Geology 101 - Deserts

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Covers

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25% of land surface

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primary control on deserts?

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plate tectonics

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unique qualities

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extreme dryness
may be hot or cold
specialized ecosystems
low human population
unique geologic processes

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deff of desert

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land that is so extremly arid that :
evaporation prevents permanent surface water

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hot desert

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low latitudes
low elevations
far from oceans

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cold desert

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high latitudes
high elevations
near cold ocean currents

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land surface features

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exposed bedrock
accumulated clasts
unweathered sediment
precipitated salt
windblown sand

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five types of deserts

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subtripical deserts
rain shadow desert
coastal deserts
continental interiors
polar desert

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subtropical desert

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earth's largest
form due to patterns of atmospheric convention
found at 20 to 30 degrees N and S latitude across geologic time

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rainshadow desert

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moist ocean winds are driven over mountains

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coastal deserts

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-cool air over ocean water hols little moisture
-air is absorbs moisture when it interacts with land
-the atacama desert is the driest place on earth

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interior deserts

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-hair loses moisture as it crosses contients
-land far from ocean moisture can be arid
ex: gobi desert

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polar deserts

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-Above 66º N and S latitude there is little moisture.
-Air circulation carries dry air to polar regions.
-It is so cold; the air can’t hold moisture.

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desert processes

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deserts are characterized by special processes:
-eronsion by water and wind
-weathering
-soil formation
-deposition

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weathering

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Physical weathering dominates.
-Rare chemical weathering leaches ions.
-Evaporation salts both break and cement grains.

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desert varnish

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Exposed surfaces develop desert varnish.
Dark surface coating of iron and manganese oxides.
Forms very slowly by bacterial activity, dust, and water.
Native Americans left petroglyphs in desert varnish.

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desert soils

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-Desert soils are thin, with poorly defined horizons.
-They usually are colored like the bedrock nearby.
-Trace elements orchestrate wide color variations.

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water erosion

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Though rare, water shapes landscapes.
-Lack of roots magnifies sediment erosion and transport.
-Torrential rains generate dangerous flash floods.
-Rapid flow of thick, muddy, and viscous water.
-Flash floods quickly infiltrate dry stream beds.

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wind erosion and transport

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-Sparsely vegetated ground is scoured by wind.
-Sand and silt-sized sediment is lifted and moved.
-Surface load – Grains moved in contact with land surface.
-Saltation – Sand skipped and bounced by grain impact.
-Suspended load – Sediment carried in the air.
-High winds can carry dust across entire oceans.

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wind erosion and transport

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-Coarse clasts cannot be lifted and moved by wind.
-Lag deposits form when finer sediment is removed.
-Form desert pavement, a surface layer of stones.

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deflation

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lowering the land surface via erosion
-concentrated wind erosion creates a blowout
- ex: mars

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desert deposition

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-desert sediments accumulate in many seetings
-erosional debris accumulates near source areas

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alluvial fans

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conical accumulations of sediment
-water exiting a canyon spreads out and drops sediment
-grow outward from source over time

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playas

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desert lakes that have no outlet streams
-internal drainage collects water from flash floods
-dissolved solids crystallize out as water exaporates

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surface load

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coarser sand-sized particles

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suspended load

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finer-grained silt-sized dust

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bedrock controls landforms

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bedrock exposed along cliffs breaks away along joints
-cliff retreat in flat rocks creats plateaus then mesas
-then buttes and finally chimneys

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stony plains

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sloping gravelly alluvial plains

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pediments

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ramp like surfaces that slow up toward a mountain front

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dunes

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windblown accumulations of sand

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dunes depend on

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wind - velocity and consistency of direction
sediment supply - is sand scarce of abundant

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desertification

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aridificcation of non-desert areas
-human activity expands desert areas

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desertification is caused by:

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overpopulation
overgrazing
careless agriculture
diversion of water supplies

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large dust storms..

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cross entire ocean basins
carry diseased organismed