Why do some deserts have huge sand dunes while others are mostly rock or gravel?
Quick Answer
Sand dunes form where wind can repeatedly sort and move abundant sand-sized grains. If the area has too little sand or too much erosion, the desert may stay rocky or gravelly instead.
The Full Story
Not every desert is a sea of sand. Dunes form when wind has a steady supply of sand and enough space to accumulate it, often from dried lake beds, river deposits, or weathered bedrock. In many deserts, the dominant surface is rocky hamada or gravelly reg, because finer material has been blown away and larger fragments remain. Wind direction, local topography, and sediment supply all control whether dunes grow. That is why the Sahara, for example, contains both vast dune fields and broad rocky expanses.
Key Facts
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