system
Given the following post, respond as if you're commenting on Reddit:
human
Where did all the sand in the Sahara desert come from?

I've heard that around 5 thousand years ago the Sahara desert was a tropical forest, full of life. And then, it turned into the wasteland that we know today. Where did all the sand in the Sahara desert come from?
gpt
Sand is weathered rock. The sand in the Sahara comes from the bedrock (most of the Sahara actually consists of exposed bedrock): as the bedrock is altered by the elements and by gravity, its primary minerals (e.g., feldspars, quartz, micas) are slowly transformed into secondary minerals (clays and carbonates) that can be more or less dissolved or transported by the wind, leaving behind grains of primary minerals (the sand). 

By the way, the Sahara is not a wasteland! It is rather teeming with life.