L8: Cracking and ice-wedge polygons

Question 1

What do the majority of processes involved in sorting revolve around?

Answer 1

An inclined freezing front.

Once you’ve got some relief, this has an impact of how it pushes material ahead of it and the clasts rising up.

Beneath cracks, freezing front penetrates quicker (air gets in quicker), coarser clasts pull will be rising up.

Ahead of the freezing front = movement of fine soil will be falling down.

Question 2

Where do inclined freezing front penetrate faster?

Answer 2

Beneath cracks, freezing front penetrates quicker (air gets in quicker), coarser clasts pull will be rising up because of thermal conductivity

Question 3

Where do inclined freezing front penetrate slower?

Answer 3

Where there is heterogenous material and some it is frost susceptible without clasts.

Lower thermal conductivity and growth of segregation ice means release of latent heat.

Question 4

Explain the processes in a double convection cell…

Answer 4

In winter, once some sorting is occurring, some material is going to be more frost susceptible. This means the growth of segregation ice = HEAVING in winter FREEZEBACK.
Any clasts on the surface fall down the ‘sides’.

Question 5

What is an ice wedge polygon?

Answer 5

An ice filled crack.

The crack forms during the very cold parts of winter when the soil between the cracks is shrinking (-10oC).

These cracks slightly open until spring snow melts.

The water from the snow runs into the cracks and freezes and slightly expands = causes soil to be pushed away.

More and more water comes in ice wedge continues to expand.

Question 6

Permafrost definition

Answer 6

Any earth material below ground which is at or below 0 degree C for two or more years.

Question 7

What is the active layer?

Answer 7

The portion of the ground that is affected by air temperatures (half a metre).
Thaws every summer, refreezes every winter.

Question 8

What is thermokarst?

Answer 8

Thermokarst is a land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws.

Question 9

What are non sorted circles?

Answer 9

You have barren patches surrounded by vegetation every winter

It gets cold so the ground freezes and vegetation slows the freezing down but bear patches freeze first.

What happens is you get water being attracted by that freezing (into ice wedges).

These rise every winter and in summer they collapse again.

This process (frost heave) prevents the roots from surviving in the centre.

Question 10

When wet soil freezes, it expands by 9% as phase change occurs, so how does thermal contraction occur with cooling?

Answer 10

By a coefficient of either expansion or contraction (summer vs winter).

This a function of the change in length of the crack and the temperature change as it warms up or cools.

Question 11

What does the amount of cracking depend on?

Answer 11

The nature of the material.

1) Ice and unfrozen water content
2) Mineralogy
3) Organic content
4) Texture
5) Thermal history

Question 12

Things crack when…

Have to have…

Answer 12

Stress > strength

RAPID cooling to prevent internal deformation (creep).

Question 13

Traditional ice wedge model

Answer 13

1. Narrow crack opens up- possibly through contraction.
2. Water fills crack, washing in fine sediment.
3. Water freezes, expanding to widen the wedge.
4. As wedge expands, adjoining sediment is buckled up

But…how can ice growing in a crack push?

Question 14

What are epigenetic ice wedges?

Answer 14

Growing into material that is already there.

These are younger than the sediments and usually are found on stable ground surfaces.

Get wider but not much deeper.

Question 15

What are syngenetic ice wedges?

Answer 15

Need to be in an area of aggregating permafrost.

Relate to changing surface conditions, with the growth of wedges parallel to accumulating sediments (tends to be episodic).

Less common.

Question 16

What are anti-syngenetic ice wedges?

Answer 16

Associated with eroding or retreating surfaces.

Wedge burrows downwards to keep pace with surface denudation.

Question 17

Where do orthogonal polygons form?

Answer 17

Best developed in heterogenous sediments where primary cracks develop randomly, and then secondary cracks progressively divide the area.

Question 18

Where do hexagonal polygons form?

Answer 18

Best developed in homogenous sediment with the frost cracks occurring more less simultaneously.

Question 19

Degradation of active low centred polygons to form inactive high centred polygons

Answer 19

Initial low centred polygon shows intact rims adjacent to troughs and ice wedges.

During the onset of thermokarst, troughs subside atop melting ice wedges as soil is shed into them and new peat accumulates by plant growth.

Erosion of polygon rims continues even as an insulative mat of soil and plant material limits further ice wedge thaw.

Question 20

Describe the external form of earth hummocks

Answer 20

Domed form
Thick vegetation mat
Grouped in fields
Show asymmetry if on slope (front steeper than back)
Height: width ratios suggest a maximum size for mature hummocks

Question 21

Describe the internal form of earth hummocks

Answer 21

Contain organic material, often an Ob horizon.

Show a range of cryoturbation features (including involutions, flame structures, diapirs).

Clear base of disturbance.

Horizons may be traced between adjacent hummocks.

Evidence of sorting fine material.

Question 22

Indirection evidence of external differential heave

Answer 22

Width: depth ratios

Domed form

Common features

Question 23

Indirection evidence of external flow during thaw

Answer 23

Implicated in maximum size

Question 24

Indirection evidence of external mass movement

Answer 24

Asymmetry, with steeper downslope side.

Question 25

Indirection evidence of external erosion

Answer 25

Erosional scars and surface ruptures.

Water channels in troughs

Question 26

Indirection evidence of internal differential heave

Answer 26

Cryoturbation features

Diapirs, sorting and injections

Question 27

Indirection evidence of internal flow during thaw

Answer 27

Possible liquefied flow features

Question 28

Indirection evidence of internal mass movement

Answer 28

Asymmetry in internal structures, especially downslope organic concentrations.

Question 29

Indirection evidence of internal erosion

Answer 29

Discontinuities in internal horizons

Question 30

Indirection evidence of micro differential heave

Answer 30

Lamellar structures
Silt capping
Preferred orientations
Shattered minerals

Question 31

Indirection evidence of micro flow during thaw

Answer 31

Vesicles

Question 32

Indirection evidence of micro mass movement

Answer 32

Preferred orientation of particles downslope.

Question 33

Main findings in terms of freeze thaw has done stuff to micromorphology of material

Answer 33

Main findings:

1. Silt cappings (with rotation)
2. Lamellar structure
3. Vesicles
4. Desiccation cracks
5. Preferred orientation of elongated grains.

Question 34

How does altitude influence the process of differential heave?

Answer 34

Determining freezing index and rate of freeze back.

Why is that important? In terms of differential heave to allow that growth of segregation ice.

Question 35

How does altitude influence the process of flow during thaw?

Answer 35

Determining freezing index and rate of freeze thaw.

Question 36

How does altitude influence the process of mass movement?

Answer 36

Climate and gelifluction processes.

Question 37

Why is vegetation important in the production of earth hummocks?

Answer 37

Critical in helping to determine the differential, thermal and hydrological regimes with the hummock trough system.

Roots help stabilise heaved form allowing internal movement of material without collapse.

Question 38

How does vegetation influence the process of differential heave?

Answer 38

Root mat to maintain form.

Preferred location for ice lenses beneath root mat.

Differentiation of thermal properties.

Question 39

How does vegetation influence the process of flow during thaw?

Answer 39

Root mat to maintain surface form and allow flow beneath.

Question 40

How does vegetation influence the process of mass movement?

Answer 40

Root mat to maintain surface form and allow asymmetry

Question 41

How does vegetation influence the process of erosion?

Answer 41

Root mat to maintain surface form- allow ruptures where vegetation broken or species different.