Geology of DesertBy (saba jamal din)

Content

• Definition• Formation• Distribution of desert• Types of desert• Erosional features• Depositional features

What is a desert?

Any region with an arid climate and a rainfall less than 25 cm/year.

How deserts form? Descending warm and moist air. Rain shadow Distance from ocean Coasts with cold ocean currents

• Most of the deserts lie along a narrow belt 30° N and S of the Equator.

• These two belts have warmer air and can hold a lot of moisture, leading to very low rainfalls.

• The two belts are characterized by clear skies, lots of sunshine, little rain and high evaporation

Distribution of deserts

Distribution of deserts

Types of Deserts

• Hot and dry desert

• Semi Arid desert• Coastal desert• Cold Desert

Seasons are warm throughout year and get very hot during the summer. • Average temps. Range from 20-25 C • When it gets really hot it ranges from about 43-49 C • Minimum temps. can drop as low as -18C • Sometimes rain evaporates before touching the ground. • Average precip. per year is about 1.5 cm. • The soil is shallow, rocky and has good drainage.

Hot Deserts

Cold Deserts

Have long cold winters with snow and some rain and sometimes in the summer. • Average temperatures are from -2-4 C in the winter • Average temperatures are from 21-26 C in the summer • The soil is heavy, salty and has good drainage so that most of the salt is leached out. • Average precipitation is about 15-26 cm. per year

Coastal Deserts

Have cool winters and long warm summers. • Temperatures range from 13-24 C in the summer • Temperatures drop as low as -5 C in the winter • The soil is some what salty and has good drainage. • Precipitation is about 8-13 cm. per year

Semiarid Desert

• The winters are cool and the summers are long and dry. • Average temperatures range from 21-27 C • Evening temperatures are around 10 C • Cool nights help the plants and animal by lowering moisture that is lost by sweating and breathing during the day. • Average precipitation is about 2-4 cm. per year • The soil is sandy, has pebbles, gravel is low in salt and has good drainage

Wind Action

Wind can be an important agent of erosion and transportation of fine sediments in desert regionsLarge daily temperature and pressure differences lead to strong winds in desert regions

Desert features

Erosional features Depositional features

Erosional features

Desert pavement:thin surface layer of closely-packed pebbles

Ventifacts: rock fragments with flat, wind-abraded surfaces

Blowout: depression on the land surface caused by wind erosion.

Depositional features

• Loess• Sand dunes

Loess

Wind can deposit thick layers of silt and clay-sized sediments to form loess deposits

Sand dunes

Mounds of loose sand grains heaped up by the wind.Composition of sand depends on;Sand sourceChemical weatheringQuartz, feldspar and calcite are generally more abundant.Well sorted and rounded.

How a dune forms

• A sand dune forms with a gentle upwind slope and a steeper slip face on the downwind side.

• Sand eroded from the upwind side is deposited on the slip face, forming cross-beds.

4 types of dunes:

1. Barchan2. Transverse3. Parabolic4. Longitudinal

Barchan: crescent shaped dune convex in the upwind direction

Transverse: relatively straight, elongate dune oriented perpendicular to the wind direction

Parabolic: similar to a barchan dune except that it is convex in the downwind direction.

longitudinal: symmetrical ridge that forms parallel to the wind direction