288

These are ripple marks in beach sand, made by the wind. Similar ripples can be seen in deserts, and even in large playground sandboxes. What causes the ripples to be so evenly spaced?

Answer:

Imagine starting with nice, flat sand, smoothed by the last tide. Even though it is flat, there will be some irregularities, small stones, shells, etc. If the wind is blowing steadily from one direction, the sand will begin to pile up in front of the irregularities, until it can blow over the top.

At that point, the sand grains are blown over the top, launching them into the air. How far they travel depends on their size, density, and the speed of the wind. Since water and wind both tend to deposit well sorted sediments, most of the grains are usually very similar in size, so with a constant wind, they will travel about the same distance.

When they land, they often stop, forming the next crest. At the same time, they dislodge more sand grains, sending them jumping to form the next crest. Recent studies have shown that the sand grains build up a negative, electrostatic charge, which causes them to repel slightly. That lets the arriving grains bump more grains away to the next crest.

Non-subscriber