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Dune collisions [5] [6] and changes in wind direction spawn new barchans from the horns of the old ones and govern the size distribution of a given field. [7] As barchan dunes migrate, smaller dunes outpace larger dunes, catching-up the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side.
Most parabolic dunes do not reach heights higher than a few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms.
Two mountain streams—Medano and Sand Creeks—also capture sand from the mountainside of the dune field and carry it around the dunes and back to the valley floor. The creeks then disappear into the sand sheet, and the sand blows back into the dune field. Barchan and transverse dunes form near these creeks. The combination of opposing winds ...
Transverse dunes, also called barchan, form when there is a unidirectional (unimodal) wind regime. [8] A third type of dune, the star dune, is typically formed in multidirectional (multimodal) wind regimes. [8] The presence of each of these dune forms, suggests the wind regime that produced it. [9] Linear and star dunes are rare on Mars. [9]
The dunes consist of almost pure quartz grains with an average diameter of less than 0.22 mm, which is fairly uniform for all of the grains. Dune types include coppice, wind-shadow, Akle, Barchan, and parabolic. Winds from the southeast in the summer, from the northwest in the winter, and the southwest during spring, influence dune shape.
Olympia Undae contains a variety of dune forms and wind-related depositional features, including sand sheets, [11] transverse dunes, simple barchan dunes, mega-barchans, and complex barchanoid ridges. [12] All of these dune types occur on Earth too. Barchans are isolated, crescent-shaped dunes with horns that point downwind. They occur is areas ...
For example, there are ergs or fields of linear dunes, of crescentic dunes, of star dunes, and of parabolic dunes, and these dune arrays tend to have consistent orientations and sizes. [10] [11] By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges.
Barchan: A type of dune that forms as a result of low sediment supply in regions where wind blows in one primary direction. This causes the dune to have a concave shape, with the tails facing downwind. [8] Transverse dunes and Seif dunes are some alternative examples of the effect of unidirectional air flow on dune formation. Barchan dune ...