Where did all the sand in the sahara desert come from?

The sand is primarily derived from weathering of Cretaceous sandstones in North Africa. When these sandstones were deposited in the Cretaceous, the area where they are now was a shallow sea. The original source of the sand was the large mountain ranges that still exist in the central part of the Sahara. These mountains are volcanic and intrusive, and the granite rock weathers out to leave behind quartz sand grains that are carried by rivers to the sea. These sand deposits eventually formed into sandstone, and as they were uplifted began to weather and break down into sand again.

Only about 10% of the Sahara is actually covered by sand, and parts of the Sahara are in fact covered by soil. More than half of the area comprises soils known as yermosols, with shallow profiles over gravel or pebble beds. These soils have been developing over the past 50 million years.