Aeolian

Question 1

Aeolian processes=

Answer 1

processes and landforms related to the action of wind

Question 2

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?

Answer 2

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!

Question 3

___ is the most turbulent medium

Answer 3

air

Question 4

t/f
erosivity has a linear relation with wind velocity

Answer 4

false

exponential!

Question 5

What are the 4 main controls on wind erosion?

Answer 5

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

Question 6

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

Equation: E= V^3p

Answer 6

20/10= 2
2^3= 8

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

Question 7

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

Answer 7

• 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

Question 8

What effect does surface cover have on wind erosion?

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

What is the zone of little or no wind called?

Answer 11

The laminar sublayer (or the boundary layer)

Question 12

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

What about in vegetated areas?

Answer 12

Exposed bed= 0.5-1.0x grain diameter

Vegetated areas= height of the continuous vegetative cover

Question 13

What does the Shield’s curve tell us?

Answer 13

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

Question 14

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

Answer 14

• 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

Question 15

If sediments are wet, can wind easily entrain them?

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

Answer 15

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

bed
suspended

Question 16

Shear stress threshold velocity=

Answer 16

force it takes to entrain a grain

Question 17

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 ____

Answer 17

deposition

transport (entrainment)

erosion

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

1. traction
2. saltation
3. suspension

Question 21

Describe the process of traction and how it moves sediment

Answer 21

rolling/ traction/ creep

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

note drag= force exerted from fluid flowing past the grain

Question 22

Describe the process of saltation and how it moves sediment

Answer 22

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

Question 23

Describe the process of suspension and how it moves sediment

Answer 23

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

Question 24

What are the 5 main erosional aeolian landforms?

Answer 24

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

Question 25

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

Answer 25

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

Question 26

Which 2 landforms are created by deflation?

Answer 26

desert pavement/ varnish and deflation hollows

Question 27

Which 3 landforms are created by abrasion?

Answer 27

yardangs, ventifacts, rock trees

Question 28

Desert pavement=

Answer 28

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

Question 29

desert varnish=

Answer 29

abrasion of desert pavement over long time frames (polishing) common texture on mars and antarctica!

Question 30

Deflation Hollow (blowout)=

Answer 30

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

Question 31

What will stop deflation hollows from continuing?

Answer 31

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

Question 32

Abrasion is also known as ___ (mechanical weathering) =

Answer 32

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

Question 33

yardings=

Answer 33

long linear features formed by abrasion - form where wind is constant direction over recessive sediment like sandstone (100s-1000s of years to form!)

Question 34

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

Answer 34

asymmetrical vast

Question 35

ventifacts=

Answer 35

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

Question 36

Faceted face of ventifacts always in ___ direction

Answer 36

upwind ie angled faces form on the windward face

Question 37

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

Answer 37

changing rounded angular

Question 38

Rock trees form when:

Answer 38

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

Question 39

List ~9 main depositional aeolian landforms.

Answer 39

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

Question 40

desert=

Answer 40

barren area with little precipitation

Question 41

ergs=

Answer 41

region with wind-born sand deposits

Question 42

loess=

Answer 42

region with wind-borne silt deposits

Question 43

Ergs include: (4)

Answer 43

sand seas, sand sheets, ripples, and dunes

Question 44

Loess accumulates in massive ___

Answer 44

sheets

Question 45

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

Answer 45

<2cm increases

Question 46

Individual dunes are __-__m tall, and they are hills of loose ___. Dunes are formed with __ velocity than ripples and require t____

Answer 46

1-10m sand (fine-med) higher turbulence

Question 47

What are compound/ complex dunes?

Answer 47

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

Question 48

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

Answer 48

false they're huge (up to many km)

Question 49

Describe how dune formation is initiated

Answer 49

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)

Question 50

Stoss side of a dune= ___ via saltation and traction Lee side of a dune= ___ at angle of repose

Answer 50

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)

Question 51

List 4-7 important variables controlling dune morphology:

Answer 51

- wind direction - wind direction consistency - surface cover (veg) - grain size - topography - sediment supply - wind speed

Question 52

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

Answer 52

parallel high low low

Question 53

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

Answer 53

perpendicular unidirectional winds high no veg to obstruct formation

Question 54

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

Answer 54

lake stable

Question 55

What type of dune is the "classical desert dune"?

Answer 55

Transverse

Question 56

Barchan dunes form at: - ___ wind intensity - ___sed supply - ___ veg The horn shape points ___

Answer 56

high moderate low downwind

Question 57

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

Answer 57

meld together barchan dunes and transverse dunes

Question 58

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

Answer 58

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

Question 59

Parabolic dunes form at: - ___ veg - ___ sediment supply - ___ wind intensity Their horns point ____ and are sheltered by ____

Answer 59

moderate mod-high medium upwind vegetation

Question 60

Deflation hollows form on ____ dunes in ___ the direction

Answer 60

parabolic dunes upwind side

Question 61

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

Answer 61

true they can even have little ponds which slows the blowout

Question 62

How can you distinguish barchan vs parabolic dunes?

Answer 62

look for vegetation and water (=parabolic) barchan will not have veg or water

Question 63

Star dunes form when: -____ dominant wind - ___ sed supply often compound dune morphologies up to ___m tall

Answer 63

no dominant wind direction high sed 300m!

Question 64

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

Answer 64

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!

Question 65

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

Answer 65

katabatic

Question 66

Loess deposition: - continuous or not? - loess= ___(good/bad) paleoenvironment record

Answer 66

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