Back 229

a valley that forms a cliff face witht the main valley it enters because its lower part has been eroded away by flacial action

Front 230

hanging wall

Back 230

the block that overlies the inclined fault plane in a dip-slip fault

Front 231

hardness

Back 231

a quality of minerals determined by teh Mohs hardness scale

Front 232

hardpan

Back 232

a layer of soil, usually the Bhorizon, that is so hard (usually cemented by CaCO3 or SiO2) that even a backhoe cannot break through it

Front 233

headland

Back 233

a rocky arm of a coastline that juts into the sea

Front 234

headward erosion

Back 234

erosion that results when a valley is extended upward above its original source by gullying, mass wasting, and sheet erosion

Front 235

headwaters

Back 235

the origination of a stream; usually in higher elevations of mountainous terrain

Front 236

heat flow

Back 236

of the earth in general, the amount of heat from the earth's interior that is lost at the surface

Front 237

heavy crude oil

Back 237

a dense, viscous petroleum that flows so slowly it is usually left behind in an oil field

Front 238

herringbone cross-bedding

Back 238

a distinctive pattern of alternating cross-bedding directions that is reflective of a rhythmic, high-energy environment, such as a tidal zone

Front 239

hinge line

Back 239

the center axis of a fold

Front 240

horn

Back 240

a sharply defined peak that has formed from erosional processes along the rim of a cirque

Front 241

horst

Back 241

a feature that results when a block that is bounded by normal faults experiences a compressive force that forces the block upward, forming mountainous terrain

Front 242

hot spring

Back 242

a spring with water 6 to 9 degrees centigrade (11 to 16 degrees F) warmer than the mean annual air temperature for the locality where it occurs

Front 243

hot springs deposit

Back 243

a disseminated metal deposit formed in response to hot spring activity at the surface of the earth

Front 244

hydraulic action

Back 244

the ability of flowing water to dislodge, pick up, and transport rock particles or sediment

Front 245

hydrogenous sediments

Back 245

sediments on the ocean floor that have chemically precipitated from seawater

Front 246

hydrologic cycle

Back 246

the continuous exchange of water between the atmosphere, oceans, continents, plants, and animals

Front 247

hydrothermal deposit

Back 247

of metallic ore, the result of the formation of rich deposits from hydrothermal solutions that circulate through fractured country rock

Front 248

hydrothermal rocks

Back 248

those rocks whose minerals crystallized from hot water or whose minerals have been altered by hot water passing through them

Front 249

hydrothermal vein

Back 249

minerals that are deposited from hydrothermal processes and fill a crack in the country rock

Front 250

iceberg

Back 250

a floating mass of ice that results from calving from a glacial face into the water of a lake or ocean

Front 251

ice cap

Back 251

a glacial ice mass that is centered on a highland area and migrates outward in all directions

Front 252

ice fall

Back 252

a blocky, piled-up ice surface that results when rapid ice movements rupture glacial crevasses

Front 253

ice sheet

Back 253

a glacier that covers a broad expanse of land and is not restricted by a channel

Front 254

igneous rock

Back 254

rock that at one time was molten and party of magmas or lavas and that then cooled

Front 255

incised meander

Back 255

a steep-walled canyon that results from the down-cutting of a meandering stream

Front 256

inclinometer

Back 256

a device on a compass used to measure dip angle

Front 257

inclusion

Back 257

a rock fragment enclosed within an intrusive rock unit

Front 258

index fossil

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a fossil of those species that lived only during a restricted period and that identifies the narrow time range during which the host rock could have formed

Front 259

inner core

Back 259

the spherical, solid, innermost part of the earth

Front 260

inselberg

Back 260

an isolated bedrock remnant of a former mountain front; may project through the pediment cover

Front 261

interior drainage pattern

Back 261

a drainage pattern in which streams empty into landlocked basins

Front 262

intermediate rock

Back 262

a general term for rock between mafic and felsic classifications

Front 263

intrusive igneous rock

Back 263

igneous rock taht formed from magmas that moved upward into craks and voids deep in the crust and that never reached the surface

Front 264

island arc

Back 264

a curved chain of islands that develops between an oceanic trench and a continental mass

Front 265

isoclinal fold

Back 265

a fold that has undegone stress great enough to compress its limbs together

Front 266

isostasy

Back 266

th equilibrium, or balance, between adjacent blocks of crust overlying the mantly

