Deserts

Definition 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)

Location Dry regions cover 30 percent of Earth’s land surface Two climatic types are commonly recognized Desert or arid Steppe or semiarid

Location of Deserts

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

Location 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

Location 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

Location 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 Deserts

Weathering & 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 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 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 in the desert

The same stream channel following heavy rainfall

Weathering & 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 & Erosion Wind erosion Mechanisms of transport Bedload Saltation – skipping and bouncing along the surface Suspended load In the air as duststorms

Weathering & Erosion Wind erosion Mechanisms of wind erosion Deflation Lifting of loose material Produces Blowouts Desert pavement

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 Environments Where does all of the eroded sediment go? Water Deposits Talus Aprons Alluvial Fans Bajada Playas and Salt Lakes

Depositional 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 Environments

Depositional Environments Where does all of the eroded sediment go? Wind deposits Dunes Types of sand dunes

Depositional 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 Landforms Butte Chimney cap rock Plateau Mesa Alluvial fan sandstone Alluvial fan Pedestal Rocky desert pavement Playa lake shale Dunes

Desert Landforms Rocky desert areas change over time

Desert Landforms Rocky desert areas change over time

Desert Landforms

Basin and Range: the evolution 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 landscape

Landscape evolution in a mountainous desert – early stage

Landscape evolution in a mountainous desert – middle stage

Landscape evolution in a mountainous desert – late stage

Inselbergs in Southern California

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