Chapter 21: Dry Regions: The Geology of Deserts Earth: Portrait of a Planet, 3 rd edition, by...

Chapter 21: Dry Regions: The Geology of Deserts Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 21Dry Regions: The Geology of Deserts

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Dry Regions:The Geology of Deserts

Dry Regions:The Geology of Deserts

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana


DesertsDeserts Deserts cover ~ 25% of land surfaces.Deserts cover ~ 25% of land surfaces. The primary control on deserts? Plate tectonics! The primary control on deserts? Plate tectonics! Unique and lovely, deserts are characterized by…Unique and lovely, deserts are characterized by…

Extreme dryness. They may be hot Extreme dryness. They may be hot oror cold. cold. Specialized ecosystems and low human populations.Specialized ecosystems and low human populations. Unique geologic processes.Unique geologic processes.


What Is a Desert?What Is a Desert? Land that is so extremely arid, that…Land that is so extremely arid, that…

Evaporation prevents permanent surface water. Evaporation prevents permanent surface water. Vegetation covers less than 15% of its surface. Vegetation covers less than 15% of its surface. Annual rainfall amounts to less than 10” (25 cm).Annual rainfall amounts to less than 10” (25 cm).

Desert types exist in both hot and cold climates.Desert types exist in both hot and cold climates.


Hot vs. Cold DesertsHot vs. Cold Deserts Cold deserts.Cold deserts.

High latitudes.High latitudes. High elevations.High elevations. Near cold ocean currents.Near cold ocean currents.

Hot deserts.Hot deserts. Low latitudes.Low latitudes. Low elevations.Low elevations. Far from oceans.Far from oceans.


Desert FeaturesDesert Features Desert land surfaces often include…Desert land surfaces often include…

Exposed bedrock.Exposed bedrock. Accumulated clasts.Accumulated clasts. Unweathered sediment.Unweathered sediment. Precipitated salt.Precipitated salt. Windblown sand.Windblown sand.


Types of DesertsTypes of Deserts 5 types – With distinctive landscapes and biota. 5 types – With distinctive landscapes and biota.

Subtropical deserts (Sahara, Arabian, Kalahari).Subtropical deserts (Sahara, Arabian, Kalahari). Rain shadow deserts (Eastern Oregon).Rain shadow deserts (Eastern Oregon). Coastal deserts (Atacama).Coastal deserts (Atacama). Continental interiors (Gobi).Continental interiors (Gobi). Polar deserts (Antarctica).Polar deserts (Antarctica).


Subtropical DesertsSubtropical Deserts Due to patterns of atmospheric convection. Due to patterns of atmospheric convection.

Equator – 0Equator – 0oo latitude. latitude.Solar energy evaporates water, which rises as hot, moist air.Solar energy evaporates water, which rises as hot, moist air.Rising air cools and expands, forming abundant rain. Rising air cools and expands, forming abundant rain. This air, stripped of moisture, flows to the N and S.This air, stripped of moisture, flows to the N and S.

Subtropics – 20Subtropics – 20oo- 30- 30oo N & S. N & S.Sinking dry air wicks water.Sinking dry air wicks water.Landscape below dries. Landscape below dries.

Earth’s largest Earth’s largest deserts. deserts.


Found at 20 to 30Found at 20 to 30oo N and S latitude across time. N and S latitude across time. African deserts bracket the equator. African deserts bracket the equator.

Sahara & Arabian to the north.Sahara & Arabian to the north. Rainforest straddling equator.Rainforest straddling equator. Namib & Kalahari to the south. Namib & Kalahari to the south.

Subtropical DesertsSubtropical Deserts


Rainshadow DesertsRainshadow Deserts Moist ocean winds are driven over mountains. Moist ocean winds are driven over mountains.

Windward air is forced to rise, expand, and cool.Windward air is forced to rise, expand, and cool. Moisture condenses, becomes rain, and creates a rain Moisture condenses, becomes rain, and creates a rain

forest. forest. Leeward air, stripped of moisture, sinks and drinks.Leeward air, stripped of moisture, sinks and drinks.

Sinking air warms, compresses, and sucks water out of land.Sinking air warms, compresses, and sucks water out of land.


Coastal DesertsCoastal Deserts Cool air over cold ocean water holds little moisture.Cool air over cold ocean water holds little moisture.

This air absorbs moisture when it interacts with land. This air absorbs moisture when it interacts with land. The Atacama desert (Peru) is the driest place on Earth.The Atacama desert (Peru) is the driest place on Earth.


