Aeolian landscapes

Question 1

Aeolian

Answer 1

= 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.

Question 2

relevance of aeolian processes

Answer 2

• 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

Question 3

hyper-arid

Answer 3

<100mm rainfall per year

Question 4

arid

Answer 4

100-250mm rainfall per year

Question 5

semi-arid

Answer 5

250-500mm rainfall per year

Question 6

deserts

Answer 6

<300mm rainfall per year

Question 7

dryland characteristics

Answer 7

• high/low surface temperatures
• low precipitation
• very little vegetation
• often have high winds
• so aeolian processes are dominant

Question 8

deflation

Answer 8

• 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)

Question 9

wind profiles

Answer 9

• 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

Question 10

wind erosion: Ventifacts

Answer 10

• 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

Question 11

Pedestal

Answer 11

• rapid/high velocity surface winds

- softer rock at the bottom, preferential erosion/abrasion (mushroom cap effect)

Question 12

what effects how easily a sediment can be moved by wind

Answer 12

• gravitational forces
• grain size/diameter
• density of grain/sediment
• air density

Question 13

sediment entrainment

Answer 13

• 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

Question 14

sediment size and density

Answer 14

• sediment size and density affect sorting processes
• stronger winds = larger/more dense grains
• concentrated
• lighter grains transported further away

Question 15

aeolian transport

Answer 15

• 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

Question 16

suspension

Answer 16

• fine sediment may be kept in suspension for days due to low settling velocity

Question 17

saltation

Answer 17

ballistic trajectory of grains as they are ejected from the grain bed and given horizontal momentum by airflow

Question 18

creep

Answer 18

rolling of coarse particles due to wind drag and collision of saltating grains

Question 19

aeolian landforms

Answer 19

• 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

Question 20

Dune initiation

Answer 20

• 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)

Question 21

crest and slipface

Answer 21

very important in terms of movement and migration

Question 22

Thomas, 1997 classification

Answer 22

Dunes can be divided into 2 major types according to form and process

• Transverse or crescentic dunes
• linear dunes

Question 23

simple linear dunes

Answer 23

• 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

Question 24

star dunes

Answer 24

• pyramid mounds
• slipface on 3 or more arms radiating out
• multidirection wind regimes
• grow upwards rather than laterally

Question 25

fixed dunes

Answer 25

• anchored to vegetation

- not mobile

Question 26

main controls on dune form and development

Answer 26

• 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

Question 27

more dune examples

Answer 27

• Transverse/crescentic ridge
• Barchan dunes
• Barchanoid Ridges
• simple and complex linear dunes (seif)
• star dunes
• fixed dunes