Chapter 21
Dry Regions: The Geology of Deserts
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Dry Regions:
The Geology of Deserts
Prepared by
Ronald Parker
Earlham College Department of Geosciences
Richmond, Indiana
Deserts
Deserts cover ~ 25% of land surfaces.
 The primary control on deserts? Plate tectonics!
 Unique and lovely, deserts are characterized by…

 Extreme
dryness. They may be hot or cold.
 Specialized ecosystems and low human populations.
 Unique geologic processes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
What Is a Desert?

Land that is so extremely arid, that…
 Evaporation
prevents permanent surface water.
 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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Hot vs. Cold Deserts

Hot deserts.
Low latitudes.
 Low elevations.
 Far from oceans.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Cold deserts.
High latitudes.
 High elevations.
 Near cold ocean currents.

Chapter 21: Dry Regions: The Geology of Deserts
Desert Features

Desert land surfaces often include…
Exposed bedrock.
 Accumulated clasts.
 Unweathered sediment.
 Precipitated salt.
 Windblown sand.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Types of Deserts

5 types – With distinctive landscapes and biota.
Subtropical deserts (Sahara, Arabian, Kalahari).
 Rain shadow deserts (Eastern Oregon).
 Coastal deserts (Atacama).
 Continental interiors (Gobi).
 Polar deserts (Antarctica).

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Subtropical Deserts

Due to patterns of atmospheric convection.

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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Subtropical Deserts
Found at 20 to 30o N and S latitude across time.
 African deserts bracket the equator.




Sahara & Arabian to the north.
Rainforest straddling equator.
Namib & Kalahari to the south.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Rainshadow Deserts

Moist ocean winds are driven over mountains.
Windward air is forced to rise, expand, and cool.
 Moisture condenses, becomes rain, and creates a rain
forest.
 Leeward air, stripped of moisture, sinks and drinks.

Sinking air warms, compresses, and sucks water out of land.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Coastal Deserts

Cool air over cold ocean water holds little moisture.
This air absorbs moisture when it interacts with land.
 The Atacama desert (Peru) is the driest place on Earth.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Interior Deserts
Air loses moisture as it crosses continents.
 Land far from ocean moisture can be arid.
 The Gobi desert in Mongolia is a prime example.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Polar Deserts

Above 66º N and S latitude there is little moisture.
Air circulation carries dry air to polar regions.
 It is so cold that the air can’t hold moisture.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Processes

Deserts are characterized by special processes.
Erosion by water and wind.
 Weathering.
 Soil formation.
 Deposition.


These processes result in unique landscapes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Weathering

Physical weathering dominates.
Rare chemical weathering leaches ions.
 Evaporation salts both break and cement grains.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Varnish

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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Soils
Desert soils are thin, with poorly defined horizons.
 They usually are colored like the bedrock nearby.
 Trace elements orchestrate wide color variations.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Water Erosion

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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Wind Erosion and Transport
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Wind Erosion and Transport

Grains in wind often “sandblast” exposed surfaces.
Ventifacts - Stones with wind-faceted upper surfaces.
 Yardangs – Wind-sculpted bedrock.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Wind Erosion and Transport
Deflation – Lowering the land surface via erosion.
 Concentrated wind erosion creates a blowout.
 Desert processes are clearly operating on Mars.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Deposition
Desert sediments accumulate in many settings.
 Erosional debris accumulates near source areas.

Talus aprons, at the angle of repose, collect cliff debris.
 Rock blocks have shapes determined by jointing.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Deposition

Alluvial fans – Conical accumulations of sediment.
Water exiting a canyon spreads out and drops sediment.
 Alluvial fans grow outward from source over time.
 Sediment characteristics depend on fan position.

Near the source channel sediments are coarse.
Sediment grains become finer away from source.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Deposition

Playas – Desert lakes that have no outlet streams.
Internal drainage collects water from flash floods.
 Dissolved solids crystallize out as water evaporates.

Halite, gypsum, borax.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Processes

Wind carries two types of sediment load.
Surface load – Coarser sand-sized particles.
 Suspended load – Finer-grained silt-sized “dust.”
 Sand forms dunes inside deserts; dust forms loess
adjacent (and downwind of) deserts.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes
Deserts exhibit a diverse set of landscape styles.
 These reflect a complex interplay of erosional and
depositional processes over long time spans.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Cliffs and mesas – Bedrock controls landforms.
Bedrock exposed along cliffs breaks away along joints.
 Cliff retreat in flat rocks creates plateaus, then mesas…
 Then buttes and, finally, chimneys.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Cliffs and mesas – Weathering dictates appearance.
Resistant – Form steep cliffs as thick as the rock unit.
 Weak – Weather to form slopes at the angle of repose.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Cliffs and mesas – Structure governs landforms.

Nonhorizontal bedding produces elongate linear ridges.
Cuesta – Steep joint-controlled cliff; less-steep dip slope.
Hogback – Steeply dipping beds create a symmetric ridge.
Inselberg – Eroded remnant of almost-complete cliff retreat.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes
Stony plains – Sloping, gravelly alluvial fans.
 Pediments – Ramplike surfaces that slope up toward
a mountain front.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Dunes – Windblown accumulations of sand.
Sand carried by wind accumulates around an obstacle.
 Over time, a dune grows and begins to move downwind.
 Dunes generate enormous crossbeds.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Dunes – Windblown accumulations of sand.

Many types; actual dune form depends on variation in…
Wind – Velocity and consistency of direction.
Sediment supply – Is sand scarce or abundant?
Dune
Wind
Sand
Barchan
Steady
1 direction
Scarce
Star
Steady
Changing directions
Scarce
Transverse
Steady, moderate
1 direction
Plentiful
Parabolic
Steady, strong
1 direction
Plentiful
Longitudinal
Steady, strong
1 direction
Abundant
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Landscapes

Dunes – Windblown accumulations of sand.

Gigantic oceans of sand (ergs) develop in some deserts.
Arabian peninsula.
Namibia.

Ergs existed in the past.
Permian.
Jurassic.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Life

Desert organisms are adapted to harsh conditions.
Dessication.
 Temperature extremes.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Problems
Desertification – Aridification of nondesert areas.
 Human activity expands desert areas.


Case study: the Aral Sea.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Problems

Desertification is caused by…
Overpopulation.
 Overgrazing.
 Careless agriculture.
 Diversion of water supplies.


Desertification leads to famine.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
Desert Problems

Desertification facilitates large dust storms that...
Cross entire ocean basins.
 Carry disease organisms.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts
This concludes the
Chapter 21
Dry Regions: The Geology of Deserts
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 21: Dry Regions: The Geology of Deserts