deserts and wind action (aeolian processes) › geology › pdfs › desert-notes.pdf · desert...

Deserts and Wind Action (Aeolian Processes)

Desert -- arid land with less than 250 mm (10 inches) precipitation per year or in which evaporation exceeds precipitation. No significant vegetation exists. It need NOT be hot and dry.

*Includes 25% of the world's land area outside the Polar Regions.

Depends on the circulation of the earth's atmosphere

•Coriolis effect -- deflection process of winds around earth

•Easterlies, westerlies, trade winds

•Most arid land lies between latitudes 10o and 30o (N. & S.)

There are several types of deserts:

1. Subtropical: Those associated with the circum-global belts of dry, subtropical air (Sahara, Kalahari, Great Australian)

2. Rainshadow: Lee side of mountain ranges in the rain shadow (Sonoran).

3. Continental interiors: with low rainfall (Gobi, Takla Makan in Asia)

4. Coastal deserts where upwelling cold seawater cools the air, lowering its ability to hold moisture (Peru, Chile, SW Africa)

5. Polar deserts (N. Greenland, ice-free areas in Antarctica)

Desert climate and vegetation

1. High temperature (+130oF)

2. Low precipitation (20-50 mm)

3. High evaporation rate (up to 250 mm/yr)

4. Frequent strong winds

Deserts and Arid Regions

Stages of Development for Desert or Arid Landscape:

1. First Stage:

•Playa lakes start to dry up; evaporation > precipitation

•Alluvial fans form -- fan-shaped sediment features

2. Middle Stage:

•Lakes dry up -- playas form

•Alluvial fans dry up -- form bajadas of pediment

3. Late Stage:

•Isolated remnants of resistant rock remain

•Drainage is internal -- never out of region except major

storm events

Erosion by wind

•Deflation

•Deflation armor or desert pavement

•Ventifacts -- rock sculpted by work of wind (sandblasting)

Types of Sand Dunes

1. Barchan – the most common dunes…constant wind…move 8-15 m/yr.

2. Transverse – Form in areas with strong winds and abundant sand.

3. Parabolic – moderate winds with some vegetation.

4. Longitudinal – high, somewhat variable winds and little sand.

Geologic processes in deserts

1. Weathering and mass wasting

2. Mostly mechanical weathering

3. Thin regolith (material overlying bedrock)

4. Desert varnish -- veneer coating of manganese oxides

5. Streams: flash floods, debris flows

6. Playa lakes, pediments, bajadas, inselbergs

7. Groundwater

8. Wind

Wind action

•Transport of sediment by wind

•Not efficient in moving great quantities of material

•Best sorter of particle size

•Suspended load

•Volcanic ash

Desert landforms

1. Fans and bajadas

2. Pediments

3. Inselbergs

Droughts and desertification

1. Droughts and soil erosion

2. Stabilization techniques

Key terms:

Deflation -- the process of removing sediment by wind erosion

Blowout -- area deflated by wind

Key Terminology

Desert Coriolis Effect

Subtropical Rainshadow

Coastal Polar

Continental Interior Playa lake

Alluvial fan Bajadas

Playas Inselberg

Internal drainage Ephemeral stream

Ventifact Blowout

Deflation Loess

Regolith Desert varnish

Barchan Barchanoid

Parabolic Longitudinal

Stellate Transverse