Patterned ground Flashcards
Patterned ground
What is Patterned Ground?
What are the 5 main types?
Ground showing patterns of stones, usually in the shape of polygons
- Circles
- Polygons
- Nets
- Steps
- Stripes
Flat -> steep
(Washburn, 1956)
How did Washburn categories the 5 patterned grounds?
Whether they were sorted or not (vegetated or not).
What is sorted vs non sorted?
Two types are distinguished: sorted patterns delimited by alternating soil and clasts, and nonsorted patterns defined by microrelief or alternating vegetated and unvegetated ground.
Where cracking is essential…
Where cracking is non essential…
heave is not dominant
heave is dominant.
Whats a difference between large and small patterns?
Small sorted patterns reflect shallow soil freezing, but large sorted patterns are often associated with permafrost.
How do larger sorted patterns form?
- when frost heave occurs the ground becomes slightly mounded
- An inclined freezing front penetrates fastest beneath undulations/ cracks/ variations in texture, causing any coarser clasts to be “pulled up” due to thermal conductivity.
- This causes the stone to roll down the side of it due to gravity
- As this happens to all the frost heaved stones they collect in circles around the bottom of the dome.
- Once that differentiation, occurs it becomes exacerbated (positive feedback) and becomes a network of polygons.
How do small sorted forms form?
Desiccation or seasonal frost cracking or a combination of the two is probably the initial cause.
Desiccation can occur for a number of reasons: Wind may promote evaporation from the ground, the ground may be subjected to a drainage change, during freezing and ice segregation.
Where small polygons form distinct nets it is likely that other processes such as differential frost heave and snowmelt erosion are also important ( Seppala 2005)
What is the basic idea behind sorting?
- Initiation…
Either through cracking, textural variations, or through snow and vegetation cover, you get the freezing front penetrating quicker in one place over another- higher thermal conductivity. - Frost susceptible material forms segregation ice and is heaved up with a buoyancy lift, causing any clasts to move laterally (outwards) due to gravity
- There is then a compensatory downward movement -> forms a double convection cell (circling).
- This grows and grows and leads to an accumulation of frost susceptible material (plug) further rising up with buoyancy
(Ballantyne, 2018).
What are sorted circles?
Fine material surrounded by coarse material.
Pattern width is related to the size of the clasts in the borders.
The heave in the winter disturbs roots of vegetation (unlikely to grow in fine material).
Frost action processes
Ice seg, frost heave, up-freezing objects, mass displacement, frost sorting, frost cracking
What is ice segregation?
Formation discrete layers/lenses as result of migration and subsequent freezing of PORE WATER
In General how do Earth hummocks form?
Their formation has been attributed to circulatory soil movement above depressions in the permafrost table, differential frost heave, and upward injection of soil through dilation cracks.
In general how do non sorted circles and stripes form ?
Nonsorted circles and stripes are thought to reflect circulatory soil movements in areas of alternating vegetated and vegetation-free ground in permafrost areas.
How do stripes form?
They probably reflect the same processes of cryoturbation that from circles but modified by the effects of slopewash and other mass- wasting processes.