Aeolian landscapes Flashcards
Aeolian
= wind blown
- key process in dryland environments
- responsible for erosion and depsosition
- significant processes in terms of human activity i.e. desertification, dust storms etc.
relevance of aeolian processes
- to understand landscape evolution e.g. dunefields, loess plateaus
- the prediction of future dunefield mobility, in response to climate change and environmental management
- understand the effect of aeolian processes on the climate system e.g. dust storms
- understand the effect of aeolian processes on human health
- to aid our interpretation of aeolian sediments in the palaeo-record; e.g. Quaternary loess deposits and dune activity
hyper-arid
<100mm rainfall per year
arid
100-250mm rainfall per year
semi-arid
250-500mm rainfall per year
deserts
<300mm rainfall per year
dryland characteristics
- high/low surface temperatures
- low precipitation
- very little vegetation
- often have high winds
- so aeolian processes are dominant
deflation
- when the wind moves particles this is entrainment
- entrainment leads to deflation
- deflation can create hollows or blowouts
- clay, silt and fine medium sand is removed
- leaves a lag deposit (Stone pavement)
wind profiles
- wind speed reduced by friction at, and close to, the ground surface (magnitude of effect depends largely on surface roughness
- surface friction consumes energy
- as the frictional effect declines away from the obstacle surface, wind velocities increase
- rapid rate of increase close to the surface
- slower at > heights
wind erosion: Ventifacts
- cobbles and pebbles on stony desert surfaces can be abraded by wind carrying silt and dust (silt blasted)
- shaped into faces or facets separated by keels
- also termed dreikanter
- bedrock exposures can also be abraded in the same manner
Pedestal
- rapid/high velocity surface winds
- softer rock at the bottom, preferential erosion/abrasion (mushroom cap effect)
what effects how easily a sediment can be moved by wind
- gravitational forces
- grain size/diameter
- density of grain/sediment
- air density
sediment entrainment
- moisture and cohesion acts to prevent entrainment:
- additional weight
- increases weight
- increases drag
- acts like a larger more dense particle
- drier = dune mobilise
- wetter = dunes stabalise
sediment size and density
- sediment size and density affect sorting processes
- stronger winds = larger/more dense grains
- concentrated
- lighter grains transported further away
aeolian transport
- wind dislodges, entrains and transports particles
- dead air close to the ground where the velocity is zero (the height of this layer is known as z0)
- shear movement at the surface is controlled by the shear velocity and z0
- vegetation reduces shear greatly
suspension
- fine sediment may be kept in suspension for days due to low settling velocity
saltation
ballistic trajectory of grains as they are ejected from the grain bed and given horizontal momentum by airflow
creep
rolling of coarse particles due to wind drag and collision of saltating grains
aeolian landforms
- a major feature of desert environments are sand dunes and sand seas (ergs)
- fine grained (silt sized) dust can also form large-scale landforms e.g. loess plateaux
Dune initiation
- dune initiation is poorly understood, but there must be a nucleus to concentrate particle accumulation e.g. topographic obstacles or vegetaiton
- bedform (i.e dune) development occurs when there is a disruption to wind velocity causing a reduction in U (& therefore shear stress)
crest and slipface
very important in terms of movement and migration
Thomas, 1997 classification
Dunes can be divided into 2 major types according to form and process
- Transverse or crescentic dunes
- linear dunes
simple linear dunes
- longer than they are wide
- can be >160km long
- multiple or isolated ridges (gravel interdune areas)
- bi-directional winds
- long axis in direction of sand movement
star dunes
- pyramid mounds
- slipface on 3 or more arms radiating out
- multidirection wind regimes
- grow upwards rather than laterally
fixed dunes
- anchored to vegetation
- not mobile
main controls on dune form and development
- wind direction
- sediment supply
- vegetation type and density
- in terms of landform development, longterm variations in these main controls are important e.g. the effect of climate change
more dune examples
- Transverse/crescentic ridge
- Barchan dunes
- Barchanoid Ridges
- simple and complex linear dunes (seif)
- star dunes
- fixed dunes
