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Ltryptophan's avatar

How does a desert/beach form?

Asked by Ltryptophan (10839points) November 28th, 2011

How does the sand collect in such massive quantities? What was the Sahara desert like before it became a vast sand lot?

What’s beneath all the sand? How deep does the sand go? Tell me about the creation and settlement of sand. Thanks

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8 Answers

CaptainHarley's avatar

Over eons of time, rocks break down from the action of wind and waves and earthquakes and rain. That’s how the sand forms. The hotter the climate, the easier it is, since moisture tends to make the grains chohere. How deep is it? Usually several hundreds of feet deep. What’s beneath it? Certainly rock, plus underground streams, forgotten cities, and all the other detrius of mankind. How did it all accumulate in one place? Mostly the action of wind over long periods of time.

ETpro's avatar

@Ltryptophan I can’t add anything to @CaptainHarley‘s thorough answer. He nailed it. If interested, here is afun link and some technical info on how dunes form.

zenvelo's avatar

The same action described by @CaptainHarley coupled with wave action against the shore also creates sand that is washed to sea and then deposited on the beach. But sand on the beach is not very thick, and in some places a whole beach of sand can disappear during a storm. It then takes a number of days for the sand to “re-appear” as sand from up the coast settles.

Beach sand along a coast moves in one general direction with the prevailing current. This becomes a contentious issue when towns build breakwaters that disrupt the deposit of sand on the next beach down the coast.

thorninmud's avatar

The mechanism for the collection of sand is different for beaches and deserts, but in both cases you have the interplay between a propulsive force—wind or surf—and gravity. Smaller particles of rock are more easily moved along by wind or water; larger particles will tend to stay put.

Sand deserts form downwind from mountain ranges. The mountains are where the erosion happens that breaks up the rock in the first place, often by glaciation. Wind then starts moving all that stuff. The finer grains get transported quite far, often hundreds of miles, because they’ll keep moving even under a relatively mild wind. It takes a strong gust to move the coarser grains. Sand deserts will form in flat areas where the winds are calm enough to let even those fine grains settle down. Once sand starts to accumulate, the process accelerates because new sand gets trapped and held in place when it hits the old sand.

Beaches are formed of grains that are light enough to be held in suspension by the churning surf.

Adirondackwannabe's avatar

@thorninmud I get the downwind thing about deserts in the US. Our deserts tend to be on the east side of the mountian ranges. What about the deserts in other continents? The Sahara for example.

thorninmud's avatar

@Adirondackwannabe The Sahara is a classic example. It lies in the wind shadow of the Atlas mountains. The Atlas Mts. were glaciated in the Quaternary period, which created the ground stone that would end up in the Sahara as sand. The prevailing winds are off the Mediterranean to the north. The mountains blunt the force of the wind, creating a zone of relative calm south of the range.

So the winds rip into the range off the Mediterranean, picking up the glacial debris. The coarser stuff is dropped on the leeward flanks of the range, but the finer stuff remains airborne for many miles to the plains below and to the south, where gravity finally gets the upper hand.

asmonet's avatar

Also, topsoil can erode leaving the land barren in a process called desertification. Things die out and the desert can creep further, helped along with the sand being carried from the winds over the desert in question. And there are tons of other causes too. Some towns have constructed sand barriers, and some villages have to sweep sand out of their area daily. So there, there is another way for ya.

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