Lecture 16 - Deserts

Definition of Desert A desert is an area with less than 25 cm (10 inches) of annual precipitation aridity index = potential evaporation/precipitation greater than 4.0 Deserts may be cold, temperate or hot. All major continents have one type of desert or the other.

Wind-Swept Sand Dunes

Desert Life Forms (Roadrunner)

Desert Life Forms (Lizard)

Types of Deserts Desert Types - Subtropical Desert – 30 o Latitude - Deserts on Leeward side of major Mountain ranges - Interior Deserts- center of continents far from ocean - Coastal desert- prevailing onshore wind cooled by cold ocean current - Polar deserts- extremely cold and dry

The major wind cells Coriolis “turns” them

Subtropical Deserts +/- 30 o latitude

A Rain-Shadow Desert

Ocean-Current Desert

Also Interior and Polar Deserts

Desert Landscape (Features) Weathering and desert streams create Desert features Weathering in Desert is mostly mechanical - A little chemical weathering produces manganese and iron-oxide stains, called desert vanish Stream Erosion - Arroyo- channel with water during periods of high discharge but dry most part of the year - Pediments- large-scale gently inclined surfaces - Inselberg- steep-sided knob of durable rock - Playa- dry lake bed

Chemical Weathering in Deserts

Desert Landforms Produced By Water

Uluru (Ayers Rock) Inselberg

Playas

Playas (cont'd)

Playas (cont'd) – A Playa in Death Valley, California

Swimmers in hypersaline Dead Sea Evaporite deposits indicate a dry climate in the geologic record

Water-Carved Canyon

Work of Winds Erosion by Wind - Deflation- wind removes finer particles from the surface - Desert pavement- layer of pebbles left behind after deflation - Abrasion- sand blasting - Ventifacts- wind-shaped stones with sharp- edge faces - Yardangs- streamlined desert ridges

Blowout Caused by Deflation

Desert Pavements

Desert Pavements (cont'd)

Desert Pavements (cont'd) – Source: Martin Miller These make good landing strips

Desert Varnish slows infiltration Causes Flash Floods

Remnants of Wind Abrasion in addition to occasional flash flood erosion (surface grains are frosted)

Origin of Ventifacts

Origin of Ventifacts (cont’d) Wind

Yardangs, White Desert, Egypt

Transport By Wind No dissolved load Suspended Load- most consist of dust (silt, clay, pollen, bacteria, salt crystals, etc.) Bed Load- sediments moved along or near the ground –Rolling or saltation- bed loads lifted off the ground momentarily due to force of collision with other grains

Transport of Wind-Borne Sediment Suspended Load Transports Sahara sediment to Caribbean and Amazon Rain Forest

Deposition of Dunes Reduced wind velocity results in sediments deposition Dunes are hills of loose wind-born sand - Size, shape, and orientation of dune are determined by available sand, vegetation, and wind

Beach Sand Dunes

Deposition of Wind’s Bed Load Rain – Shadow Desert in Lee Of Mountains

Large Scale Dunes (Gobi Desert)

Dune Migration Just like ripples in a stream

Dune Migration (cont'd)

Deposition and Dune Types Dune Types - Transverse- ridges that are perpendicular to prevailing wind direction - Longitudinal- ridges that are parallel to prevailing wind direction - Barchans- crescent-shaped with horns pointing downwind - Horseshoe (Parabolic)- crescent-shaped with horns pointing upwind - Star- winds from three or more directions

Transverse Dunes

Longitudinal Dunes

Barchan Dunes

Barchan Dunes in Baja California

Parabolic Dunes

Star Dunes

Lithified Sand Dunes (Jurassic Navajo Sandstone)

Loess Loess formed by windblown deposits of glacial outwash silt Loess from the Columbia River Basin

Desertification Desertification- invasion of desert conditions into formerly non-desert areas Drought and overpopulation are main causes Signs - Lowering of water table - Marked reduction of water supply - Increased salinity in water and soil - Progressive destruction of native vegetation - Accelerated soil erosion

Map of the Sahel

End of Lecture 16