Like water, wind can carry sediment in suspension or saltation, and very limited “bed load”.

• Saltation: hopping motion seen in leaves in wind, sand particles in wind, sand and some gravel in water.

– Fine- to very-fine-grained sand can be carried by wind by saltation.

– Unlike water, normal winds tend to carry only one size of material: coarser sand is not picked up at all, and finer material is usually held together by water cohesion and inter-particle charges, but very-fine to fine sand is easily carried. As wind speed increases or decreases (up to gales, anyway), it does not start transporting larger or smaller particles, but more or fewer particles.

– Wind-transported sand becomes very rounded, very well sorted, and frosted.

– Air carries sand usually in only the lower 1 to 1.5 meters.

– Don’t lie down on the ground to escape a sandstorm: stand up, and hold onto something, and maybe keep a bandana across your mouth and nose to keep sand out.

• Suspension

– Very fine particles (clay and silt) can be carried by wind in suspension for great distances.

– Silt can often be easily eroded, especially when dry. Clay either gets picked up from a mixture or when it is very, very dry or when they are kicked up by something else.

– Airborne silt and clay provide nucleation sites for clouds.

• Bedload: At very high speeds, wind can pick up large objects, like trailers.

Wind

• There is not much connection between deserts and eolian features.

• Wind, like water, can erode, transport, and deposit sediment.

• Eolian (= wind-related processes, also spelled aeolian) processes are most noticeable

– where other processes are slow (like stream processes in deserts),

– where winds are high and constant and there's lots of sediment (like Indiana Dunes) or

– where vegetation is minimal (wind-at-ground is high)

• Eolian erosion occurs on the windward side and deposition in the leeward side or other places where wind speed drops, such as in wind shadows leeward of vegetation.

16. Describe Eolian relief, Eolian deposits, Eolian landscape (types of dunes), Dune Migration.

Eolian Deposition > Fines

• Clay

– Clay stays aloft a long time and often deposits uniformly over an area. Eolian clay deposition might be important on land, but it does not leave distinctive deposits, so it is hard to say how significant it is. Eolian clay is a significant source of sediments in deep oceans, where there isn’t much else.

• Loess (Silt)

– Air-borne silt comes mainly from deserts and from dry riverbeds draining continental glaciers. At continental glaciers, the sediment is largely unweathered, finely ground rock known as rock flour (includes both clay and silt). The airborne silt deposits more in low spots than high spots and rounds/softens the topography. This airborne silt, known by the German word loess, accounts for the fertility of the US Corn Belt, as well as northern Europe and China.

Eolian Deposition

• Dunes(sand) are sub-aerial ripples, and form by the same process: sediment is eroded on the upstream side, moves downstream, and deposits in the leeward, low-velocity zone; as more sediment is deposited, the slope gets too steep and the sediment slides down the slipface. The shape of the dunes is mainly influenced by winds and how constant in direction, supply of sand, and amount of vegetation.

• Heavier material drops out of the wind before lighter materials

• Sand deposited before silt and clay particles

• Deposition of sand forms sand dunes

• Deposition of clay and silt forms loess

Movement of dunes

– Free Dunes

– Tied Dunes

Date: 2015-12-11; view: 761