Unit 49
WIND AS A GEOMORPHOLOGICAL AGENT
Unit OverviewThis unit examines the potential for wind to shape the physical landscape. The main sections are as follows:
- Wind erosion
- Degradational landforms
- Aggradational landforms
- Sand dunes
- Loess
Compared to fluvial, glacial, and coastal erosion, eolian erosion is a more subtle process. Wind plays a larger role in
less-vegetated landscapes. Only when wind picks up particles does its power to erode become important. Wind transports a
significant amount of matter. This process occurs through suspension, saltation, and surface creep. Aggradational landforms
include sand dunes and loess.
Unit Objectives
- To examine the mechanisms of wind erosion and the landforms produced by this process
- To relate various types of sand dunes to environmental controls
- To note the importance and environmental significance of loess
Glossary of Key Terms
Backslope | The windward slope of a sand dune. |
Barchan | A crescent-shaped sand dune with its points lying downwind; convex side of this dune is the windward side. |
Deflation | The process whereby wind sweeps along a surface and carries away the finest particles. |
Deflation hollow | Shallow desert basin created by the wind erosion process of deflation. |
Desert pavement | A smoothly weathered, varnish-like surface of closely packed pebbles that has developed on the upper part of an alluvial fan or bajada; no longer subject to stream braiding, such a surface is stable and may support desert vegetation. |
Dune | An accumulation of sand that is shaped by wind action. |
Eolian | Pertaining to the action of the wind. |
Erg | A sand sea; a large expanse of sandy desert. |
Loess | A deposit of very fine silt or dust laid down after having been blown some distance (perhaps hundreds of kilometers) by the wind; characterized by its fertility and ability to stand in steep vertical walls. |
Longitudinal dune | A long ridge-like sand dune that lies parallel to the prevailing wind. |
Parabolic dune | A crescent-shaped sand dune with its points lying upwind; concave side of this dune is the windward side. |
Slip face | The leeward slope of a sand dune. |
Surface creep | The movement of fairly large rock fragments by the wind, actually pushing them along the ground, especially during windstorms. |
Transverse dune | A ridge-like sand dune that is positioned at a right angle to the prevailing wind; usually straight or slightly curved. |
Wind abrasion | The erosion of rock surfaces by windborne sand particles. |
Yardang | A desert landform shaped by wind abrasion in the form of a low ridge lying parallel to the prevailing wind direction; most common in dry sandy areas underlain by soft bedrock. |
Unit Outline
- Wind erosion
- Eolian processes are wind-related and shape the Earth's surface
- Wind is both an aggradational and degradational force
- Wind is only an erosive agent when it carries sand particles and performs wind abrasion
- Degradational landforms
- Deflation is the process of wind sweeping away fine particles of sand
- deflation hollows are produced by this process
- desert pavement is the remaining closely-packed pebbles on the surface
- yardangs are low ridges that form parallel to wind direction, due to abrasion
- Wind transportation (Fig. 49.4)
- finest particles carried high in the air in suspension
- larger particles bounce between the air and the ground, in saltation
- largest rock fragments pushed along at the surface by surface creep
- Deflation is the process of wind sweeping away fine particles of sand
- Aggradational landforms
- Erg, or sand sea, is the main feature in arid environments
- Ripples may be formed by saltation or surface creep
- Sand dunes
- An accumulation of sand that is shaped by the wind
- Dune is active when the wind is constantly moving it, and it has no vegetation
- Dunes become stable (fixed) when they enter a moister area, and support vegetation
- Dune features (Fig. 49.6)
- backslope (or windward slope)
- crest (or top)
- slip face (or leeward slope)
- Dune forms (Fig. 49.7)
- barchans are crescent-shaped dunes, the windward side is convex
- parabolic dunes are crescent-shaped dunes, the windward side is concave
- transverse dunes are straight or slightly curved, at right angles to wind direction, the windward side is concave
- longitudinal dunes are long ridges, parallel to wind direction
- Dune landscape research
- dune morphology can provide information about past climates
- Loess
- Fine-grained sedimentary deposits originally left by glaciers, then picked up by wind
- Distribution of loess deposits
- extensive in Eurasia, some in North America, southern South America
- Properties of loess
- loess produces very fertile soil for agriculture
- consists of quartz, feldspars, carbonates, clays, and other minerals
- fertile through its entire profile, not just its upper layer
- a homogeneous, porous substance that has vertical strength once water has developed passages within it
- China's Loess Plateau
Review Questions
- Name the four major forms of sand dunes, using Fig. 49.7.
- Describe the processes of suspension, saltation, and soil creep.
- Explain the global distribution of loess, using Fig. 49.9 as a guide.