These undulating patterns of sands in the desert are typically 1 to 2 centimeters wide. However, they form quite differently from bigger dunes, another type of bedform – sand ripples are in a different league.
Ripple marks go into a process of saltation. It is the movement of particles through fluid, such as wind or water. In the desert, sand bounces across the ground. As the sand grains strike the ground, they force sand particles on the surface to recoil a few centimeters.
Particles on the smaller cavities on the ground are generally protected from these striking sand grains and remain in their position. Those that are exposed are affected more in the process. They then create ridges perpendicular to where the wind is blowing, and each mark is nearly equidistant from both sides.
Sand ripples can also be found on beaches during windy days, though they develop and disintegrate quickly, in a few minutes to a couple of hours. They are typically helpful in gauging wind directions. Their massive counterparts, the dunes, on the other hand, are useful in finding how the wind was in the last few months of years. Large ones can tell wind directions from the previous decades or millenium.
Ripple marks can be classified based on the patterns they make. Straight ripples have layers or ridges that go in the same locations, implying that there is a unidirectional flow of the wind or current. On the other hand, sinuous ripples cause wavy or curvy ridges caused by the bi-directional flow of current, like what we feel on the beach floor.
In terms of grain sets, ripples marks can be classified into mega ripples, giant ripples, or coarse-grained ripples. Some sets transport and create marks from sand-sized grains, which are typically 0.1 to 1 millimeter in diameter. Other sets contain larger grains, whose diameter can vary from millimeters and even to centimeters.
Coarse-grained ripples that have uneven sizes of grains work in the same manner sand ripples in the desert do. The difference is that larger grains tend not to be bounced off by impacting sand grains. Instead, larger and heftier grains get poked each time and are slowly transported each time the wind blows along the ground. Geologists refer to this process as creep motion or the gradual movement of particles in the steady surface flow.
If the wind or water isn’t strong enough to nudge the larger grains, they tend to stay put on the surface of the ripples. Thus, serving as a shield and preventing erosion. Some recent fieldwork showed that some ripples never move their particles at all. The larger grains sitting on the surface tend to get heftier, preventing the wind from pushing the ripple downwind.
However, if no large grains are present, desert erosion or advancement can be avoided by planting shrubs and trees with broad foliages along the edges of the sandy ground. They can serve as windbreakers and produce the first form of greenbelt, and soon enough, other rows of vegetation will follow. Thus, preventing the expansion of the desert, containing its core, and allowing us to continue being amused by its eccentric beauty.
Ripple Marks (Wikipedia)
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