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EAS 225 (Earth Surface/ Landforms) > Aeolian > Flashcards

Aeolian Flashcards

1
Q

Aeolian processes=

A

processes and landforms related to the action of wind

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2
Q

Wind moves a lot of material yearly (60-200 tons of Saharan dust per year!). What is an implication of wind-blown sediments in context of the deep ocean?

A

Wind can’t transport sediments very far usually (because it’s very hard to keep particles entrained in air), so it’s hard to get any sediments to the deep ocean
- some gets out there though!

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3
Q

___ is the most turbulent medium

A

air

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4
Q

t/f
erosivity has a linear relation with wind velocity

A

false

exponential!

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5
Q

What are the 4 main controls on wind erosion?

A
  1. wind velocity
  2. topography
  3. surface cover and roughness
  4. grain size
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6
Q

How would you calculate the erosivity change if the wind speed changed from 10km/hr to 20km/hr?

Equation: E= V^3p

A

20/10= 2
2^3= 8

8x more erosive if the wind speed increases from 10 to 20km/hr!

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7
Q

Explain the effect of topography on erosion in terms of:
- hills
- snow deposition on a hill

A
  • at the end of a hill, wind velocity increases= more erosive power
  • at the bottom of a hill (right before going up), velocity decreases= less erosive power
  • wind “erodes” snow too! Loose snow will be picked up from the windward (stoss) side of a hill, and deposited on the lee side
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8
Q

What effect does surface cover have on wind erosion?

A

More surface cover= less erosion

  • tall stand of trees= wind velocity low= less erosion
  • no veg (eg a field with exposed soil)= high wind velocity= high erosion
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9
Q

How could you decrease erosion on a field with little vegetation?

A

Plant hedges!
Adding surface cover and roughness will decrease wind velocity and therefore decrease erosion (farmers do this)

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10
Q

Why is the height of surface cover (eg trees) so important in influencing erosion?

A

Because there is little to no wind erosion on a tall, well vegetated surface
- b/c the veg. slows down the wind

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11
Q

What is the zone of little or no wind called?

A

The laminar sublayer (or the boundary layer)

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12
Q

On an exposed bed, what is the laminar boundary layer height?

What about in vegetated areas?

A

Exposed bed= 0.5-1.0x grain diameter

Vegetated areas= height of the continuous vegetative cover

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13
Q

What does the Shield’s curve tell us?

A

grain size vs threshold velocity needed to entrain that certain grain size

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14
Q

List some differences in the Shields Curve when the medium is wind instead of water

A
  • wind needs higher velocity to entrain sediment, so the whole curve shifts up
  • wind is completely unable to move big boulders usually
  • silt to fine sands are moved by wind
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15
Q

If sediments are wet, can wind easily entrain them?

Wind has a higher component of __ load compared to ___ load

A

No! So wind moves much less if it’s not really dry

bed
suspended

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16
Q

Shear stress threshold velocity=

A

force it takes to entrain a grain

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17
Q

On Sheilds curve:
- Below the impact threshold, you’ll see ___
- Between the impact threshold and the friction threshold, you’ll see ____
- Above the friction threshold, you’ll see ____

A

deposition

transport (entrainment)

erosion

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18
Q

Why is it hard for wind to erode clay? What about larger material like boulders?

A

clay= hard to move due to cohesion, especially when wet

boulders: hard to move larger material by a low density fluid like wind, because they have high mass

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19
Q

What are the 4 mechanisms at work in aeolian processes? Describe their forces

A
  1. gravity: downward force
  2. Friction: caused by roughness of surface
  3. Drag: forward force due to motion of fluid
  4. Lift: caused by Bernoulli principle: negative pressure behind the particle
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20
Q

There are 3 main ways that aeolian processes can transport sediments:

A
  1. traction
  2. saltation
  3. suspension
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21
Q

Describe the process of traction and how it moves sediment

A

rolling/ traction/ creep

  • motion along bed of course sands and pebbles
  • can’t move far (cm scale)
  • gravity&raquo_space; lift (caused by Bernoulli effect, very little of it here)
  • drag > friction

note drag= force exerted from fluid flowing past the grain

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22
Q

Describe the process of saltation and how it moves sediment

A

saltation= bouncing
- moves a few m horizontally, 1-2m vertically

At first, lift exceeds gravitational force
- negative pressure equalizes & gravity then exceeds lift
= up and down movement

  • as the particle falls, it impacts other particles and they become entrained = positive feedback
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23
Q

Describe the process of suspension and how it moves sediment

A

lift force exceeds gravitation

  • this predominately occurs for silt and fine sand because they’re lighter and easier to lift

can transport long distances (eg in jet stream)–> sahara dust has been found in Antarctic ice cores, carried by suspension at some point

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24
Q

What are the 5 main erosional aeolian landforms?

A
  1. desert pavement/ varnish
  2. deflation hollows
  3. yardangs
  4. ventifacts
  5. rock trees
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25
Q

What are the 2 key processes that form erosional aeolian landforms?

A
  1. abrasion= mechanical erosion of surfaces by wind blown sediment
  2. deflation= removal of fine grained sediments + net lowering of the surface
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26
Q

Which 2 landforms are created by deflation?

A

desert pavement/ varnish and deflation hollows

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27
Q

Which 3 landforms are created by abrasion?

A

yardangs, ventifacts, rock trees

28
Q

Desert pavement=

A

wind erodes fine silts, deflation continues to remove finer particles until the surface is lowered and course clasts are left sticking out

29
Q

desert varnish=

A

abrasion of desert pavement over long time frames (polishing)

common texture on mars and antarctica!