Front 267

isotope

Back 267

an atom of an element that contains a different number of neutrons in its nuclei than does another atom of the same element

Front 268

jetty

Back 268

a wall that is built on both sides of a harbor and that extende into the ocean to protect the harbor from sedimentation and destructive waves

Front 269

joint

Back 269

an opening in a rock along which the rock exhibits no displacement; generally an equilibrium response to cooling or unloading

Front 270

joint set

Back 270

a series of roughly parallel joints that occur in one direction

Front 271

jovian planets

Back 271

those outer planets that have densities less than 2g/cm3 (Saturn, Jupiter, Uranus, Neptune)

Front 272

kame

Back 272

a steep-sided mound of stratified till that was deposited by meltwater in depressions or openings in the ice or as short-lived deltas or fans at the mouths of meltwater streams

Front 273

karst topography

Back 273

an irregular land surgace dotted with numerous sinkholes and depressions related to underlying cave systems

Front 274

kettle

Back 274

a depression in glacial till that results when a buried block of glacial ice melts

Front 275

kettle lake

Back 275

a body of water occupying a kettle

Front 276

komatiite

Back 276

a typical ultramafic extrusive rock that is mostly olivine and pyroxene, with lesser feldspar

Front 277

laccolith

Back 277

an intrusive feature similar to a sill but formed from a more viscous magma that creates a lens-shaped mass between sedimentary layers, arching the overlying strata upward

Front 278

lahar

Back 278

a mudflow originating on a volcanic slope

Front 279

landslide

Back 279

a destructive, rapid mass-wasting event

Front 280

lateral erosion

Back 280

erosion that occurs when a stream meanders or braids back and forth across its valley floor or channel, undercutting and eroding its banks

Front 281

lateral moraine

Back 281

a moraine sonsisting of rock debris and sediment that have worked loose from the walls beside a velley glacier and have accumulated in ridges alog the sides of the glacier

Front 282

laterite

Back 282

a typically bright red, highly leached, residual soil that forms in tropic regions

Front 283

lateritic weathering

Back 283

weathering that results in residual deposits that become enriched through the chemical breakdown and removal of more reactive elements of a rock

Front 284

Laurasia

Back 284

the paleocontinent that once included the present-day landmasses of N. America and Eurasia

Front 285

lava

Back 285

magma that is extruded at the earth's surface, as from a volcano

Front 286

lava flood

Back 286

nonvolcanic lava that vents from deep cracks in the continental crust

Front 287

law of faunal succession

Back 287

a law that states that fossil species succeed one another in undisturbed rocks in a definite and recognizable order around the world

Front 288

law of original horizontality

Back 288

a law that states that most sedimentary rocks formed as nearly horizontal layers

Front 289

law of superpositition

Back 289

a law that states that in an undisturbed sequence of sedimentary rocks or lava flows, the overlying rock is younger than the underlying rock

Front 290

left-lateral strike-slip fault

Back 290

a strike-slip fault in which teh block across the fault appears to have moved to the left

Front 291

lignite

Back 291

a soft, brown coal produced by increasing temperature and pressure on peat

Front 292

limb

Back 292

one side of a fold

Front 293

liquefaction

Back 293

of a landslide, an occurence in which water-saturated soil moves downslope like a liquid

Front 294

lithification

Back 294

the hardening of sediment into a rock

Front 295

lithosphere

Back 295

an area composed of the crust and the uppermost part of the mantle

Front 296

loam

Back 296

soil that contains about equal amounts of sand, silt, and clay, as well as abundant organic matter

Front 297

loess

Back 297

silt and clay deposited by wind and weakly cemented by calcite

Front 298

longitudinal dune

Back 298

a large, symmetrical ridge of sand that parallels the wind direction; it can be over 100m high and over 100km long

Front 299

longshore current

Back 299

a strong current resulting from water bing pushed parallel to the shore by repeated wave action; primary transporter of sand in the shoreline environment

Front 300

longshore drift

Back 300

the movement of sediment parallel to the shore by wave action

Front 301

losing stream

Back 301

a stream whose channel lies above the water table and loses water into the unsaturated zone through which it is flowing

Front 302

luster

Back 302

the appearance or quality of light reflected from a mineral's surface

Front 303

mafic rock

Back 303

an igneous rock containing approximately 50& silica and relatively high percentages of iron, magnesium, and calcium

Front 304

magma

Back 304

molten rock that forms below the surface of the earth, usually at depths of 100km or greater

