Deserts & Wind: Eolian Processes

A desert is defined by dryness — typically less than 250 mm of rain per year — not by heat. Antarctica is a desert. In these dry regions, sparse vegetation leaves the ground exposed, and wind becomes the primary sculptor of the landscape.

Eolian (wind-driven) processes have three components, just like water:

• Erosion: Wind picks up loose particles (deflation) and grinds exposed rock (abrasion).
• Transport: Wind carries sediment in three ways.
• Deposition: When wind slows or encounters an obstacle, sediment settles into dunes or blanket deposits.

• Deflation: Wind lifts and removes loose, fine-grained particles, leaving behind a lag deposit of coarser pebbles and cobbles. Over time, deflation can lower the land surface, creating large, shallow depressions called deflation basins.
• Abrasion: Wind-blown sand acts like sandpaper, etching and polishing exposed rock. Features shaped by abrasion include yardangs (elongated ridges) and ventifacts (rocks with flat, wind-faceted surfaces).

• Suspension: The finest particles (silt and clay) are lifted high into the atmosphere and can travel thousands of kilometres. Saharan dust regularly reaches the Americas.
• Saltation: Sand grains bounce along the surface in short hops. This is the dominant mode of sand transport and accounts for most dune-building.
• Surface creep: Larger grains are nudged along by the impacts of saltating grains. It is slower than saltation but moves the coarsest material.

Dunes form where sand supply is abundant and wind is strong enough to move it but not so strong that it blows everything away. Four major types are controlled by wind direction and sand supply:

• Barchan dunes: Crescent-shaped dunes with horns pointing downwind. They form where sand supply is limited and wind blows from one dominant direction.
• Transverse dunes: Long ridges perpendicular to the wind, forming where sand supply is abundant and wind is unidirectional.
• Longitudinal dunes: Ridges parallel to the wind, forming where wind direction varies or where sand supply is moderate. They can stretch for tens of kilometres.
• Star dunes: Pyramidal dunes with multiple arms, forming where wind comes from several directions. They are the tallest dune type and can exceed 200 m in height.

Loess is a thick deposit of windblown silt, typically derived from deflated desert basins or glacial outwash plains. In arid regions, dry winds pick up fine silt from exposed surfaces and deposit it downwind as fertile, yellow-brown blankets. Glacial outwash is a major source in some regions (e.g. the US Midwest), but desert basins are equally important (e.g. the Chinese Loess Plateau).

Loess is agriculturally valuable because its fine texture retains water and nutrients, and its vertical cliffs (due to calcareous cementation) create distinctive topography. The Loess Plateau of China and the Midwestern United States are classic examples.