30
Q

Deflation Hollow (blowout)=

A

shallow depressions that form in dunes by net erosion + removal of fines

31
Q

What will stop deflation hollows from continuing?

A

their depth is limited by coarse lag gravel or the water table

32
Q

Abrasion is also known as ___
(mechanical weathering)

=

A

sandblasting

= impact of entrained sand grains against rock surfaces and other grains

33
Q

yardings=

A

long linear features formed by abrasion

  • form where wind is constant direction over recessive sediment like sandstone

(100s-1000s of years to form!)

34
Q

The cross section of yardangs is often ____, and they are found across ____ scales

A

asymmetrical

vast

35
Q

ventifacts=

A

abrasion from multiple dominant wind directions over long time scales leads to polished facets (like carved jems!)

36
Q

Faceted face of ventifacts always in ___ direction

A

upwind

ie angled faces form on the windward face

37
Q

Ventifacts:
with ____direction of dominant winds, different facets merge along sharp ridges to transform ___ stones to ____ ventifacts

A

changing

rounded

angular

38
Q

Rock trees form when:

A

there is a lot of wind, but in no dominant direction (like cyclonic)

  • long term abrasion
39
Q

List ~9 main depositional aeolian landforms.

A
  1. ergs
  2. loess
  3. sand sheets (basically ergs)
  4. transverse dunes
  5. barchan dunes
  6. barchanoid ridges
  7. parabolic dunes
  8. linear dunes
  9. star dunes
40
Q

desert=

A

barren area with little precipitation

41
Q

ergs=

A

region with wind-born sand deposits

42
Q

loess=

A

region with wind-borne silt deposits

43
Q

Ergs include: (4)

A

sand seas, sand sheets, ripples, and dunes

44
Q

Loess accumulates in massive ___

A

sheets

45
Q

Ripples are ___cm tall, and their height ___ with grain size and increasing velocity

A

<2cm
increases

46
Q

Individual dunes are __-__m tall, and they are hills of loose ___.

Dunes are formed with __ velocity than ripples and require t____

A

1-10m
sand (fine-med)

higher
turbulence

47
Q

What are compound/ complex dunes?

A

dunes on top of other dunes or yardings
up to 100s of m high!

48
Q

t/f

sand sheets and loess deposits can only be 100s of m big

A

false
they’re huge (up to many km)

49
Q

Describe how dune formation is initiated

A

Turbulence is key!

topographic irregularities lead to change in the wind’s velocity profile. A sudden decrease in velocity= deposition

  • creates more topographic irregularity, so more turbulence= more deposition= dune grows (or other dunes form)
50
Q

Stoss side of a dune= ___ via saltation and traction

Lee side of a dune= ___ at angle of repose

A

erosion (sand saltates up the gentler windward side of the dune)

deposition
(sand cascades down and settles at the face of the steeper slip face of the dune)

51
Q

List 4-7 important variables controlling dune morphology:

A
  • wind direction
  • wind direction consistency
  • surface cover (veg)
  • grain size
  • topography
  • sediment supply
  • wind speed
52
Q

Longitudinal dunes form ____ to the direction of the wind, with __ wind speeds, ___ sediment supply, and ___ vegetation

A

parallel
high
low
low

53
Q

Transverse dunes form ____ to the direction of the wind
- high + persistent wind speeds (__directional)
- __ sed supply
- __ vegetation

A

perpendicular
unidirectional winds
high
no veg to obstruct formation

54
Q

Changes of wind velocity where there are transverse dunes can form ___ between dunes. This implies a ___ landscape

A

lake
stable

55
Q

What type of dune is the “classical desert dune”?

A

Transverse

56
Q

Barchan dunes form at:
- ___ wind intensity
- ___sed supply
- ___ veg

The horn shape points ___

A

high
moderate
low

downwind

57
Q

Barchanoid ridges form when the horns of barhcan dunes ___ ____. They form a mosaic landscape.

Intermediate between:

A

meld together

barchan dunes and transverse dunes

58
Q

How would it be possible for barchan dunes, barchanoid ridges, and transverse dunes to exist on the same landscape, close to each other?

A

this happens because of local differences in wind intensities and sediment supply

59
Q

Parabolic dunes form at:
- ___ veg
- ___ sediment supply
- ___ wind intensity

Their horns point ____ and are sheltered by ____

A

moderate
mod-high
medium

upwind
vegetation

60
Q

Deflation hollows form on ____ dunes in ___ the direction

A

parabolic dunes

upwind side

61
Q

T/F
parabolic dunes are usually stable on the landscape and don’t change quickly

A

true

they can even have little ponds which slows the blowout

62
Q

How can you distinguish barchan vs parabolic dunes?

A

look for vegetation and water (=parabolic)

barchan will not have veg or water

63
Q

Star dunes form when:
-____ dominant wind
- ___ sed supply

often compound dune morphologies

up to ___m tall

A

no dominant wind direction

high sed

300m!

64
Q

Why do we see loess deposits in the locations we do?

A

It forms at past ice sheet margins! Because past glaciers deposit lots of silt sized material, so there’s a high supply of silt in these areas

note till= silt and cobbles!

65
Q

_____winds from glaciers/ icesheets (cold–>warm) flow over glaciocustrine + proglacial fluvial + tills with silt and redistribute the silt in suspension

A

katabatic

66
Q

Loess deposition:

  • continuous or not?
  • loess= ___(good/bad) paleoenvironment record
A

semi-continuous

excellent paleoenvironment record (volcanic ash, pollens, fossils) –> even better record if there is also permafrost

67
Q
A

EAS 225 (Earth Surface/ Landforms) (13 decks)

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