Sand dunes are large, natural accumulations of loose sand that are sculpted by the wind into various shapes and sizes. These geological features are common in arid desert environments, along sandy coastlines, and even on the beds of large rivers or lakes. They represent depositional landforms, meaning they are built up by the material the wind carries rather than carved out by its erosive power. The study of how wind shapes the land and moves sediment, known as aeolian geomorphology, helps explain the formation of these migrating sand mounds.
Prerequisites for Dune Formation
The creation of a sand dune requires the combination of three elements. First, there must be an abundant source of loose, dry sand to serve as the building material. This sand is often sourced from dried-up riverbeds, ancient lake bottoms, or directly from coastal beaches where tides allow the material to dry out.
Second, a persistent and strong wind is necessary to act as the energy source for transportation. The wind must reach a sufficient velocity to lift and move the individual sand grains across the surface. Finally, an obstruction is required to initiate the sand’s deposition, causing the wind speed to drop in a localized area. This obstruction can be as minor as a small pebble, a clump of stabilizing vegetation, or a slight change in the local topography. Once the wind slows down, the carried sand settles, marking the beginning of a dune.
The Mechanics of Sand Movement
Wind transports sand grains through a process called aeolian transport, which occurs in three distinct ways depending primarily on the size of the particle. The most common form of movement is saltation, which accounts for the majority of sand transport. This involves medium-sized sand grains bouncing along the ground in short hops or skips, rarely rising more than a centimeter above the surface.
These saltating grains strike the surface upon landing, dislodging other particles and causing a chain reaction that continues the movement. The impact of the saltating particles powers the second method, known as surface creep. Surface creep involves the rolling or sliding of the largest sand grains that are too heavy to be lifted into the air. These heavier particles are pushed forward by the kinetic energy transferred from the smaller, bouncing grains.
The third mode of transport is suspension, which affects the finest dust and silt particles less than 0.2 millimeters in diameter. These very light particles are lifted and carried high into the atmosphere by upward air currents, sometimes traveling vast distances. While suspension contributes little to the actual building of a dune structure, it is a significant component of wind erosion.
The Process of Accumulation and Growth
Once the wind carrying the sand encounters an obstruction, its velocity decreases, causing the transported sediment to be deposited. Sand accumulates on the upwind side, known as the stoss slope, which tends to have a gentle incline. As more sand is continually blown up this gentle slope, it eventually reaches the dune’s crest.
The sand then spills over the crest and falls onto the steeper, downwind side, which is called the slip face. This deposition continues until the slope of the slip face reaches a stable angle, known as the angle of repose.
When the sand accumulation causes the slope to exceed the angle of repose, the sand avalanches downward in small landslides, reforming the stable angle. This continuous cycle of wind erosion on the windward side, transport over the crest, and deposition on the leeward side causes the entire dune structure to migrate downwind. The dune essentially “rolls” along, maintaining its characteristic asymmetric shape as it moves across the landscape.
Primary Dune Classifications
The shape and size of a sand dune are determined by the combination of sand supply and the variability of the wind direction.
Barchan Dunes
Barchan dunes are crescent-shaped, characterized by horns that point downwind. They form in areas with a limited sand supply and a wind blowing predominantly from a single direction. They are known for migrating quickly across the desert surface.
Transverse Dunes
Transverse dunes form as long, linear ridges that are oriented perpendicular to the main wind direction. These dunes develop in environments where the sand supply is abundant and the wind remains unidirectional. They often form when multiple barchan dunes merge laterally.
Star Dunes
The most complex formations are Star dunes, which are pyramidal mounds with three or more arms radiating from a central peak. Star dunes require a substantial supply of sand and are created in regions where the wind direction shifts frequently throughout the year. Unlike other types that migrate horizontally, star dunes tend to grow upward, often becoming the tallest dunes in a given area.