Hobbies And Interests
Crescentic Dunes
These dunes are the most common on Earth -- and on Mars as well, according to the U.S. Geological Survey. As the name suggests, they are crescent-shaped formations built by prevailing winds from a constant direction. One of the most striking crescentic dunes is the barchan, which has sweeping wings tapered downwind from the dune's high point, forming a deeply concave arc on the leeward slipface. Barchans, which commonly migrate over non-sandy flats as winds spill sand over both the slipface and the ends of the wings, are the swiftest of all dunes. Another crescentic model is the transverse dune, somewhat less defined than the barchan and often manifesting as parallel chains of curved dune-crests.
Seif Dunes
"Seif" is Arabic for "sword," which well describes these knife-edged, straight or gently sinuous sand crests, also called linear or longitudinal dunes. Usually seifs are oriented in parallel formation in large groups, a configuration that may spring from competing directions of wind. Converging seifs form Y-shaped dune complexes. Great stretches of stony or sandy flats may separate each seif, which at their largest may be several hundred feet high and dozens of miles long.
Star Dunes
Among the most striking of sand formations, star dunes form under the influence of multidirectional winds and are defined by a central summit from which curved arms radiate. Growing vertically, these dunes may be well over 1,000 feet high in places like the Badain Jaran Desert of China, and often form along the margins of sand seas, or ergs, as in portions of the Sahara and El Gran Desierto in Mexico's Pinacate country.
Parabolic Dunes
Parabolic or U-shaped dunes are similar to barchans and other crescentic sand formations except their trailing arms extend in the upwind direction, and the slipface is set on the outside of the concave crest. Vegetation usually anchors the trailing arms. These dunes may form along coastal areas and extend inland, or may stem from blowouts in interior deserts.
Dome Dunes
These simple mounds of sand, which may be circular or oval in shape, tend to lack an active slipface. They sometimes form upwind of ergs or tracts of barchans, as well as downwind of linear-dune fields.