Deserts 23

Front 1

Distribution of Dry Regions

Back 1

Dry regions (deserts and steepes) cover 30% of earth's land surface (roughly 42 million square kilometers

Front 2

Dry Climate Zones

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-yearly precipitation is not as great as loss of water by evaporation

Front 3

Desert

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arid, driest of the dry climate zones

Front 4

Steppe

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semi-arid, marginal zone that borders arid deserts

Front 5

low-latitude Deserts

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-two bands of deserts are concentrated at low-latitudes, between 20degrees-30degrees north and 20degrees-30degrees south latitude
-belts of arid and semiarid climate stretch across nearly 9300 km at the Tropics of Cancer and Capricorn
-two high pressure melts at low latitudes
-in high pressure zones, dry air descends from high altitudes, resulting in warm, dry climate

Front 6

MId-Latitude Deserts

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-mid-latitude deserts and steppes occur in areas sheltered by mountains
-high mountains in the path of prevailing winds produce rain shadow deserts

Front 7

Rain shadows

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-winds move warm, moist air up the windward slope of a mountain, temp decreases and water vapor condenses and precipitates
-when air descends on leeward side, it is extremely dry, resulting in arid region on the leeward side of mountain

Front 8

continental interior deserts

Back 8

-form on inland of large land masses
-wind moving onto continent loses moisture by precipitation
-as air moves further inland, air increasingly dry, produces little to no precipitation

Front 9

dry channels

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beds of ephemeral streams that contain water only during heavy rainfall

Front 10

desert floods

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-rain is rare but typically occurs as heavy showers
-sparse vegetation and poorly integrated drainage systems contribute to unhindered runoff
-flash floods along valley floors are common and a single, short-lived rain storm can cause massive erosion

Front 11

desert rivers

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-few permanent rivers, originate often in mountains
-water table usually very deep, few desert streams can draw on it as steady supply of water
-combination of infiltration and evaporation rapidly deplete streams

Front 12

desert groundwater

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-water table usually very deep, forming oasis where it intersects surface
-few desert streams gain form groundwater

Front 13

interior drainage

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-a discontinuous pattern of intermittent streams that do not flow to the ocean

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early landscape evolution

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-initially driven by block faulting and tectonic uplift of mountain rangers
-alluvial fans form at moths of canyons and playa lakes form in flat central area of undrained desert basins

Front 15

Late landscape evolution

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-ongoing erosion lowers mountains and fills basins w sediment
-alluvial fans coalesce to form bajada
-eroded mountains may form inselbergs

Front 16

bajada

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an apron of sediment along a mountain front

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inselbergs

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isolated mountain remnants

Front 18

suspended load

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fine particles transported within the air

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saltation

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transport of particles through series of bounces

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bedload

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particles moved along the ground surface by wind

Front 21

wind erosion

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as wind lifts fine, loose sediment from the desert floor (deflation) it leaves behind erosional features such as:
-blowouts-shallow depressions
-desert pavement-surface of coarse pebbles and cobbles

Front 22

wind deposits

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-dunes
-loess

Front 23

loess deposits

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-forms in area where windblown silt accumulates
-western and northern China
-midwest USA-silt from glacial deposits

Front 24

dune formation

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-forms where wind speed slows, sand from bedload accumulates in wind shadows
-asymmetrical profile

Front 25

no dunes

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-forms in areas with low wind, little sand, and extensive vegetation

Front 26

transverse dunes

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-series of long ridges oriented perpendicular to prevailing wind
-form where vegetation is sparse
-sand is very plentiful
-wind is strong

Front 27

longitudinal dunes

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-long ridges of sand oriented parallel to prevailing wind
-dunes form where sand supplies are limited

Front 28

barchan dunes

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-sand dunes shaped like crescents with tips pointing downwind
-minimal vegetation
-medium wind
-medium sand

Front 29

parabolic dunes

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-crescent shaped dune
-similar in shape to barchan dune except tips point into wind
-often form along coasts

Front 30

dune migration

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-continued sand accumulation, w periodic slides down the slip face, results in slow migration of dune in direction of wind

Front 31

changing wind direction

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changes in prevailing wind direction is recorded by changes in layers of cross beds

Front 32

lithifies dunes

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-dunes buried under layers of sediment and turned to sandstone (lithified), asymmetrical shape is destroyed, cross bedding is often prominent

Front 33

deserts as climate indicators

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-dunes and crossbedding in sandstones indicates and arid paleoclimate during time of deposition, regardless of area's modern climate
-expanding dunes and deserts indicates modern climate shifts-aridification leads to desertification

Front 34

desertification

Back 34

caused by climate change, overgrazing, drought