Question

about sand dunes in 200 words

Answer 1

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Sand dune, any accumulation of sand grains shaped into a mound or ridge by the windunder the influence of gravity. Sand dunes are comparable to other forms that appear when a fluid moves over a loose bed, such as subaqueous “dunes” on the beds of rivers and tidal estuaries and sand waves on the continental shelves beneath shallow seas. Dunes are found wherever loose sand is windblown: in deserts, on beaches, and even on some eroded and abandoned farm fields in semiarid regions, such as northwest India and parts of the southwestern United States. Images of Marsreturned by the U.S. Mariner 9 and Viking spacecrafts have shown that dunes are widely distributed on that planet both in craters and in a sand seasurrounding the north polar ice cap.

During the past two million years or so the conditions of very low rainfall under which true dunes form expanded beyond the margins of the Sahara and other present-day arid regions into areas that are now more humid. The best evidence for these changes is the presence of sand seas that are immobilized by vegetation. Dunes formed under similar climates in the geologic past and at certain times occupied deserts as extensive as modern ones. Rocks formed by the solidification of ancient sand seas occur, for example, in the walls of the Grand Canyon in the southwestern United States, in the West Midlands of England, and in southern Brazil.
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Answer 2

Answer:

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