How deep is the sand of the Sahara desert and what is directly beneath it? The Sahara is subject to considerable wind erosion and its sand is found thousands of miles away. Is it replenished by some means?
Not all the Sahara is sandy, but wherever wind-blown sand settles for any reason, more is likely to collect in the same place. Sand accordingly forms wind-driven dunes, whose behaviour is astonishingly complex.
Surface sand seldom piles higher than a couple of hundred metres above underlying earth or bedrock, except by filling ancient valleys or lakes. Such deep sands and spongy sandstone form important groundwater reservoirs. And yes, sand does form and re-form constantly as water erosion, frost and wind-driven particles flake grains off rocks. Conversely, deep moist layers of sand become cemented into sandstone, which in turn may go through the same cycle after millions of years.
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Far deeper sand occurs in submarine detritus fans which build up at the mouths of rivers. Even more intriguingly, the Mediterranean has dried up repeatedly in the last tens of millions of years. Each time that happened, rivers flowing into the basin eroded their fans into massive canyons, which silted up again whenever the sea returned. The bed of the lower reaches of the Nile consists of silt, more or less compacted, kilometres deep, turning it into an underworld canyon that has dwarfed the Grand Canyon in the past.
Jon Richfield, Somerset West, South Africa
At more than 9 million square kilometres, the Sahara is the world鈥檚 largest desert but it has no uniform topography. About 15 per cent of the area is covered by sand dunes, 70 per cent is made up of stone deserts of denuded rock and coarse gravel, while the rest is oases and mountain ranges.
Beneath the sand dunes lies rock of varying types. In Algeria and Libya, deposits of oil and gas have also been discovered, although inaccessibility has hampered their exploitation. The same forces that remove sand from the Sahara are also largely responsible for it being replenished. The wind not only sweeps sand from one area, but causes erosion of rocks in others which leads to it being replaced.
Ian Smith, London, UK