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