Aeolian Flashcards
Aeolian processes=
processes and landforms related to the action of wind
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?
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!
___ is the most turbulent medium
air
t/f
erosivity has a linear relation with wind velocity
false
exponential!
What are the 4 main controls on wind erosion?
- wind velocity
- topography
- surface cover and roughness
- grain size
How would you calculate the erosivity change if the wind speed changed from 10km/hr to 20km/hr?
Equation: E= V^3p
20/10= 2
2^3= 8
8x more erosive if the wind speed increases from 10 to 20km/hr!
Explain the effect of topography on erosion in terms of:
- hills
- snow deposition on a hill
- 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
What effect does surface cover have on wind erosion?
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
How could you decrease erosion on a field with little vegetation?
Plant hedges!
Adding surface cover and roughness will decrease wind velocity and therefore decrease erosion (farmers do this)
Why is the height of surface cover (eg trees) so important in influencing erosion?
Because there is little to no wind erosion on a tall, well vegetated surface
- b/c the veg. slows down the wind
What is the zone of little or no wind called?
The laminar sublayer (or the boundary layer)
On an exposed bed, what is the laminar boundary layer height?
What about in vegetated areas?
Exposed bed= 0.5-1.0x grain diameter
Vegetated areas= height of the continuous vegetative cover
What does the Shield’s curve tell us?
grain size vs threshold velocity needed to entrain that certain grain size
List some differences in the Shields Curve when the medium is wind instead of water
- 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
If sediments are wet, can wind easily entrain them?
Wind has a higher component of __ load compared to ___ load
No! So wind moves much less if it’s not really dry
bed
suspended
Shear stress threshold velocity=
force it takes to entrain a grain
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 ____
deposition
transport (entrainment)
erosion
Why is it hard for wind to erode clay? What about larger material like boulders?
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
What are the 4 mechanisms at work in aeolian processes? Describe their forces
- gravity: downward force
- Friction: caused by roughness of surface
- Drag: forward force due to motion of fluid
- Lift: caused by Bernoulli principle: negative pressure behind the particle
There are 3 main ways that aeolian processes can transport sediments:
- traction
- saltation
- suspension
Describe the process of traction and how it moves sediment
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
Describe the process of saltation and how it moves sediment
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
Describe the process of suspension and how it moves sediment
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
What are the 5 main erosional aeolian landforms?
- desert pavement/ varnish
- deflation hollows
- yardangs
- ventifacts
- rock trees
What are the 2 key processes that form erosional aeolian landforms?
- abrasion= mechanical erosion of surfaces by wind blown sediment
- deflation= removal of fine grained sediments + net lowering of the surface
Which 2 landforms are created by deflation?
desert pavement/ varnish and deflation hollows
Which 3 landforms are created by abrasion?
yardangs, ventifacts, rock trees
Desert pavement=
wind erodes fine silts, deflation continues to remove finer particles until the surface is lowered and course clasts are left sticking out
desert varnish=
abrasion of desert pavement over long time frames (polishing)
common texture on mars and antarctica!
Deflation Hollow (blowout)=
shallow depressions that form in dunes by net erosion + removal of fines
What will stop deflation hollows from continuing?
their depth is limited by coarse lag gravel or the water table
Abrasion is also known as ___
(mechanical weathering)
=
sandblasting
= impact of entrained sand grains against rock surfaces and other grains
yardings=
long linear features formed by abrasion
- form where wind is constant direction over recessive sediment like sandstone
(100s-1000s of years to form!)
The cross section of yardangs is often ____, and they are found across ____ scales
asymmetrical
vast
ventifacts=
abrasion from multiple dominant wind directions over long time scales leads to polished facets (like carved jems!)
Faceted face of ventifacts always in ___ direction
upwind
ie angled faces form on the windward face
Ventifacts:
with ____direction of dominant winds, different facets merge along sharp ridges to transform ___ stones to ____ ventifacts
changing
rounded
angular
Rock trees form when:
there is a lot of wind, but in no dominant direction (like cyclonic)
- long term abrasion
List ~9 main depositional aeolian landforms.
- ergs
- loess
- sand sheets (basically ergs)
- transverse dunes
- barchan dunes
- barchanoid ridges
- parabolic dunes
- linear dunes
- star dunes
desert=
barren area with little precipitation
ergs=
region with wind-born sand deposits
loess=
region with wind-borne silt deposits
Ergs include: (4)
sand seas, sand sheets, ripples, and dunes
Loess accumulates in massive ___
sheets
Ripples are ___cm tall, and their height ___ with grain size and increasing velocity
<2cm
increases
Individual dunes are __-__m tall, and they are hills of loose ___.
Dunes are formed with __ velocity than ripples and require t____
1-10m
sand (fine-med)
higher
turbulence
What are compound/ complex dunes?
dunes on top of other dunes or yardings
up to 100s of m high!
t/f
sand sheets and loess deposits can only be 100s of m big
false
they’re huge (up to many km)
Describe how dune formation is initiated
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)
Stoss side of a dune= ___ via saltation and traction
Lee side of a dune= ___ at angle of repose
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)
List 4-7 important variables controlling dune morphology:
- wind direction
- wind direction consistency
- surface cover (veg)
- grain size
- topography
- sediment supply
- wind speed
Longitudinal dunes form ____ to the direction of the wind, with __ wind speeds, ___ sediment supply, and ___ vegetation
parallel
high
low
low
Transverse dunes form ____ to the direction of the wind
- high + persistent wind speeds (__directional)
- __ sed supply
- __ vegetation
perpendicular
unidirectional winds
high
no veg to obstruct formation
Changes of wind velocity where there are transverse dunes can form ___ between dunes. This implies a ___ landscape
lake
stable
What type of dune is the “classical desert dune”?
Transverse
Barchan dunes form at:
- ___ wind intensity
- ___sed supply
- ___ veg
The horn shape points ___
high
moderate
low
downwind
Barchanoid ridges form when the horns of barhcan dunes ___ ____. They form a mosaic landscape.
Intermediate between:
meld together
barchan dunes and transverse dunes
How would it be possible for barchan dunes, barchanoid ridges, and transverse dunes to exist on the same landscape, close to each other?
this happens because of local differences in wind intensities and sediment supply
Parabolic dunes form at:
- ___ veg
- ___ sediment supply
- ___ wind intensity
Their horns point ____ and are sheltered by ____
moderate
mod-high
medium
upwind
vegetation
Deflation hollows form on ____ dunes in ___ the direction
parabolic dunes
upwind side
T/F
parabolic dunes are usually stable on the landscape and don’t change quickly
true
they can even have little ponds which slows the blowout
How can you distinguish barchan vs parabolic dunes?
look for vegetation and water (=parabolic)
barchan will not have veg or water
Star dunes form when:
-____ dominant wind
- ___ sed supply
often compound dune morphologies
up to ___m tall
no dominant wind direction
high sed
300m!
Why do we see loess deposits in the locations we do?
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!
_____winds from glaciers/ icesheets (cold–>warm) flow over glaciocustrine + proglacial fluvial + tills with silt and redistribute the silt in suspension
katabatic
Loess deposition:
- continuous or not?
- loess= ___(good/bad) paleoenvironment record
semi-continuous
excellent paleoenvironment record (volcanic ash, pollens, fossils) –> even better record if there is also permafrost