Interior DesertsInterior Deserts Air loses moisture as it crosses continents.Air loses moisture as it crosses continents. Land far from ocean moisture can be arid. Land far from ocean moisture can be arid. The Gobi desert in Mongolia is a prime example. The Gobi desert in Mongolia is a prime example.


Polar DesertsPolar Deserts Above 66º N and S latitude there is little moisture.Above 66º N and S latitude there is little moisture.

Air circulation carries dry air to polar regions.Air circulation carries dry air to polar regions. It is so cold that the air can’t hold moisture.It is so cold that the air can’t hold moisture.


Desert ProcessesDesert Processes Deserts are characterized by special processes.Deserts are characterized by special processes.

Erosion by water and wind.Erosion by water and wind. Weathering.Weathering. Soil formation.Soil formation. Deposition. Deposition.

These processes result in unique landscapes.These processes result in unique landscapes.


WeatheringWeathering Physical weathering dominates.Physical weathering dominates.

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


Desert VarnishDesert Varnish Exposed surfaces develop desert varnish.Exposed surfaces develop desert varnish.

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


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


Water ErosionWater Erosion Though rare, water shapes landscapes.Though rare, water shapes landscapes.

Lack of roots magnifies sediment erosion and transport. Lack of roots magnifies sediment erosion and transport. Torrential rains generate dangerous flash floods. Torrential rains generate dangerous flash floods.

Rapid flow of thick, muddy, and viscous water. Rapid flow of thick, muddy, and viscous water. Flash floods quickly infiltrate dry stream beds.Flash floods quickly infiltrate dry stream beds.


Sparsely vegetated ground is scoured by wind.Sparsely vegetated ground is scoured by wind. Sand and silt-sized sediment is lifted and moved.Sand and silt-sized sediment is lifted and moved.

Surface load – Grains moved in contact with land surface.Surface load – Grains moved in contact with land surface. Saltation – Sand skipped and bounced by grain impact.Saltation – Sand skipped and bounced by grain impact. Suspended load – Sediment carried in the air.Suspended load – Sediment carried in the air.

High winds can carry dust across entire oceans.High winds can carry dust across entire oceans.

Wind Erosion and TransportWind Erosion and Transport


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

Desert pavement is resistant to erosion. Desert pavement is resistant to erosion. Often coated with desert varnish.Often coated with desert varnish.



Grains in wind often “sandblast” exposed surfaces. Grains in wind often “sandblast” exposed surfaces. Ventifacts - Stones with wind-faceted upper surfaces. Ventifacts - Stones with wind-faceted upper surfaces. Yardangs – Wind-sculpted bedrock.Yardangs – Wind-sculpted bedrock.



Deflation – Lowering the land surface via erosion.Deflation – Lowering the land surface via erosion. Concentrated wind erosion creates a blowout.Concentrated wind erosion creates a blowout. Desert processes are clearly operating on Mars.Desert processes are clearly operating on Mars.



Desert sediments accumulate in many settings.Desert sediments accumulate in many settings. Erosional debris accumulates near source areas. Erosional debris accumulates near source areas.

Talus aprons, at the angle of repose, collect cliff debris. Talus aprons, at the angle of repose, collect cliff debris. Rock blocks have shapes determined by jointing.Rock blocks have shapes determined by jointing.

Desert DepositionDesert Deposition


Alluvial fans – Conical accumulations of sediment.Alluvial fans – Conical accumulations of sediment. Water exiting a canyon spreads out and drops sediment. Water exiting a canyon spreads out and drops sediment. Alluvial fans grow outward from source over time. Alluvial fans grow outward from source over time. Sediment characteristics depend on fan position. Sediment characteristics depend on fan position.

Near the source channel sediments are coarse.Near the source channel sediments are coarse.Sediment grains become finer away from source. Sediment grains become finer away from source.

Desert DepositionDesert Deposition


Desert DepositionDesert Deposition Playas – Desert lakes that have no outlet streams.Playas – Desert lakes that have no outlet streams.

Internal drainage collects water from flash floods.Internal drainage collects water from flash floods. Dissolved solids crystallize out as water evaporates.Dissolved solids crystallize out as water evaporates.

Halite, gypsum, borax.Halite, gypsum, borax.


Wind carries two types of sediment load. Wind carries two types of sediment load. Surface load – Coarser sand-sized particles. Surface load – Coarser sand-sized particles. Suspended load – Finer-grained silt-sized “dust.”Suspended load – Finer-grained silt-sized “dust.” Sand forms dunes inside deserts; dust forms loess Sand forms dunes inside deserts; dust forms loess

adjacent (and downwind of) deserts.adjacent (and downwind of) deserts.

