THE WORK OF WIND Flashcards
difference in physical laws of fluid motion of water and wind
wind is less contrained by gravity and unconified
e.g. wind can blow uphill
wind is less effective agent of erosion, deposition and transport
what are the exceptions to this
arid zone – some times whole landscapes are product of aeolian processes
what and were is the Rub al Khali?
Saudi Arabia
surface water free dunefield
where is wind action most important
drylands
deserts
coasts
water as a sculpting agent in deserts
also played a significant role
what are deserts?
places where surface water is limited in quantity
arid zone geomorphology
what is aridity?
combination of low moisture input and high moisture output doe to low precip and high evap
aridity index
ratio between precip and evap
precip/potential evaptrans
P/PET
what is aridity characterized by?
as net surface water deficit caused by global atmospheric, oceanographic and topographical factors
global atmospheric, oceanographic and topographical factors
1) Atmospheric stability due to global air mass dynamics
2) Atmospheric stability due to cold ocean currents
3) Continentality
atmospheric stability arising from global air mass dynamics
Arid areas form in zones of dry, descending, stable air over the sub-tropical high pressure belts.
atmospheric stability induced by ocean currents
Cold ocean currents associated with descending stable, dry air
The cold Benguela current and associated upwelling ‘causes’ the Namib desert
aridity induced by ‘continentality
Distance from the oceans prevents penetration of rain-bearing winds,
Aridity induced by topography
So-called rain shadows, where mountains act as barriers to the penetration of rain-bearing winds, e.g. Tanqua (Ceres) Karoo.
where are deserts?
one third of arid zones falls within africa
Global extent of drylands Dry-subhumid 10 Semi-arid 18 Arid 12 Hyperarid 7
Total 47
where are landform features shaped by the wind are found
where wind blows frequently and at high velocities and where entrained particles are blasted against easily disintergrated surfaces
what are four types of landforms produced by wind erosion
a) deflation hollows
b) desert pavements
c) ventifacts
d) yardangs
what is the key process in creating wind produced landforms
abrasion/ attrition
deflation hollows
removal by wind of sand and silt particles producing a local lowering of the land surface
‘Blow-out’ may then expands over time due to collection of runoff and intensification of weathering associated with fluctuating groundwater (results in a ‘pan’).
where do deflation hollows occur?
where vegetation cover is broken
desert pavement
deflation may remove finer grained particles and leave behind a lag layer of gravel
ventifacts
abrasive action of etrained material causes grooving and polishing of surface rocks - wind faceted stones
are results from impact by suspended and saltating particles - sandblasing
complex shapes and sculpturing
yardangs
duplicates of ventifacts on a much larger scale
inverted boat
well developed in soft sediments
wind transport of sand
transport is maximized in grain size of 0.04mm - 0.4mm
grains have characteristic physical feature
wind turbulence promotes uplift and entrainment
how does sand move
creep
saltation
hjulstrom equivalent for wind
see slides
creep and saltation of sand
creep = rolling along bed saltation = jumping
what is dust
generalised term that accounts for fine particles
dust grain size
10-100 microns
transport of dust
dust can travel very far
dust from sahara may be principal supplier of nutrients to amazon rain forest
can remain in suspension for very long time
dust storms
what causes aeolian deposition
reduction in wind energy - loss of competence
what happens if the entrained particles are sand sized?
formation of discrete landforms dunes
what are the controlling factors of dune types
sand supply
grain size
wind velocity, frequency and direction
main dune types
barchan transverse longitudinal star parabolic
barchan
barchanoid dune cresent shape often in groups horns point in wind direction commonest in areas of limited sand supply high migration rates
transverse dunes
can result from barchan coalescence to form irregular ridges – ridges are perpendicular to wind direction
found in areas of abundant sand supply and no vegetation
slower migration rates
linear/longitudinal or seif dunes
long straight ridges parallel to prevailing wind direction
moderate sand supply
dunes are sediment accumulators
migrate slowly
parabolic dunes
like barchans but their horns face opposite to wind direction
environmental implications of desertification
Human activities may induce the spread of desert-like conditions and, in the classical model, this involves the invasion of non-desert areas by mobile dune forms or the reactivation of stabilised dunes.
many and complex (include ‘drought’, grazing and crop mismanagement, climate change) but impacts are often severe (e.g. famine) and the consequences dire.
Kalahari case study
largest area of inactive sand dunes (vegetated or fossile dunes)
Active in the relatively recent geological past under conditions drier/windier than present
What about the future under predicted climate conditions in a warmer earth?
widespread reactivation of ‘fossil’ dune forms
