Presentation on theme: "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."— Presentation transcript:
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.
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
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
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
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
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
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
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