name the active agent of erosion in desert. explain the formation of all features made by erosion and deposition in dessert
Answers
Explanation:
Wind Erosion
Wind is a stronger erosional force in arid regions than it is in humid regions because winds are stronger. In humid areas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectively picked up and transported.
Deflation
As small particles are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement (Figure below), a surface covered by gravel-sized particles that are not easily moved by wind.
Desert pavement formed as a result of deflation
This desert pavement formed in the Mojave Desert as a result of deflation.[Figure3]
Abrasion
Particles moved by wind do the work of abrasion. As a grain strikes another grain or surface it erodes that surface. Abrasion by wind may polish natural or human-made surfaces, such as buildings. Stones that have become polished and faceted due to abrasion by sand particles are called ventifacts (Figure below).
A ventifact formed by wind abrasion
As wind blows from different direction, polished flat surfaces create a ventifact.[Figure4]
Desert Varnish
Exposed rocks in desert areas often develop a dark brown or black coating called desert varnish. Wind transports clay-sized particles that chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides (Figure below).
Petroglyphs carved into desert varnish
Ancient people carved these petroglyphs into desert varnish near Canyonlands National Park in Utah.[Figure5]
Wind Deposition
The main features deposited by wind are sand dunes. Loess are wind deposits of finer sediments.
Sand Dunes
Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes are usually made of quartz because quartz is what's left in humid areas as other minerals weather into clays. Sand dunes may be composed of calcium carbonate in tropical areas. But in deserts, sand dunes are composed of a variety of minerals because there is little weathering.
Dune sands are usually very uniform in size and shape. Larger particles are too heavy for the wind to transport by suspension and smaller particles can't be picked up. Particles are rounded, since rounded grains roll more easily than angular grains.
Sand dunes in Death Valley
This sand dune in Death Valley, California shows secondary sand ripples along its slip face.[Figure6]
For sand dunes to form there must be an abundant supply of sand and steady winds. A strong wind slows down, often over some type of obstacle, such as a rock or some vegetation, and drops its sand. As the wind moves up and over the obstacle, it increases in speed. It carries the sand grains up the gently sloping, upwind side of the dune by saltation. As the wind passes over the dune, its speed decreases. Sand cascades down the crest, forming the slip face of the dune. The slip face is steep because it is at the angle of repose for dry sand, about 34o (Figure below).
How a sand dune forms
Sand dunes slope gently in the upwind direction. Downwind, a steeper slip face forms.[Figure7]
Wind deposits dune sands layer by layer. If the wind changes directions, cross beds form. Cross beds are named for the way each layer is formed at an angle to the ground (Figure below).
Sandstone that shows crossbedding
This sandstone in Zion National Park, Utah, shows crossbedding.[Figure8]
The type of sand dune that forms depends on the amount of sand available, the character and direction of the wind, and the type of ground the sand is moving over. Dunes may be crescent-shaped, star-shaped, parabolic, linear, or barchan.