Desert ProcessesDesert Processes


Deserts exhibit a diverse set of landscape styles. Deserts exhibit a diverse set of landscape styles. These reflect a complex interplay of erosional and These reflect a complex interplay of erosional and

depositional processes over long time spans.depositional processes over long time spans.

Desert LandscapesDesert Landscapes


Desert LandscapesDesert Landscapes Cliffs and mesas – Bedrock controls landforms. Cliffs and mesas – Bedrock controls landforms.

Bedrock exposed along cliffs breaks away along joints.Bedrock exposed along cliffs breaks away along joints. Cliff retreat in flat rocks creates plateaus, then mesas…Cliff retreat in flat rocks creates plateaus, then mesas… Then buttes and, finally, chimneys. Then buttes and, finally, chimneys.


Cliffs and mesas – Weathering dictates appearance.Cliffs and mesas – Weathering dictates appearance. Resistant – Form steep cliffs as thick as the rock unit.Resistant – Form steep cliffs as thick as the rock unit. Weak – Weather to form slopes at the angle of repose.Weak – Weather to form slopes at the angle of repose.



Cliffs and mesas – Structure governs landforms. Cliffs and mesas – Structure governs landforms. Nonhorizontal bedding produces elongate linear ridges.Nonhorizontal bedding produces elongate linear ridges.

Cuesta – Steep joint-controlled cliff; less-steep dip slope.Cuesta – Steep joint-controlled cliff; less-steep dip slope.Hogback – Steeply dipping beds create a symmetric ridge. Hogback – Steeply dipping beds create a symmetric ridge. Inselberg – Eroded remnant of almost-complete cliff retreat. Inselberg – Eroded remnant of almost-complete cliff retreat.



Stony plains – Sloping, gravelly alluvial fans.Stony plains – Sloping, gravelly alluvial fans. Pediments – Ramplike surfaces that slope up toward Pediments – Ramplike surfaces that slope up toward

a mountain front.a mountain front.



Dunes – Windblown accumulations of sand.Dunes – Windblown accumulations of sand. Sand carried by wind accumulates around an obstacle. Sand carried by wind accumulates around an obstacle. Over time, a dune grows and begins to move downwind.Over time, a dune grows and begins to move downwind. Dunes generate enormous crossbeds.Dunes generate enormous crossbeds.



Dunes – Windblown accumulations of sand.Dunes – Windblown accumulations of sand. Many types; actual dune form depends on variation in…Many types; actual dune form depends on variation in…

Wind – Velocity and consistency of direction.Wind – Velocity and consistency of direction.Sediment supply – Is sand scarce or abundant?Sediment supply – Is sand scarce or abundant?


DuneDune WindWind SandSand

BarchanBarchanSteadySteady

1 direction1 directionScarceScarce

StarStarSteadySteady

Changing directionsChanging directionsScarceScarce

TransverseTransverseSteady, moderateSteady, moderate

1 direction1 directionPlentifulPlentiful

ParabolicParabolicSteady, strongSteady, strong

1 direction1 directionPlentifulPlentiful

LongitudinalLongitudinalSteady, strongSteady, strong

1 direction1 directionAbundantAbundant


Dunes – Windblown accumulations of sand.Dunes – Windblown accumulations of sand. Gigantic oceans of sand (ergs) develop in some deserts. Gigantic oceans of sand (ergs) develop in some deserts.

Arabian peninsula.Arabian peninsula.Namibia.Namibia.

Ergs existed in the past.Ergs existed in the past.Permian.Permian.Jurassic.Jurassic.



Desert organisms are adapted to harsh conditions. Desert organisms are adapted to harsh conditions. Dessication.Dessication. Temperature extremes.Temperature extremes.

Desert LifeDesert Life


Desert ProblemsDesert Problems Desertification – Aridification of nondesert areas. Desertification – Aridification of nondesert areas. Human activity expands desert areas. Human activity expands desert areas.

Case study: the Aral Sea.Case study: the Aral Sea.


Desert ProblemsDesert Problems Desertification is caused by… Desertification is caused by…

Overpopulation.Overpopulation. Overgrazing.Overgrazing. Careless agriculture. Careless agriculture. Diversion of water supplies. Diversion of water supplies.

Desertification leads to famine.Desertification leads to famine.


Desert ProblemsDesert Problems Desertification facilitates large dust storms that...Desertification facilitates large dust storms that...

Cross entire ocean basins. Cross entire ocean basins. Carry disease organisms.Carry disease organisms.


Chapter 21Dry Regions: The Geology of Deserts

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

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