Question

there is a vast sea of sand in western part of sahara .why

Answer 1

The theory states that the planet Mars entered into hundreds of catastrophic close encounters with earth during historical times. During these encounters an incandescent molten Mars internally convulsed and ejected immeasurable quantities of vaporised rock, volatiles, dust and debris out into space – a natural by-product of planetary chaos. Vast swaths of rock vapour fell to earth (along with tons of other sedimentary material) where it condensed out of the atmosphere as tiny quartz grains. In other words, it rained sand!
As the sand fell it ‘choked’ what was previously a verdant green paradise.
The reason why sand is relatively uniform in size is because, like rain, it fell from the sky!
As the sand grains fell they were coated in a submicroscopic coating of iron rich clay drawn from moisture in earth’s atmosphere – this is where sand gets its colour. Any variations would be down to density and saturation variations in the atmosphere. Post depositional processes would of course play a part, but quartz sand has nothing to do with the absurd “billions of years of erosion” from granite rock.
Hope it helps! ^^