Wind

Wind

1. Wind erosion

   • Deflation
     • the wind removes loose surface material and carries it away
   • Abrasion
     • material is carried in the wind and it hits the exposed rock surfaces and carves them into different shapes
   • Erosion definition- the breakdown and the removal of rocks.
   • Ventifacts
     • Formed by abrasion
       • rocks sand blast the surface causing a abraded surface which appears shinny and this also forms sharp edges
   • deflation hollows
     • formed by deflation
       • wind removes sediment from the surface and forms a depression
         • an example of a deflation hollow is Qattara Depression.
           • which now lies 134 metres below sea level
             • causes an oasis to be formed as it breaks down below the water table
   • rock pedestals
     • formed when abrasion undercuts the hard rock on the top
       • horizontal layers of hard and soft rock
         • eddy currents usually undercut at the base of the rock pedestal
   • yardang
     • the alternating layers of hard and soft rock are parallel to the wind direction
       • the less resistant rock is abraded first and some of it is undercut
         • this forms troughs where the rock has been eroded
     • yardangs are formed when where water is scarce and the prevailing winds are strong , uni-directional and carrying an abrasive load of sediment
   • zeugens
     • the layers of rock are horizontal and are perpendicular to the wind direction
     • They are often formed where the surface has developed a hard duricrust.

2. DESERT LANDFORMS

   • Water
     • Sources of water
       • Exogenous
         • exogenous rivers are a constant supply of water and are sources that are from outside of the desert
           • for example the Nile
       • Endoreic
         • short rivers that flow regularly that are inland and never reach the sea
           • for example River Jordan
           • terminate inland
       • Ephemeral
         • have high discharge, usually occur when there is a high level of precipitation
           • flash floods or high level of rainfall
         • most common type of river in the desert
     • Water landforms
       • Pediments
         • desert surface that is slightly sloped (3 degrees to 7)
       • Inselbergs
         • an elevated hill that has rounded edges due to further erosion
       • Mesas
         • An elevated area of land that has a very steep sides and a flat top.
           • they are formed by weathering and erosion of horizontal layered rocks that are uplifted by tectonic activity
             • this is because some types of rocks erode at different rates
         • Mesas and buttes are formed by mechanical weathering
           • exfoliation
             • leaves the surface exposed
               • The steep sides are caused due to the differential erosion as mesas and buttes have both soft and hard rock
                 • They are eroded at times of high energy by sheet floods, which transport material through saltation and traction.
       • Buttes
         • an isolated hill with steep sides and a flat top
         • (a smaller mesa)
           • buttes can be formed by mesas after a long time period
       • Playas
         • they are often found at the foot of the pediment. they are ephemeral lakes which are fed by endoreic streams which flood occasionally and then dry up leaving behind a bed of salt and fine sediment
       • Salt lakes
         • the water has been evapourated leaving the original salt from the water on the desert surface
         • Salt lakes form when Desert Rivers which are rich in salt, are endoreic (end in an inland sea). The water can’t leave the lake and evaporation is high, resulting in the lake resulting in a lake with an extremely high salt content
           • Some salt lakes are ephemeral – the evaporation is so high that they completely dry up at some points in the year, leaving behind the salt to form duricrust
       • Wadis
         • dry valley with steep sides containing an ephemeral (occasional) stream
         • When the river meets the wadi exit, there is a rapid decrease in energy due to the decrease in gradient. The heaviest particles are therefore deposited at the beginning. The lighter particles don't need as much energy, so they travel further, but the lightest material travels the farthest.
       • Alluvial fans
         • Bajada
         • usually found at the exit of a wadi
   • Weathering
     • Exfoliation (onion skin weathering)
       • Type of mechanical weathering
         • The diurnal temperature range experienced in deserts is up to 40°C. The direct heating of the sun on the surface layers of the desert contrasts to low night temperature, and this puts severe strain upon the rock surface. Rocks are composed of differing minerals which react differently to these extreme changes of temperature, leading to stress fractures occuring. The surface layers peel, in a process known as exfoliation. Aided by the wind this can produce features often referred to as ‘mushroom rocks’, where exfoliation has caused the rock to peel and the wind has abraded the lower base of the rock.
           • differential expansion is when the rocks are made up off different minerals and they expand at different rates this builds up more stress on the rock.
             • lightly coloured minerals reflect the heat whereas dark coloured minerals absorb the heat and are therefore more likely to expand
       • different layer of rocks are cooled and heated at different rates so this expansion and contraction results in the layers becoming weakened falling off
         • this then leaves the surface exposed so there is likely to be eroded
     • Salt weathering
       • Type of mechanical weathering
         • Salts percolate upwards by capillary action and form crystals which expand with further evaporation. This expansion can lead to the crystals breaking away from the main body of the rock. Rainfall may cause the crystals to hydrate, or swell, or change their structure. Such accumulations produce a naturally cemented duricrust
     • Definition- The breakdown of rocks or soil in situ
     • Thaw weathering
       • water enters crevices in the rocks so that when the temperature falls below freezing. The water turns to ice which needs more room and pushes the crevices making them larger. Over time the crevice gets so large the rick cracks and breaks into smaller fragments.
     • Mechanical weathering involves the physical breakdown and disintegration of rock in situ. The pieces of rock are smaller, but they are the same as they were when attached to the main part of the rock. There is no chemical change or change in appearance has taken place.

3. Wind transport

   • Suspension
     • the finest material is picked up and carried by the wind
       • diameter less than 0.15mm
   • Surface creep
     • coarser material rolls along the surface as they are heavier and can not be transported by the wind so much
   • Saltation
     • Moves sand grains forward as they bounce along the floor
       • Sediment stays close to the surface
         • Maximum height 2m

4. Wind deposition

   • Dunes
     • Barchan dune
     • Transverse dune
     • Dome dune
     • Seif (linear)
     • Parabolic dune
     • Star dune
     • Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind.
     • dunes can move by saltation
   • desert surface
     • erg
       • sandy desert
     • reg
       • stony desert
     • hamada
       • rocky desert