How do sand dunes migrate across a desert?
Quick Answer
Wind pushes sand grains up the gentle windward side of a dune, and they avalanche down the steep slip face, moving the dune forward. Some dunes can shift by several meters a year.
The Full Story
Sand dunes move because wind lifts and rolls grains up the windward slope until they reach the crest, where they tumble down the lee side. This process, called saltation and avalanching, gradually shifts the whole dune in the direction of the prevailing wind. The speed of movement depends on wind strength, sand supply, and vegetation, so some dunes crawl only centimeters per year while others move several meters annually. In places like the Namib and Sahara, dune fields can form large migrating landscapes over long periods. The steep slip face is typically near the angle of repose for dry sand, about 30 to 34 degrees.
Key Facts
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"Watch a desert mountain of sand slowly walk across the land."
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