BARCHAN SAND DUNES

Barchans are among the most distinctive sand dunes because of their crescent shape. In order to form, they need winds of moderate velocity that blow in a constant direction. They can be found in deserts of Earth, and they occur also on planet Mars. 

Aerial view of a single barchan dune, Mojave Desert, California. Dune length is approximately 100 m.

Three chains of barchans dunes on the surface of Mars (Credit:NASA/JPL-Caltech/University of Arizona). Each dune chain stretches many kilometers.

As shown in the above sketch, barchan dunes have a crescentic ground plan: a convex side (the windward side) faces the wind and a concave slip face faces downwind (the leeward side). The slip face  has a slope angle of approximately 34° (with a range between 30 and 35°). The sand blows over the apex of the convex side of the dune and  avalanches down the concave slip face. At each far end of the slip face, there is a “horn,” which always points downwind. Thus, in an aerial photograph, it is easy to determine which way the dominant wind direction is/was by seeing which way the “horns” point toward.

The “horns” always point downwind. Barchans move horizontally, at the rate of from 1 to 100 meters/year.

With shifts in the wind direction, one horn can become larger than the other one. Barchan dunes are typically isolated dunes, but they can occur in a chainlike fashion (like those shown above, in the Mars image). If one horn becomes much more elongate than the other, then a barchan dune can transition into a linear-shaped dune.

Vegetation stabilizes (stops) the horizontal movement of barchans. 

The Namib Desert in Namibia of southwest Africa has some well developed dune fields with abundant barchans (Google Earth, 2023). 

Examples of barchan dunes in the coastal region of the Namib Desert (Google Earth, 2023). As viewed, north is at the top of the image; the winds that formed these dunes blew to the north. 

References Consulted:

Bates, R.L. and J. A. Jackson. 1984. Dictonary of geological terms. Anchor Press/Doubleday, Garden City, New York. 571 pp.

Hamblin, W.K. and E.H. Christiansen. 1995. Earth’s dynamic systems. Prentice-Hall, Inc. New Jersey. 740 pp.

Wikipedia.org (2023)