Simply put, sand dunes move because of the wind. When the wind reaches about 15 mph, it's powerful enough to pick up grains of sand. These grains bounce around and strike each other, causing them to move. Eventually they deposit against an object and build into a dune. When a dune grows large enough, it will collapse under its own weight and sand will fall down the dune's slip face. The sand will then begin to pile up in this new spot and grow again until the process repeats itself. Over time the dune can be seen to roll along. Dunes can pick up so much sand along the way that they have been known to creep up on a village and envelop the whole thing.
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