DesertsDeserts

DefinitionDefinition

• Desert:– A region so arid that it contains no permanent

streams except for those that bring water in from elsewhere, and has very sparse vegetation cover.

– NOT related to temperature!– Deserts can be

• Hot (>35 °C)

• Cold (< 20 °C)

LocationLocation

• Dry regions cover 30 percent of Earth’s land surface

• Two climatic types are commonly recognized– Desert or arid– Steppe or semiarid

Location of DesertsLocation of Deserts

Earth’s dry regions coincide with the Earth’s dry regions coincide with the subtropical high pressure belts & solar heating subtropical high pressure belts & solar heating

LocationLocation

• Types of deserts– Classified by location

• Subtropics– Low-latitude deserts

– In the vicinities of the Tropics of Cancer and Capricorn

– Areas of high pressure and sinking air that is compressed and warmed

LocationLocation

• Types of deserts– Classified by location

• Middle-latitudes– Located in the deep interiors of continents

– High mountains in the path of the prevailing winds produce a rainshadow desert

LocationLocation

• Types of deserts– Classified by environment in which they are formed

• subtropical: in the hot dry latitudes between 20 and 30°, both north and south

• rain shadow: on the landward side of coastal mountain ranges

• coastal: along coasts bordering cold ocean currents

• continental interior: deep within continents, far from major water sources

• polar: in the cold dry polar regions, both north and south

Formation of Coastal DesertsFormation of Coastal Deserts

Weathering & ErosionWeathering & Erosion

• Not as effective as in humid regions

• Mechanical weathering forms unaltered rock and mineral fragments

• Some chemical weathering does occur – caliche-rich soils – desert varnish

Weathering & Erosion Weathering & Erosion

• Water Erosion– Desert rainfall

• Rain often occurs as heavy showers

• Causes flash floods

• Poorly integrated drainage

• Most erosional work in a desert is done by running water

Weathering & Erosion Weathering & Erosion

• Water Erosion – Streams are dry most of the time

• Desert streams are said to be ephemeral – Flow only during periods of rainfall

– Different names are used for desert streams including wash, arroyo, wadi, donga, and nullah

A dry stream channel A dry stream channel in the desertin the desert

The same stream channel The same stream channel following heavy rainfallfollowing heavy rainfall

Weathering & ErosionWeathering & Erosion

• Wind erosion– Differs from that of running water in two ways

• Wind is less capable of picking up and transporting coarse materials

• Wind is not confined to channels and can spread sediment over large areas

Weathering & ErosionWeathering & Erosion

• Wind erosion– Mechanisms of transport

• Bedload– Saltation – skipping and bouncing along the surface

• Suspended load– In the air as duststorms

Weathering & Erosion Weathering & Erosion

• Wind erosion – Mechanisms of wind erosion

• Deflation– Lifting of loose material

– Produces

» Blowouts

» Desert pavement

Weathering & Erosion Weathering & Erosion

• Wind erosion – Mechanisms of wind erosion

• Abrasion– Produces ventifacts (stones with flat faces) and yardangs

(wind sculpted ridges)

– Limited in vertical extent

Depositional EnvironmentsDepositional Environments

• Where does all of the eroded sediment go?– Water Deposits

• Talus Aprons

• Alluvial Fans

• Bajada

• Playas and Salt Lakes

Depositional EnvironmentsDepositional Environments

• Where does all of the eroded sediment go?– Wind deposits

• Dunes– Mounds or ridges of sand

– Often asymmetrically shaped

– Characteristic features

» Slip face

» Cross beds

Depositional EnvironmentsDepositional Environments

Depositional EnvironmentsDepositional Environments

• Where does all of the eroded sediment go?– Wind deposits

• Dunes– Types of sand dunes

Depositional EnvironmentsDepositional Environments

• Where does all of the eroded sediment go?– Wind deposits

• Loess – Deposits of windblown silt

– Extensive blanket deposits

– Primary sources are deserts and glacial stratified drift

Desert LandformsDesert Landforms

PlateauMesa

ButteChimney cap rock

Pedestal

Playa lake

Alluvial fan

Rocky desert pavement

Dunes

shale

sandstone

Desert LandformsDesert Landforms

• Rocky desert areas change over time

Desert LandformsDesert Landforms

• Rocky desert areas change over time

Desert LandformsDesert Landforms

Basin and Range: the evolution Basin and Range: the evolution of a desert landscape of a desert landscape • Uplifted crustal blocks • Interior drainage into basins produces:

– Alluvial fans and bajadas– Playas and playa lakes

• Erosion of mountain mass causes local relief to continually diminish

• Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment filled basin

Basin and Range landscapeBasin and Range landscape

Landscape evolution in a Landscape evolution in a mountainous desert – early stagemountainous desert – early stage

Landscape evolution in a mountainous Landscape evolution in a mountainous desert – middle stagedesert – middle stage

Landscape evolution in a Landscape evolution in a mountainous desert – late stagemountainous desert – late stage

Inselbergs in Southern CaliforniaInselbergs in Southern California

~ End ~~ End ~