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Aeolian Geomorphology > Aeolian > Flashcards

Aeolian Flashcards

1
Q

Distribution of Aeolian Landforms

A

Mostly contained in deserts’ hot regions

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2
Q

What are dunes?

A

Accumulation of wind deposited particles, composed of mainly sand but silt and clayin some certain conditions.

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3
Q

What are ergs?

A

Sand is not evenly spread in desert regions, they’re concentrated in ergs (sand seas)

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4
Q

Present-day aeolian sands are mostly what?

A

Alluvium as opposed to previously quartz from bedrock weathering.

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5
Q

What wind dominate hot desert regions?

A

Sub-tropical high pressure wind systems.

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6
Q

What generates pressure gradients and generate local winds?

A

Contrasts in thermal properties of desert surfaces and differential heating (variations in albedo).

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7
Q

Vertical variation of wind speed above ground

A

Close to surface = wind speeds reduced by friction = magnitude varies with surface roughness.

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8
Q

As wind speed and surface roughness increase…

A

Airflow becomes more turbulent and erosion may increase.

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9
Q

Wind speed increases with

A

Height

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10
Q

Susceptibility for soils to erode is related to…

A

Water/moisture content

Cohesiveness.

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11
Q

Movement is restricted by…

A

Weight of particles
Friction
Interparticle cohesion.

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12
Q

Movement is induced by…

A

drag and lift forces and impact of grains already in motion (ballistic impact).

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13
Q

LIFT forces - can cause…

A

particles to jump into airflow by pressure differences.

High velocity across top of particles
Lower velocity on lower parts
= particle vibrates and more when shear stress and grain surface exceeds threshold velocity/critical shear stress value = grain lifted vertically into air.

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14
Q

DRAG forces - can cause…

A

difference in fluid pressure on windward and leeward sides of a grain in airflow.
Grains move downwind by rolling or sliding (surface creep).
Can initiate movement at lower wind velocities than those necessary for lift and particles too heavy to be lifted by wind

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15
Q

What is is the fluid threshold velocity?

A

The wind speed needed to promote movement through lift or drag.

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16
Q

Airflows consist of…

A

bursts of higher velocity flow, creating a pocket of low presure above surface and potential for lift or drag.

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17
Q

Variations in wind velocity are important because

A

drag force varies with second power of velocity = doubling of wind velocity = fourfold increase in drag

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18
Q

A particle will move at a given wind speed depending on different factors in addition to its…

A

size, density and shape, packing of grains around it and cohesion between particles.

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19
Q

Grains larger than 1mm…

A

rarely moved by drag - wind speeds hardly attained in natural conditions

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20
Q

Grains >0.6mm…

A

threshold velocity for movement (shear stress/critical shear stress) increases as particle size decreases - reduced surface roughness therefore lower level turbulence from finer material.

Also, greater cohesion increases resistance in finer particles. Smaller particles can be packed and sheltered from airflow by larger grains

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21
Q

Grains >0.1mm

A

Suspended once in motion

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22
Q

Grains >0.4mm

A

Temporarily lifted and returned (saltation) - grains can be hit downwind by those already in motion (ballistic impact) = additional forward momentum means grains of particular size can be set in motion at wind velocity lower than required for movement.

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23
Q

Difference between impact threshold and fluid threshold velocity is

A

small for fine particles but significant for larger particles.

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24
Q

Once a gust of wind has caused initial movement in some particles, further motion can be temporarily sustained by…

A

ballistic impact even after wind has dropped below fluid threshold velocity.

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25
Q

Total movement occurring by surface creep becomes less with…

A

decreasing grain size.

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26
Q

What is saltation?

A

Saltation is major process of sand movement.

Once in airflow, carried downwind with increased wind speeds, return to surface at a trajectory determined by balance between gravity, drag and strike surface at angle
Grains may bounce on impact and cause other grains to saltate
In general, height increases with greater wind speeds and particles, but larger particles may bounce better therefore rise higher into airflow.

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27
Q

Aeolian erosion involves two processes

A

Deflation - removal of loose particles by the wind

Abrasion - bombardment of rock and other surfaces by particles in the airflow

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28
Q

Deflation of sand sized particles is localised as…

A

they can’t be moved greater distances except over long periods of time.

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29
Q

Most deflation is removed by…

A

dust storms and redeposited elsewhere forming loess (silt-formed sediment) and finer materials can contribute to ocean sediments.

