WIND AND EOLIAN PROCESSES.

The movement of the atmosphere in response to pressure differences produces wind. Wind is a geomorphic agent of erosion, transportation, and deposition. Eolian processes modify and move sand accumulations along coastal beaches and deserts. Wind’s ability to move materials is small compared with that of water and ice.

EOLIAN EROSION AND RESULTING LANDFORMS.

 

Two principal wind-erosion processes are deflation, the removal and lifting of individual loose particles by the force of the air, and abrasion, the “sandblasting” of rock surfaces with particles captured in the air. Fine materials are eroded by wind deflation and moving water, leaving behind concentrations of pebbles and gravel called desert pavement. Wherever wind encounters loose sediment, deflation may remove enough material to form basins. Called blowout depressions, they range from small indentations less than a meter wide up to areas hundreds of meters wide and many meters deep. Rocks that bear evidence of eolian erosion are called ventifacts. On a larger scale, deflation and abrasion are capable of streamlining rock structures, leaving behind distinctive rock formations or elongated ridges called yardangs.

EOLIAN TRANSPORTATION (saltation and surface creep).

 

Wind exerts a drag or frictional pull on surface particles until they become airborne. Only the finest dust particles travel significant distances, so the finer material suspended in a dust storm is lifted much higher than the coarser particles of a sand storm. Saltating particles crash into other particles, knocking them both loose and forward. The motion called surface creep slides and rolls particles too large for saltation.

EOLIAN DEPOSITIONAL LANDFORMS.

 

In arid and semiarid climates and along some coastlines where sand is available dunes accumulate. A dune is a wind-sculpted accumulation of sand. An extensive area of dunes, such as that found in North Africa, is characteristic of an erg desert, or sand sea. When saltating sand grains encounter small patches of sand, their kinetic energy (motion) is dissipated and they start to accumulate into a dune. As height increases above 30 cm (12 in.) a steeply sloping slipface on the lee side and characteristic dune features are formed. Dune forms are broadly classified as crescentic (barchan), linear (enlongated), and star.

LOESS DEPOSITS.

 

Eolian transported materials contribute to soil formation in distant places. Windblown loess deposits occur worldwide and can develop into good agricultural soils. These fine-grained clays and silts are moved by the wind many kilometers, where they are redeposited in unstratified, homogeneous deposits. The binding strength of loess causes it to weather and erode in steep bluffs, or vertical faces. Significant accumulations throughout the Mississippi and Missouri valleys form continuous deposits 15–30 m (50–100 ft) thick. Loess deposits also occur in eastern Washington State, Idaho, much of Ukraine, central Europe, China, the Pampas–Patagonia regions of Argentina, and lowland New Zealand.

 

DESERT LANDSCAPES

 

Dry and semiarid climates occupy about 35% of earth’s land surface. The spatial distribution of these dry lands is related to subtropical high-pressure cells between 15° and 35° N and S, to rain shadows on the lee side of mountain ranges, or to areas at great distance from moisture-bearing air masses, such as central Asia. Water events are rare, yet running water is still the major erosional agent in deserts. Precipitation events may be rare, but when they do occur, a dry streambed may fill with a torrent called a flash flood. Depending on the region, such a dry streambed is known as a wash, an arroyo (Spanish), or a wadi (Arabic). As runoff water evaporates, salt crusts may be left behind on the desert floor. An intermittently wet and dry low area in a region of closed drainage is called a playa, site of an ephemeral lake when water is present.

 

In arid climates, a prominent landform is the alluvial fan at the mouth of a canyon where it exits into a valley. The fan is produced by flowing water that abruptly loses velocity as it leaves the constricted channel of the canyon and deposits a layer of sediment along the mountain block. A continuous apron, or bajada, may form if individual alluvial fans coalesce. In a dry region, weak surface materials may weather to a complex, rugged low topography, called a badland. A province is a large region that is characterized by several geologic or physiographic traits. The Basin and Range Province of the western United States consists of alternating basins and mountain ranges. A slope-and-basin area between the crests of two adjacent ridges in a dry region of interior drainage is termed a bolson. Desertification is the process that leads to unwanted expansion of the Earth’s desert lands.