ES313 Geomorphology and Earth Surface Processes
Dry Regions:
The Geology of Deserts
Part 1
• Deserts cover ~ 25% of land surfaces.
• The primary control on deserts? Plate tectonics!
• Unique and lovely, deserts are characterized by…
Deserts • Extreme dryness. They may be hot or cold.
• Specialized ecosystems and low human populations.
• Unique geologic processes.
• Land that is so extremely arid, that…
What Is a • Evaporation prevents permanent surface water.
Desert? • Vegetation covers less than 15% of its surface.
• Annual rainfall amounts to less than 10” (25 cm).
• Desert types exist in both hot and cold climates.
Hot vs. Cold Deserts • Hot deserts.
• Low latitudes.
• Low elevations.
• Far from oceans.
• Cold deserts.
• High latitudes.
• High elevations.
• Near cold ocean currents.
Desert Features
• Desert land surfaces often
include…
• Exposed bedrock.
• Accumulated clasts.
• Unweathered sediment.
• Precipitated salt.
• Windblown sand.
Types of Deserts
• 5 types – With distinctive
landscapes and biota.
• Subtropical deserts - Sahara,
Arabian, Kalahari, Thar.
• Rain shadow deserts - Eastern
Oregon).
• Coastal deserts - Atacama
(Peru).
• Continental interiors - Gobi
(Mongolia).
• Polar deserts (Antarctica).
• Due to patterns of atmospheric
convection.
Subtropical Deserts • Equator – 0o latitude.
• Solar energy evaporates
water, which rises as hot,
moist air.
• Rising air cools and
expands, forming
abundant rain.
• This air, stripped of
moisture, flows to the N
and S.
• Subtropics – 20o- 30o N & S.
• Sinking dry air wicks
water.
• Landscape below dries.
• Earth’s largest deserts.
Subtropical
Deserts
Arabian
Sahara
desert
desert
Namib and
Kalahari
desert
• Moist ocean winds are driven over mountains.
• Windward air is forced to rise, expand, and cool.
Rain shadow • Moisture condenses, becomes rain, and creates a rain
Deserts forest.
• Leeward air, stripped of moisture, sinks and drinks.
• Sinking air warms, compresses, and sucks water
out of land.
• Cool air over cold ocean water holds little
moisture.
• This air absorbs moisture when it
Coastal Deserts interacts with land.
• The Atacama desert (Peru) is the driest
place on Earth.
• Air loses moisture as it crosses continents.
Interior Deserts • Land far from ocean moisture can be arid.
• The Gobi desert in Mongolia is a prime
example.
• Above 66º N and S latitude there is little moisture.
Polar Deserts • Air circulation carries dry air to polar regions.
• It is so cold that the air can’t hold moisture.
Desert Processes • Deserts are characterized
by special processes.
• Erosion by water and
wind.
• Weathering.
• Soil formation.
• Deposition.
• These processes result in
unique landscapes.
• Physical weathering dominates.
Weathering • Rare chemical weathering leaches ions.
• Evaporation salts both break and cement grains.
Prehistoric rock paintings (petroglyphs) in rock shelters at
Bhimbetka, a world heritage site
• Exposed surfaces develop desert varnish.
• Dark surface coating of iron and manganese oxides.
Desert • Forms very slowly by bacterial activity, dust, and water.
• Native Americans left petroglyphs in desert varnish.
Varnish • Rock paintings at Bhimbetka, Bhopal.
• Petroglyph rock art at Daraki-Chattan near Bhanpura in Mandsaur, MP –
world’s oldest rock rock art (2-5 lakh years old)
• Desert soils are thin, with poorly defined horizons.
Desert Soils • They usually are colored like the bedrock nearby.
• Trace elements orchestrate wide color variations.
• Though rare, water shapes landscapes.
• Lack of roots magnifies sediment erosion and
transport.
Water Erosion • Torrential rains generate dangerous flash floods.
• Rapid flow of thick, muddy, and viscous water.
• Flash floods quickly infiltrate dry stream beds.
• Sparsely vegetated ground is
Wind Erosion and scoured by wind.
Transport • 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.
Wind Erosion and
Transport
• 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.
• Desert pavement is resistant to
erosion.
• Often coated with desert varnish.
Wind Erosion and Transport Ventifacts
Yardangs
• Grains in wind often “sandblast”
exposed surfaces.
• Ventifacts - Stones with wind-faceted
upper surfaces.
• Yardangs – Wind-sculpted bedrock
(also called Pedestal Rock).
Wind Erosion and
Transport • Deflation – Lowering
the land surface via
erosion.
• Concentrated wind
erosion creates a
blowout.
• Desert processes are
clearly operating on
Mars.