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30
Q

Rate of abrasion is maximised where…

A

High velocity winds transport a big amount of grains upwards.

As they’re only carried close to the surface, abrasion ony occurs within 1-2m of the surface

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31
Q

Where are aeolian erosional landforms present?

A

Only in arid areas, where other denudational agents are lacking.

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32
Q

A lot of deserts are covered by?

A

lag deposits rather than sand (gravel or coarser) and can lie over finer sediment.

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33
Q

Where stone cover is continuous, known as?

A

Desert/stone pavements

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34
Q

Small scale forms =

Intermediate scale forms =

A

Ventifacts

Yardangs

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35
Q

Characteristic features of stony surfaces

A

Rresence of faceted cobbles and pebbles, thought to be due to abrasion by dust and silt rather than sand.

Facets are produced at 30-60 degrees facing the prevailing wind and separated from protected lee side by a sharp edge (sharp edge indicated where wind erosion direction is in).

36
Q

Yardangs

A

A grooved form - streamlined powerful ridge (can be 10m high, 100m length) aligned with or away from direction of prevailing wind
Predominantly in soft lithologies
Also weathered etc to form shape, much controversy on origin, but highly likely to be wind.

37
Q

What are deflation hollows?

A

Shallow depressions, can vary widely in size - caused by deflation processes and high wind velocities and have relatively bare surfaces.

38
Q

How are deflation hollows formed?

A

Varied local conditions such as lower sediment moisture content or disturbed vegetation cover

39
Q

What are Pans/saltpans (dry lake)?

A

Large enclosed basins/grooves - many processes acting upon it as well as aeolian such as tectonic activity

40
Q

Depositional landforms

Ripples

A

Regular, wave-like undulations orientated at right angles to the direction of the prevailing wind.

41
Q

Depositional landforms

Bagnold

A

Develop from slight irregularities in sand surface through combination of creep and saltation.

42
Q

Saltating grains impact

A

strike surface at shallow angle, disturb other grains windward side of an irregularity than on leeward side which is protected from the flight path of saltating grains.

43
Q

Ballistic impacts…

A

release grains from windward side which either creep up windward or saltate to add to sand accumulation forming the next ripple.

44
Q

Depositional landforms

Dunes - patch of sand accumulates…

A

Wind speed is reduced but theres an increase in surface roughness.
Saltating grain become trapped and are unable to rebound on impact.
At least 1-5m width to become a dune.
Separation and deceleration of airflow on leeside of sand body, sand accumulates more rapidly than is removed, therefore size builds.

45
Q

Wind direction and velocity, sand supply and presence of vegetation are crucial factors…

Free dunes:

Impeded dunes:

A

Free dunes: due to wind characteristics primarily

Impeded dunes: due to influences of vegetation

46
Q

Transverse

Wind regime

A

Unidirectional

47
Q

Transverse

Slip-face orientations

A

One; unidirectional

48
Q

Barchanoid/Barchan

Wind regime

A

Unidirectional

49
Q

Barchanoid/Barchan

Slip-face orientations

A

One; unidirectional

50
Q

Linear

Wind regime

A

Bidirectional/obliquely converging

51
Q

Linear

Slip-face orientations

A

Two; opposing

52
Q

Star

Wind regime

A

Multidirectional

53
Q

Star

Slip-face orientations

A

Three or more; multidirectional

54
Q

What are compound dunes?

A

Two or more of the same basic dune type which have coalesced or superimposed

55
Q

What are complex dunes?

A

Where different basic dune types are associated.

56
Q

What are nebkas dunes?

A

One plant grows, accumulates sand around it, grows - both communicate and grow to form vegetative dune.

Dunes can’t form as vegetation growing close together prevents sand being blown.

More spread out vegetation causes sand to be blown between.

57
Q

What are Parabolic dunes?

A

Trailing arms upwind side, bare sand exposed nodes downwind.
Low topography arms.

58
Q

What are Linear dunes?

A

Winds coming from two directions.

