L8: Cracking and ice-wedge polygons Flashcards
What do the majority of processes involved in sorting revolve around?
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.
Where do inclined freezing front penetrate faster?
Beneath cracks, freezing front penetrates quicker (air gets in quicker), coarser clasts pull will be rising up because of thermal conductivity
Where do inclined freezing front penetrate slower?
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.
Explain the processes in a double convection cell…
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’.
What is an ice wedge polygon?
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.
Permafrost definition
Any earth material below ground which is at or below 0 degree C for two or more years.
What is the active layer?
The portion of the ground that is affected by air temperatures (half a metre).
Thaws every summer, refreezes every winter.
What is thermokarst?
Thermokarst is a land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws.
What are non sorted circles?
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.
When wet soil freezes, it expands by 9% as phase change occurs, so how does thermal contraction occur with cooling?
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.
What does the amount of cracking depend on?
The nature of the material.
1) Ice and unfrozen water content
2) Mineralogy
3) Organic content
4) Texture
5) Thermal history
Things crack when…
Have to have…
Stress > strength
RAPID cooling to prevent internal deformation (creep).
Traditional ice wedge model
- Narrow crack opens up- possibly through contraction.
- Water fills crack, washing in fine sediment.
- Water freezes, expanding to widen the wedge.
- As wedge expands, adjoining sediment is buckled up
But…how can ice growing in a crack push?
What are epigenetic ice wedges?
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.
What are syngenetic ice wedges?
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.
What are anti-syngenetic ice wedges?
Associated with eroding or retreating surfaces.
Wedge burrows downwards to keep pace with surface denudation.
Where do orthogonal polygons form?
Best developed in heterogenous sediments where primary cracks develop randomly, and then secondary cracks progressively divide the area.
Where do hexagonal polygons form?
Best developed in homogenous sediment with the frost cracks occurring more less simultaneously.
Degradation of active low centred polygons to form inactive high centred polygons
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.
Describe the external form of earth hummocks
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
Describe the internal form of earth hummocks
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.
Indirection evidence of external differential heave
Width: depth ratios
Domed form
Common features
Indirection evidence of external flow during thaw
Implicated in maximum size
Indirection evidence of external mass movement
Asymmetry, with steeper downslope side.
Indirection evidence of external erosion
Erosional scars and surface ruptures.
Water channels in troughs
Indirection evidence of internal differential heave
Cryoturbation features
Diapirs, sorting and injections
Indirection evidence of internal flow during thaw
Possible liquefied flow features
Indirection evidence of internal mass movement
Asymmetry in internal structures, especially downslope organic concentrations.
Indirection evidence of internal erosion
Discontinuities in internal horizons
Indirection evidence of micro differential heave
Lamellar structures
Silt capping
Preferred orientations
Shattered minerals
Indirection evidence of micro flow during thaw
Vesicles
Indirection evidence of micro mass movement
Preferred orientation of particles downslope.
Main findings in terms of freeze thaw has done stuff to micromorphology of material
Main findings:
- Silt cappings (with rotation)
- Lamellar structure
- Vesicles
- Desiccation cracks
- Preferred orientation of elongated grains.
How does altitude influence the process of differential heave?
Determining freezing index and rate of freeze back.
Why is that important? In terms of differential heave to allow that growth of segregation ice.
How does altitude influence the process of flow during thaw?
Determining freezing index and rate of freeze thaw.
How does altitude influence the process of mass movement?
Climate and gelifluction processes.
Why is vegetation important in the production of earth hummocks?
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.
How does vegetation influence the process of differential heave?
Root mat to maintain form.
Preferred location for ice lenses beneath root mat.
Differentiation of thermal properties.
How does vegetation influence the process of flow during thaw?
Root mat to maintain surface form and allow flow beneath.
How does vegetation influence the process of mass movement?
Root mat to maintain surface form and allow asymmetry
How does vegetation influence the process of erosion?
Root mat to maintain surface form- allow ruptures where vegetation broken or species different.
