Geomorphology

The word "Geomorphology" is derived from the Greek words γη, ge, "earth"; μορφή, morfé, "form"; and λόγος, logos, "knowledge". A simple definition is "The form of the earth, the general configuration of its surface, and the changes that take place in the evolution of land forms. Put another way, "Geomorphology takes into account the landforms and geological history of an area, the processes that have shaped the landscape, and the time period over which these processes occur. In other words, geomorphology can be used to explain the complex evolution of the landscape as we see it today."

Geomorphologists study the processes of weathering and erosion, sediment transport and deposition, the characterisation of landforms and the materials making up their composition.

Fluvial geomorphology, for example, studies how human use impacts natural settings in a watershed and determines the shape of river channels. Fluvial geomorphology attempts to predict what physical changes will occur to a water channel in response to alterations in watershed conditions; and how changes will impact human infrastructure and fish habitat.

Aeolian Geomorphology

Aeolian geomorphology is the study of the effects of wind erosion, aeolian processes, on the lithosphere. Winds erode, transport, and deposit materials world wide. Winds have a greater impact on the lithosphere in areas with sparse vegetation and unconsolidated, or loose sediments. Water has a much greater impact overall than wind but eolian processes are very important in arid environments.

Aeolioan (or Eolian) refers to wind, a word taken from the name of the Greek god, Æolus, the keeper of the winds (Latin, from the Greek Aiolos αιολος). The word itself in ancient Greek literally means rapid or changeable, quick moving or nimble.

Wind erosion, or deflation, is created by eddying action that picks up loose, fine grains of material, exposing surfaces and carrying loose material away, depositing it over varying distances. In conjunction with deflation, wind also wears down surfaces by grinding action, abrasion, and literally sand blasting solid surfaces with wind born particles. Areas subjected to deflation are termed aeolian deflation zones.

Aeolian landforms

Deflation and abrasion form deflation basins, also called blowouts, which are hollows formed by aeolian processes. Although generally small, they may be up to several kilometers in diameter.

Abrasion by particles carried in the wind create grooves or small depressions in solid surfaces. Ventifacts are characteristic of wind abrasion. Ventifacts are rocks which have been cut, grooved, pitted and polished by abrasion,

Larger sculpted landforms known as yardangs, are formations that have been streamlined by desert winds. They may be up to tens of meters high and kilometers long. Some of the largest and highest are the yardangs of the Lut Desert of Iran with almost 100 meters of relief (height above the surrounding land). One theory of the Sphinx of Egypt is that it was originally a yardang later modified by human sculpting.

Deflation exposes a sheet-like surface of rock fragments, desert pavement, that remains after wind and water have removed the loose particulate matter. Nearly half of the Earth's desert surfaces are stony deflation zones. The remaining rock mantle, or desert pavement, protects the underlying material from further deflation.

After extensive deflation and abrasion, desert or rock varnish remains on the surface of rocks in aeolian zones. Manganese, iron oxides, hydroxides, and clay minerals form most varnishes which appear as a dark shiny stain.

Aeolian transportation

Winds transport solid particulate matter in three ways, suspension, saltation, and creep.

suspension

Commonly surface winds may suspend particulate matter less than 0.2 millimeters in diameter and carry it aloft as dust or haze in the atmosphere for indefinite periods.

saltation

Saltation moves small particles downwind, in the direction of the wind, in a series of short hops or skips. This process usually only lifts sand sized particulate matter up to one centimetre and carries it about one-half to one-third the speed of the wind. Saltating grains strike other grains which continue the process.

creep

Saltating grains also strike grains that are too heavy to hop, but can be knocked forward, slowly creeping as they are pushed by saltating grains. Approximately twenty-five percent of grain movement in a desert is through the creeping process

Aeolian turbidity currents

Turbidity currents, or dust storms are created when the air is cooled significantly, by rain for example. Cooler air is denser and sinks toward the desert surface where it is deflected forward, sweeping up surface debris, in turn creating turbulence as a dust storm. Most of the dust is in the form of silt-size particles. Windblown silt is deposited over extensive distances. These deposits are known as loess (pronounced ‘lers’). The thickest known deposit of loess on the Loess Plateau in China, is 335 meters. Elsewhere, for example Europe and in the Americas, loess deposits are generally 20 to 30 meters thick.

Whirlwinds

Small whirlwinds, also known as dust devils, typically occur in arid areas. Theoretically they are caused by intense local heating of air which then rises and results in highly localised instabilities. Small whirlwinds may be as much as one kilometer high.

Aeolian deposition

Deposits of sand form sand sheets, ripples and dunes.

sand sheets

Sheets are flattened areas of sand that are too large to be carried by saltation and form an estimated forty percent of aeolian surface deposits. Sheets may be covered by ripples or dunes.

ripples

Winds form surface ripples with long axes perpendicular to the wind direction, with one side of a ripple to the wind. The distance between the crest of each ripple is the average length of jumps that sand grains make during saltation. The coarsest materials are collected at the crest of the ripple.

dunes

Windblown sediments accumulate and also form mounds or ridges called dunes. These dunes have an upwind and a downwind side and are longitudinal as are ripples. The upwind side (facing the wind) is much less steep than the down wind side (the lee slope) which forms a slip-face. The coarsest materials are generally in the troughs between the dunes. This distinguishes dunes from small ripples where the coarsest materials are collected on the ridge.

Dunes move down wind through a series of small avalanches. Wind-blown material moves up the windward side of a dune by saltation or creep. As the materials accumulate on the ridge of a dune the build-up exceeds the angle of repose and small avalanches of sand slid down the slip-face. In this way the dune slowly moves down wind.

References

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