Dunes

Sand dunes are the classic desert structure: picture a herd of camels and you’ll probably also imagine a dune in the background. You’ll find them anywhere there’s enough sand around that isn’t trapped by vegetation or moisture, and where the wind blows strong enough to move that sand. On Earth, dunes range in height from about 1 to 500 meters. That’s a pretty wide range!

All bedforms are created by instabilities, in which small disturbances are amplified as the wind blows over the ground. A pile of sand more than about 20 meters across affects the wind blowing above it – its height creates a disturbance in the wind that leads to an instability. Below this size, the pile of sand doesn’t affect the wind enough to create an instability, and so you’ll never see dunes smaller than about 20 meters across. Once the instability catches hold, though, the dunes can grow to be a kilometer across or more, provided there’s enough sand around and nothing stops the dune’s growth.

Large dunes are easily seen from space. Their shape and alignment are controlled by the winds that formed them, and so dunes can be used to determine the direction and relative strengths of the winds that created them. The larger the dune, the more sand it’s got in it, and so the longer it takes to respond to changes in the wind. The smallest dunes may form fresh every year, building up in one season (say, summer) and being destroyed in another (say, winter). But the biggest dunes may be thousands of years old, having been formed over a long time. These dunes tell us what wind patterns were like long ago, perhaps during the last ice age. Some dunes are buried and turn into sandstones – many of these are millions of years old and contain fossils. So not only can we tell what critters lived on those dunes when they were still being blown by the wind, we can tell what the wind patterns were like. That’s pretty awesome, if you ask me.

Ripples

Sand ripples are smaller than dunes, with widths ranging from about 1 to 20 cm. Because they are so small, a ripple doesn’t affect the wind blowing above it, and so they are not formed by the same instability that creates dunes. Ripples are a different beast altogether.