Aeolian systems

Question 1

Define aeolian

Answer 1

Processes of erosion, entrainment, transportation, and deposition by the wind

Question 2

Why is it harder to move particles on land than it water?

Answer 2

The difference in density is greater

Question 3

How are grains on land held together?

Answer 3

Gravity, friction, vegetations, cements, moisture

Question 4

What forces promote entrainment of grains on land?

Answer 4

Ballistic impacts and aerodynamic lift

Question 5

What forces prevent entrainment of grains on land?

Answer 5

Weight, friction, drag, cohesive inter-particle forces

Question 6

How are sand grains primarily transported? What does their transport and deposition allow?

Answer 6

As a bedload
Allows the construction of dunes

Question 7

How are silt and clays primarily transported?

Answer 7

In suspension

Question 8

What is a deflation lag deposit?

Answer 8

Formed by the grains that are too heavy to move e.g. gravel

Question 9

Give the three ways (or combination of these ways) that sand can be carried as a bedload

Answer 9

Rolling/creep due to drag on the bed
Saltation
Reptation

Question 10

Describe saltation in the bedload transport of sand grains

Answer 10

Particles are temporarily lifted into the airflow, collide with other particles, introducing a chain reaction of interparticle collisions
This process is the most important

Question 11

Describe reptation in the bedload transport of sand grains

Answer 11

Saltating particles return to the bed and impacting other particles, causing them to hop short distances downwind

Question 12

Where are aeolian processes most active and widespread?

Answer 12

Arid, hot/cold deserts
These cover ~30% of land surface

Question 13

Why are grains in deserts so suseptible to aeolian processes?

Answer 13

They lack vegetation (to provide strength and cover) and moisture

Question 14

Describe the back-erg of a dunefield

Answer 14

Represents the trailing, upwind margin of the erg

Question 15

Describe the fore-wind of a dunefield

Answer 15

Represents the leading, downwind margin of the dunefield

Question 16

Define an erg

Answer 16

Wind-formed desert landscapes characterised by extensive sand dunes that cover the terrain

Question 17

Give three examples of geomorphic elements that can be associated with dunes

Answer 17

Zibars, sandsheets, interdunes

Question 18

What are the six types of sand dune?

Answer 18

Barchan, transverse, linear, star, dome, parabolic

Question 19

What is a stabilising aeolian system?

Answer 19

Develop where physical, chemical, or biogenic agents promote aeolian system construction and accumulation

Question 20

What are the three classifications of aeolian systems?

Answer 20

Dry, wet, stabilising

Question 21

Describe a dry aeolian system

Answer 21

Accumulation is controlled by aerodynamic configuration
Airflow deceleration causes sediment-laden winds to deposit some of their load

Question 22

Describe a wet aeolian system

Answer 22

Accumulation is controlled by progressive water-table rise concomitant with ongoing aeolian activity

Question 23

Where do siliciclastic coastal dunefield typically occur?

Answer 23

Along humid, mid-latitude coasts

Question 24

Where do carbonate dunefields typically form?

Answer 24

Preferentially along mid-low latitude coasts, esp those bordering productive shallow marine platforms (supply of carbonate sediment)

Question 25

How are aeolian processes inhibited in humid (inc. glacial regions) settings?

Answer 25

Vegetation and moisture

Question 26

What is aeolian loess?

Answer 26

A layer of fine, wind-blown silt and clay particles

Question 27

Where are loess deposits most typically found?

Answer 27

Areas that have experienced extensive glacial activity during past ice age Areas where strong winds carry fine grains from exposed surfaces (e.g. glacial outwash plains, river valleys, deserts)

Question 28

What impacts do dust/sand storms have?

Answer 28

Reduced visibilty, productivty, resp diseases Fertisation of oceans (stimulates growth of phytoplankton)

Question 29

What are the three scales of aeolian bedform?

Answer 29

Ripples, dunes, megadunes

Question 30

What are the three types of small-scale ripple structures that occur on wind-blown surfaces?

Answer 30

Impact ripples, megaripples, adhesion ripples

Question 31

What is the most widespread ripple type?

Answer 31

Wind ripples

Question 32

Describe wind ripples

Answer 32

Low relief and form preferentially form the coarser-grained fraction of sand in transport

Question 33

What are the three stages in the formation of wind ripples?

Answer 33

Initiation of development from perturbation on a bed Impact ripple spacing determined by saltation path length Upwind-facing stoss slope (subject to impact of infoming grains)

Question 34

What generates a grain flow sand avalanche?

Answer 34

Gravitational collapse of dry, loose sand on aeolian dune/megadune lee slopes inclined at the angle of repose (~32-36deg)

Question 35

What is an aeolian grain flow strata?

Answer 35

Thick package of multiple stacked aeolian grain flow avalanche deposits

Question 36

When does gravity-driven grain fall occur?

Answer 36

Where wind carrying clouds of saltating grains over a dune brink decelerates as it passes into the lee-side depression, resulting in a reduction in sediment carrying capacity

Question 37

Give another example of when gravity-driven grain fall can occur

Answer 37

On the apron of a dune where secondary cross-wind airflow is insufficient to form ripples

Question 38

Describe what happens when net aeolian sediment budget is positive

Answer 38

Accumulation occurs where bedforms may climb over one another at varying angles Sets and cosets of cross strata are produced

Question 39

Describe the sets of cross strata formed when bedforms climb over one another

Answer 39

Contain smaller-scale facies units of wind-ripple, grainflow, grainfall, and plane-bedded strata Therefore, dune elements are larger scale strata units

Question 40

Why is one bedform truncated by another?

Answer 40

They climb at low angles, the lower bedform is accumulated as the second overrides it

Question 41

Define aeolian interdunes

Answer 41

Elongated, relatively flat and sandy landforms that exist between larger sand dunes in desert and arid environments

Question 42

Describe a dry interdune

Answer 42

Water table is not in contact with depositional surface

Question 43

Describe a damp interdune

Answer 43

Capillary fringes of the water table are in contact with the depositional surface

Question 44

Describe a wet interdune

Answer 44

Water table is in contact with the depositional surface

Question 45

How do adhesion strata arise?

Answer 45

Arise from the adhesion of moving grains to a damp surface e.g. a damp interdune

Question 46

Describe adhesion strata

Answer 46

Low relief Sub-horizontal structures with irregular surfaces

Question 47

Describe megadunes (draas)

Answer 47

Support development of superimposed dune-scale bedforms

Question 48

Describe a compound draas

Answer 48

Where a smaller dune is superimposed on a larger dune of the same morphological type

Question 49

Describe a complex draas

Answer 49

Where a smaller dune is superimposed on a larger dune of a different morphological type

Question 50

What is an aeolian sand sheet?

Answer 50

Areas of sand accumulation characterised by flat areas of impact wind ripples Limited by sediment supply

Question 51

What is a zibar?

Answer 51

Low relief ridges and domes with gently inclined sloping surfaces

Question 52

What is required for the long-term preservation of aeolian sequences?

Answer 52

The strata is placed below a regional baseline, below which erosion does not occur

Question 53

What does subsidence allow?

Answer 53

It is the principal agent that generates accommodation and promotes preservation

Question 54

Give two examples of how aeolian strata can achieve long-term preservation

Answer 54

Accumulation occurs in a subsiding basin Accumulation occurs below the water table and is protected from deflation