Front 305

magmatic arc

Back 305

a general term for belts of andesitic island arcs or inland andesitic mountain ranges (volcanic arcs) that develop along continental edges

Front 306

magmatic deposit

Back 306

of metallic ore, the result when the minerals settle to the bottom of an intrusive body and form thin, high-grade layers

Front 307

magmatic water

Back 307

water derived from magmas

Front 308

magnetic anomaly

Back 308

an area of magnetism that is either higher or lower than the average magnetic field for that region

Front 309

magnetic field

Back 309

of a planet, a magnetic force that surrounds the planet and probably originates from its metallic core

Front 310

magnetic pole

Back 310

a locality at which magnetc lines of force converge to create the strongest point in the magnetic field

Front 311

magnetometer

Back 311

a device for measuring the intensity of the magnetic field at the earth's surface

Front 312

mantle

Back 312

the middle zone o fthe earth, between the core and the crust

Front 313

mantle plume

Back 313

a "hot spot" in the crust where hot mantly material has ascended along deep penetrating cracks in the crust

Front 314

marine terrace

Back 314

a broad, gently sloping platfor offshore from a beach face

Front 315

marker beds

Back 315

those distinctive layers in a sedimentary sequence that allow exposures in defferent areas to be definitely correlated, or linked

Front 316

mass wasting

Back 316

the process of erosion whereby rock, soil, and other earth materials move down a slope because of gravetational forces

Front 317

meander

Back 317

a hairpinlike feature of a stream caused by erosion on the outside of a curve and deposition on the inside

Front 318

mechanical weathering

Back 318

the process by which rocks are physically broken down into smaller pieces by external contitions, such as the freezing and thawing of water in cracks in the rock

Front 319

medial moraine

Back 319

a long ridge of till that results when lateral maraines join as two tributary glaciers merge to form a single glacier

Front 320

Mediterranean-Himalayan belt

Back 320

a zone that runs through the Mediterranean region eastward through Asia and to the East Indies marked by frequent earthquake and volcanic activity

Front 321

member

Back 321

a distinctive rock layer that is part of a larger rock formation

Front 322

mesa

Back 322

a remnant flat-topped tower or column resulting from the weathering and erosion of a platueau's slopes

Front 323

metamorphic grades

Back 323

the different groups of minerals that crystallize and are stable at the different pressure and temperature ranges during regional metamorphism

Front 324

metamorphic rock

Back 324

a rock created by solid-state transformation of a rock mass into a rock of generally the same chemistry but with different textures and minerals

Front 325

metasomatism

Back 325

the process by which hot-water solutions carrying ions from an outside source move through a rock mass via fractures or pore space

Front 326

meteoric water

Back 326

water that is derived from the atmosphere as rain or snow and that moves down i8nto the bedrock from the earth's surface

Front 327

microseism

Back 327

a very small seismic tremor

Front 328

midoceanic ridge

Back 328

a deep crustal fault on the ocean floor that separates crustal plates and generates new ocean crust

Front 329

mineral

Back 329

a combination of elements that forms an inorganic, naturally occurring crystalline solid of a definite chemical composition

Front 330

Missippi Valley-type deposit

Back 330

a concentration of lead and zinc thought to be deposited in porous limestones and sandstones by low-temperature water that was driven out of deeper sediments by compaction

Front 331

modified Mercalli scale

Back 331

a ranking for the intensity of an earthquake, ranking it from 1 to 12 depending on the amount of resulting damage

Front 332

Mohorovicic discontinuity

Back 332

(the Moho) the first major boundary of the earth's interior; separates the crust from the underlying mantle

Front 333

Mohs hardness scale

Back 333

a scale from 1 to 10 on which the relative hardness of minerals is measured; named after its originator, Friedrich Mohs, a German mineralogist

Front 334

moraine

Back 334

an accumulation of till either lefter behind when a glacier recedes or carried on top of alpine glaciers

Front 335

mountain belt

Back 335

long chains of mountain ranges; typically thousands of km long

Front 336

mountain range

Back 336

a group of mountain peaks or ridges that form a discrete topographic area

Front 337

mountain root

Back 337

a bulge of continental crust downward into the mantle beneath a mountain

Front 338

mouth

Back 338

the terminus of a stream

Front 339

mudpot

Back 339

a kind of hot spring that produces boiling mud and realeases sulfurous gas