The diagram illustrates the three main stages involved in the formation of sand dunes, showing how wind, dry sand, and patches of wet sand interact over time.
Overall, sand dunes develop gradually as wind transports loose sand particles, which then accumulate on small areas of wet sand and eventually build up into larger, stable dune structures. The key factor enabling dune formation is the presence of wet sand patches that serve as anchors for accumulating particles.
In the first stage, wind blows across a flat surface covered mostly with dry sand but containing scattered patches of wet sand. As the wind moves sand particles across this surface, some of these particles begin to settle on the wet areas because the moisture helps them adhere.
During the second stage, these adhered particles accumulate further, forming small mounds on the wet patches. Although wind continues to move across the surface, the mounds become resistant to displacement, allowing additional sand to build up on them.
In the final stage, the mounds grow into fully formed dunes as more sand is deposited. The wind continues to carry particles over the crests of the dunes, leading to gradual enlargement. Meanwhile, small ripples appear on surrounding dry sand, indicating ongoing wind activity.