Q1.
What is the shape of a sand dune? While mountains are made of rock and their shape is fixed, why do you think sand dunes also have a similar shape, even though they are made of sand?
Answer
A sand dune is a hill-like heap of sand — a long gentle slope on the side the wind comes from, a sharp crest at the top, and a short steep slope on the sheltered side. Seen from above, many dunes are crescent-shaped.
Why sand takes a hill shape at all: the wind does the work that rock and time do in a mountain.
- Wind rolls and bounces grains of sand up the gentle windward slope.
- At the crest the grains tumble over into the still air behind and settle on the steep sheltered side.
- Sand can only pile up to a certain steepness (about 30–34°) before it slides. So the dune builds itself into a stable hill shape and stops there.
The big difference: a mountain's shape is fixed — it is solid rock and changes only over millions of years. A dune's shape is repeated, not fixed. The wind constantly removes grains from one side and adds them to the other, so the whole dune slowly moves across the desert while keeping the same outline. It is like a queue of people: the shape of the queue stays the same even though the people in it keep changing.
Try this: blow gently and steadily across a thin layer of dry sand on a tray. A tiny dune forms, with a gentle slope facing you and a steep drop behind — the same shape as a 150-metre dune in the Thar.