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DESERTS, GLACIERS AND

CLIMATE CHANGE

Objectives

• Identify five different geologic settings for deserts.

• Describe the structure of a sand dune.

• Distinguish between several different kinds of glaciers

and ice formations.

• Describe how ice in a glacier changes form,

accumulates, ablates, and moves.

• Explain how geologists learn about past climatic

conditions.

• Examine evidence for anthropogenic climate change.

Deserts and Drylands

• Types of deserts

– Desert

• An arid land that receives less than 250 millimeters of rainfall or

snow equivalent per year

• Sparsely vegetated unless it is irrigated

• Different types depending on location and global air circulation

patterns

– Subtropical desert: located between the 30°N and 30°S latitudes

– Polar desert: located in polar regions; precipitation is primarily snow

which does not melt

– Continental interior desert

– Rain-shadow desert

– Coastal desert

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Deserts and Drylands

Deserts and

Drylands

Deserts and Drylands

• Erosion- Wind blown sediment

– Surface creep

• sediment transport in which the wind causes particles to roll along

the ground

– Saltation

• Sediment transport in which particles move forward in a series of

short jumps along arc-shaped patterns

– Suspension

• Sediment transport in which the wind carries very fine particles over

long distances and periods of time

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Deserts and Drylands

• Wind erosion continued

– Mechanics of wind erosion

• Abrasion

• Airborne particles chip

small fragments off rocks

protruding above the

surface

• Deflation

• Loose particles of sand

and dust are removed by

wind

• Leaves coarser particles

behind

Ventifacts (South Sinai)

Ventifacts-Iceland

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Yardangs

Iran-largest on earth

White Desert, Egypt

White Desert, Egypt

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Deserts and Drylands

Desert pavement

• Theories for origin

– Winnowing of fines by

wind or water

– Heave of larger clasts

due to swelling of

clays or formation of

salt crystals.

– Others

Desert varnish

• Coating of

manganese oxides on

exposed rock

surfaces. Involves

oxidation and

precipitation by

microorganisms that

can live in very

alkaline conditions.

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Desert varnish on

Cretaceous sandstone,

Kurkur plain, southern

Egypt.

Dark surfaces on inselbergs and surface pebbles is

desert varnish

Landforms

• Stream valleys-wide shallow channels, high

bedload transport

• Desert plains and plateaus

– Bare surfaces (lack of vegetation)

– Stony-lag concentrate from wind erosion (reg)

• Desert pavement, rocks in close contact, may be covered

with desert varnish

– Barren rock (hammada)

– Sand seas (ergs)

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Reg surfaces

Giza Plateau El-Qaa Plain,

Sinai

Hammadas

Erg: Sand Sea

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Deserts and Drylands

• Wind deposits and landforms

– Dune

• A hill or ridge of sand deposited by winds

– Slip face

– Cross beds

– Angle of repose

Deserts and Drylands

Slip face; Mt. Baldy,

Indiana Dunes

National Lakeshore

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Deserts and Drylands

Cresent=Barchan

Dunes

Incr.

Sand

supply

Barchans

Wind from one direction; limited

sand supply; horns point

downwind; slip face is always

steeper

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Barchans near the 3rd cataract: Dongola,

Sudan

1 mile

Longitudinal (linear) dunes

Namibia; USGS

Wind from 2 directions:

limited sand supply

Great Sand Sea, Western Desert

5 miles

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Transverse dunes

Wind from one

direction:

abundant sand

supply

Parabolic (blowout) dunes

Erosion of existing

dunes; horns point

upwind.

Deserts and Drylands

Wind from more than 2

directions; abundant sand

supply

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Deserts and Drylands

• Stream erosion and

deposition

– Arroyos

– Flash floods

– Alluvial fan

• Desertification

– Invasion of desert

conditions into non-desert

areas

• “Dust Bowl”

Flash floods are common in

mountainous desert areas

because:A) Steep slopes

B) Sparse vegetation

C) Intense rainfalls

Flash floods

• En Gedi

• Wadi Zin

• El Arish, 2010

Shortcut to En Gedi floods.lnk

Shortcut to Zin2.lnk

Wadi e l A resh.flv

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Deserts and Drylands