Sand dunes are massive mounds of sand sculpted by wind, yet they possess a surprising mobility that allows them to traverse vast desert landscapes. These geological formations, which can reach heights of hundreds of feet, are constantly moving structures, not static features. Dune migration represents a continuous, large-scale transfer of material driven by the simplest interaction between air and granular particles. This movement is the result of billions of tiny, sequential displacements of individual sand grains.
How Wind Moves Sand
The fundamental reason dunes migrate lies in how wind interacts with the desert surface. Wind transfers its kinetic energy to the sand through three distinct modes of transport that occur once the wind velocity exceeds a certain threshold. The largest sand particles, typically those greater than 0.24 millimeters in diameter, move by a process called creep. Creep involves these heavy grains slowly sliding or rolling along the surface, nudged by the force of the wind and the impact of other moving grains.
The most dominant mode of sand transport is saltation, accounting for approximately 50 to 70 percent of all sand movement. This process involves medium-sized sand grains (0.15 to 0.24 millimeters) being lifted into the air in a short, ballistic trajectory. When a saltating grain falls back to the surface, its impact dislodges other stationary grains, propelling them into the air in a chain reaction. This transport is a series of low-level hops that effectively moves a large volume of sand across the ground.
Only the finest particles, typically smaller than 0.15 millimeters, are light enough to be fully lifted and carried high above the desert surface. This third mode is known as suspension, where particles are fully entrained in turbulent air currents for long distances. While suspension is responsible for dust storms, it contributes the least to the bulk movement of the sand dune itself. The combined action of creep and saltation provides the constant supply of sand necessary to fuel the advancement of the entire dune structure.
The Process of Dune Migration
The continuous movement of individual grains translates into the migration of the massive dune structure through a cycle of erosion and deposition. A migrating dune is defined by its asymmetry, possessing a gentle windward slope (stoss side) and a steep leeward slope (slip face). On the stoss side, the wind pushes sand grains up the slope, which typically has a shallow angle of only 2 to 10 degrees.
As sand grains travel up the gentle windward slope by saltation and creep, they reach the dune’s crest, or brink, where wind velocity rapidly decreases. The sudden loss of speed causes the airborne sand to drop out of the air stream and accumulate. Once the buildup of sand at the brink exceeds the angle of repose—the maximum angle at which loose sand can remain stable (approximately 32 to 33 degrees)—a small avalanche occurs.
This cascading sand flows down the steep slip face, creating a layer of freshly deposited sand at the base of the leeward slope. The dune structure does not move as a solid object; rather, it advances downwind as material is continuously eroded from the front and deposited at the back. With each small avalanche on the slip face, the dune profile shifts forward, leaving behind a characteristic cross-bedded layering preserved deep within the desert floor.
What Controls Speed and Direction
The rate and path of dune migration are not uniform but are controlled by external environmental factors and internal dune properties. Wind is the primary driving force, and its characteristics are the most influential variable in determining migration speed. Dune movement requires the wind to consistently exceed the fluid threshold velocity, and speed is directly proportional to the frequency and intensity of these sand-moving winds.
The size of the dune is inversely related to its speed; smaller dunes are typically faster-moving than larger ones. For example, small to medium-sized dunes can migrate at an average annual speed of over 13 meters per year, while larger dunes in the same region move around 11 meters per year. This occurs because a smaller volume of sand requires less time to be entirely transported from the stoss slope to the slip face.
The availability of sand supply and the variability of the wind regime dictate the ultimate shape and direction of the dune’s path. Dunes formed in areas with a limited sand supply and a single, consistent wind direction tend to form crescent shapes that migrate rapidly. Factors that stabilize the sand, such as increased soil moisture or the presence of vegetation, can suppress migration speeds. Even small amounts of plant cover can anchor the sand, effectively slowing or halting the migration process.