What are the characteristics of dunes formed by wind action?

Dunes formed by wind action are characterised by their shape, size, orientation, and the type of sand they contain.

Wind-formed dunes, also known as aeolian dunes, are fascinating geological features that are primarily shaped by the action of wind on loose sand. The wind's strength and direction, the sand's availability, and the presence of vegetation all play a role in determining the dune's characteristics.

The shape of a dune can vary greatly, from simple mounds to complex ridges and star shapes. The most common types include crescentic, linear, star, dome, and parabolic dunes. Crescentic dunes, also known as barchans, are crescent-shaped mounds with the points or 'horns' facing downwind. Linear dunes are long, straight or slightly sinuous sand ridges typically much longer than they are wide. Star dunes are pyramid-shaped and have several arms radiating from a central point. Dome dunes are circular or oval in plan view and do not have a slip face, while parabolic dunes are U-shaped mounds of sand with convex noses trailed by elongated arms.

The size of a dune can range from less than a metre to several tens of metres in height. The size is largely determined by the amount of sand available and the wind's strength and consistency. Larger dunes are typically found in desert environments where sand is abundant and winds are strong and persistent.

The orientation of a dune is determined by the prevailing wind direction. The windward side of the dune is typically a gentle slope, while the leeward side is a steep slip face where sand is deposited.

Lastly, the type of sand in a dune can tell us about its origin. For example, quartz-rich sand is typically associated with beach environments, while gypsum or limestone sand indicates a desert origin. The sand's grain size and sorting can also provide clues about the wind's strength and the dune's maturity.