Part of the show Naked Science Q & A and the World of Chemistry
Connor in Tillingham asked:
Where does all the sand in the Sahara desert come from?
Sand is tiny fragments of rock. When rock wears down you get smaller bits of rock, or pebbles, and when they wear down you get even smaller bits and eventually you get down to sand, or silica. The reason it ends up washing up on the beach is that the sea or wind can move sand around very easily while rocks are more likely to stay put. This separates things by size, and the sand ends up on the beach and the rocks end up on the seabed. There were lots of sand stones in the Sahara which have weathered and broken down over time from rain, and sun and wind. This has produced accumulations of sand which have built up over time to produce this massive desert. Rocks are made up of lots of different things other than silica, or quartz. But the silica is the toughest material which is why it gets left over after a lots of weathering, when everything else has dissolved or turned to dust.