These are isolated, pyramidal, somewhat irregularly shaped to symmetrical mounds of sand with three or more arms radiating from a central high point. The arms can vary in length, width, number, and shape, but each has a slip face. These dunes frequently have an approximate radial symmetry, i.e., in plan view the symmetry of the pattern is oriented around a point. The simple forms have smoothly curvilinear arms, usually three, and because they have a high degree of radial symmetry, they resemble a pinwheel. Compound forms usually have more arms, some sinusoidal, and are less regular with less radial symmetry. In complex star forms, the basal mound shape is also less regular, at places domelike, and overall symmetry is poor because the arms are also irregular: some may be shorter and thicker than others, or they may be elongated and extend downwind in linear or barchanoid dunes.
Star dunes also occur as secondary elements on top of, or in combination with, other dunes, especially complex linear or crescentic ridges. In these cases, star dunes perhaps signify a change in effective wind strength or direction (or both) since the basal (Pleistocene?) dunes were formed. Other star dunes occur in chains, like beads on a string, or in fields of isolated but almost regularly spaced star shapes.
We do not yet know the details of the wind regime and the sequence of events that are needed to develop and maintain these dunes, either as individuals or in groups. There is a paucity of observations and measurements, and therefore the processes that form these dunes have only been inferred from their shapes. They appear to result from winds that blow from several opposing directions, either as a result of seasonal shifts, or where secondary windflow patterns are produced by topographic barriers. Such patterns may occur in valleys, adjacent to mountain fronts, or in topographic basins where funneling effects can interact with regional winds to produce multidirectional winds. For whatever reasons, as the directions of the sand-moving winds shift around the compass, the end result is a constant shepherding of the sand back to a central point of deposition. Consequently, more than other dune types, isolated star dunes grow mostly upward; it is not unusual for them to reach heights of 200 to 300 m in the great deserts of the world and to represent true sand mountains. Their forward or lateral rates of migration, however, appear to be very low, probably only centimeters per year, and their interdunal areas tend to be swept clear of loose sand except for rippled aprons on the dune margins. No particular side of a star dune thus can be considered its "lee" or "upwind" slope, except in a temporary or local sense.
The grain size of these loose, well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Shelter from wind in a star dune field may depend on temporal or local wind conditions and cannot be predicted from dune shape alone. On the other hand, lateral migration of dune sand should not be a problem.
Trafficability depends on whether the star dunes are isolated or massed, as is common in valleys, or linked in aligned chains. Movement through fields of isolated star dunes on broad, open plains is easy, because interdunal plains are generally swept clear of loose sand except where rippled (see ripples, sand and ripples, granule). These interdunal spaces are continuous across the star dune field, generally either in a down-field or cross field direction. Star dunes in chains, however, have difficult trafficability characteristics similar to those of massive linear dunes (Linear/Seif). Experiences in Peru, Iran, and the southwestern U.S. indicate that fields of star dunes that are massed or in chains, i.e., not isolated, are not crossable in any practical application of the term.
(common names are in bold) Pyramidal dunes, oghurd, draa, uruq (some), demkha (some), sand mountains.
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