59
Q

Parabolic dune development (coastline environment)

A

Salt content reduces with distance from the coast; changed conditions enable vegetation to start growing at coastline: foredune ridge with limited vegetation

Further inland: exposure of pockets of bare sand and may allow dune to become mobile

Blow outs - blow more mobile sand and form dune like landforms (sand piling up on downwind side) - vegetation many grow around

Vegetation stabilises dunes

60
Q

Parabolic dunes in transitional enivonments

A

An area not near the coast, where same processes are occurring:
Rivers carry sediment to dune field and blown by wind
Close to river - moisture - variations in vegetation can grow alongside
Agriculture can change the landscape too - controlling livestock from trampling surface and vegetation
Dunes transition to potential parabolic
Can activate from barchan to parabolic and can reactivate from parabolic back to barchan
Quick transitions can occur after floods/heavy rains but only temporarily - arid conditions return

61
Q

Lancaster index =

A

M = W / (P/PET)
M - mobility of landscape
W - windiness
P/PET - precipitation/potential evapotranspiration

62
Q

Based on Lancaster index for a fully active dune activity =

A

Need low P/PET and wind. For in active, stable dunes, a lot of moisture must be present for vegetation to grow

63
Q

Where do we get dust events happening?

A

Erosional environments, not a lot of sand, small sediment - finer sediment needs to be present and wind

Atmospheric conditions: aridity and wind but also fine particles and nothing protecting them

64
Q

What makes areas more dusty?

A

Somewhere windy
Most dust from dry lakes/salt pan/playa - low-lying depression, dry most/all of the year = no sediment availability limitations or supply limitations

65
Q

Forces determining saltation trajectory:

A
  • Gravity
  • Aerodynamic drag
  • magnus effect
  • aerodynamic lift
66
Q

Dune Types

A

Barchan, Barchanoid , Linear, Transverse, Star, Falling and climbing

67
Q

Speed of dunes

A

Small dunes will travel faster than bigger dunes but proto dunes will actually travel faster than the smaller dunes because they don’t have the same dynamics as the big dunes.

68
Q

Supply limited means…

A

Low sediment source, wind above threshold

69
Q

Transport limited means…

A

High sediment availability, low wind strength

70
Q

Barchan dune dynamics

A

Small dunes merging with the big dunes, increasing sediment supply, increase size of dune= slow down the movement of dune. Shredding sediment, horn where the sediment would be lost.

71
Q

What are simple dunes?

A

Individual dunes, spatially separated

72
Q

What are compound dunes?

A

Two or more dunes of the same type that are superimposed (draa)

73
Q

What are complex dunes?

A

Two or more dunes of different types that are superimposed (draa)

74
Q

Different types of deserts

A

Stony, sand, hard rock, salt lakes, etc.- reg. ergs. Hamada.

75
Q

Nebkhas, Chihuahuan Desert, USA.

A

Example of nebkhas dunes, plant grows and accumulate sand around it.

76
Q

Parabolic Dunes, Madagascar

A

Semi arid- parabolic dunes- point in up wind direction. Bare sand nodes. Low topography.

77
Q

Linear Dunes, Kalahari, South Africa

A

Winds coming from two different directions. Different amount of vegetation means different sizes of linear dunes.

78
Q

Israel- Egypt boarder- agricultural practises- changed landscape…

A

Animals moved from the area, so stopped stamping on vegetation, so now the vegetation can grow again.

79
Q

Rub’ al khali, Oman and Saudi Border

A

It rained so vegetation started to grow rapidly.

80
Q

Caprivi strip, Namibia- Long term climate influences- quite tropical now…

A

Elephant population is huge- swampy. Linear dunes round there. The dunes are stabilised there.

81
Q

Mobility Index- moisture, vegetation. Lancaster index: M= W(windiness) /P(precipitation) /PET

A

(potential evapo- transpiration) – a lot of moisture means you wont move very far- but you can grow more vegetation so the dunes become more stable. Less than 50= dunes stable. 200+= unstable dunes.

82
Q

Where do we get dust events?

A

We need a erosional landscape.
Fine sediment.
Same atmospheric conditions to create dunes but also fine particles with nothing to stop them.

83
Q

Atmospheric dust climate impacts

A
  • Influence air temps (absorption and scattering of solar radiation)
  • Affect cloud formation and convectional activity
  • Influence atmospheric sulphur dioxide levels (physical absorption or heterogenous reactions)
  • Influence marine primary productivity (indirectly carbon dioxide levels)
84
Q

Reasons for deposition of dust

A
  • Wind velocity and turbulence decrease, particle can no longer remain suspended
  • Collision with moist, rough or charged surface
  • Aggregates form with increased mass
  • Rain
85
Q

Dust- process events (Spatial scales)

A
  • Dust devils- small scale. Last for a few minutes

* Dust storms large scale. Last serval days.

Aeolian Geomorphology (1 decks)

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