Life-cycles of pingos, palsas and patterned ground. Flashcards
Pingos
What are pingos?
(Mackay, 1998)
Pingos can be defined as ‘perennial, intrapermafrost, ice-cored hills, typically conical in shape, that can grow and persist only in a permafrost environment’ and contain a massive ice core (ground ice)
Pingos
Where are pingos commonly found?
(AMAP, 2011)
In periglacial areas that are characterised by continuous and discontinuous permafrost and a seasonally changing active layer.
Pingos
Pingos can be classified on the basis of origin (genesis) into what 3 types?
Hydrostatic (formerly ‘closed system’)
Hydraulic (formerly ‘open system’)
Polygenetic
Pingos
Pingos form through what?
They develop through pressurized groundwater flow mechanisms to form massive ice-cored mounds which are covered by an overburden of 1-10m (Burr et al., 2009).
Pingos
How do Hydrostatic Pingos form?
Closed-system pingos, as in the Mackenzie Delta region, typically form in recently drained lake basins or old drainage channels, and are the result of hydrostatic pressures that develop as unfrozen saturated sediment progressively freezes.
Pingos
What are hydraulic pingos?
Open system pingos that occur in areas of discontinuous permafrost where there are interspersed areas of permafrost (land frozen for at least 2 years) and talik.
Pingos
How do hydraulic pingos form?
The active layer continually freezes and melts year on year above the permafrost and talik.
Over winter, as the active layer freezes down over water can become trapped between the descending freezing plane of the active layer and the permafrost that surrounds it.
This promotes the growth of an ice lens which pushes the land up above it as it expands.
Water underneath the permafrost can move through the talik between the permafrost areas because of capillary action (the movement of water through the soil because of ) and hydraulic pressure.
This water migrates to the ice lens and freezes, swelling the ground above further.
Pingos
Hydraulic Pingos require a long-term balance between three factors:
(French 2007):
- Water Pressure
- Overburden strength
- Rate of Freezing
Pingos
How do Pingos Decay?
This usually occurs through the rupture of the thermally protective overburden (Gurney 1998).
The collapse of pingos leaves characteristic signs in a landscape, including ramparts and shallow rimmed depressions (French 2007).
Pingo Scars can form due to slumping down the side of the pingo (Burr et al., 2009).
Patterned ground
What is Patterned Ground?
What are the 5 main types?
Patterned ground is terrain exhibiting surface patterning.
- Circles
- Polygons
- Nets
- Steps
- Stripes
(Washburn, 1956)
Patterned ground
What are the two types of patterned ground?
Sorted patterns delimited by alternating soil and clasts.
Nonsorted patterns defined by microrelief or alternating vegetated and unvegetated ground.
Patterned ground
How do most patterns form?
Most patterns form through recurrent freezing and thawing of moist soil in periglacial environments.
Patterned ground
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)
Patterned ground
How do larger sorted patterns form?
Larger sorted patterns are probably produced by a combination of differential (annual) frost heave and buoyancy-driven soil circulation during thaw.
An inclined freezing front penetrates fastest beneath undulations/ cracks/ variations in texture, causing any coarser clasts to be “pulled up” due to thermal conductivity.
Once that differentiation, occurs it becomes exacerbated (positive feedback).
As this happens time and time again you get a plug of frost susceptible material.
Eventually, in the thaw it will rise in soil circulation.
Patterned ground
What’s a difference between large and small patterns?
Small sorted patterns reflect shallow soil freezing, but large sorted patterns are often associated with permafrost.
Ice wedge polygons
What are they?
Ice-wedge polygons are archetypal polygonal patterns that are feasibly the most ubiquitous feature of ground ice in the top 2–3 metres of this permafrost (Christiansen et al., 2016), particularly in areas where the permafrost is shallow and the topography is flat (Cable et al., 2016., Frost et al., 2018).
Ice wedge polygons
How are they formed?
The thermal contraction theory of wedge growth, suggested by Leffingwell (1915., Gell, 1978), states that ice wedges are produced as an outcome of soil between cracks in the ice shrinking and enabling the infiltration of meltwater in spring (Fortier and Allard 2005., Opel et al., 2018). This requires severe ground frost in winter, predominantly enabled via a combination of low ambient temperatures (at least -10oC) and a thin snow cover (Wolter et al., 2018).
An inclined freezing front penetrates fastest beneath these cracks, causing any coarser clasts to be “pulled up” due to thermal conductivity. Over centuries to millennia of repeating this process, it eventually leads to the growth of a wedge-shaped ice body as the soil above the ice is pushed up, forming ridges (Mackay 1984).
Ice wedge polygons
Ice wedge polygons are categorically divided into two types:
High and low-centre, based on their microrelief and their relationship with vegetation (Mackay 2000., Steedman, 2014).
Low-centre polygons are outlined by peaty elevated ridges, which reduce water flow, with an (often pond filled) depression in the polygon centre.
High-centre polygons, considered to be suggestive of earlier ice-wedge degradation (Mackay, 2000), are outlined by subsided troughs, which enable efficient drainage of water, overlying the ice wedges and have elevated centres (Steedman, 2014).
Patterned ground
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.
Patterned ground
In general how do non sorted circles and stripes form?
These circular forms of patterned ground are likely the result of cryoturbation.
Typically, the are composed of fine grained, frost susceptible, soil.
The upward displacement of material is caused by the freeze thaw of ice lenses at the top and bottom of the active layer, with a gravity induced cell like movement (Mackay 1979).
Palsas
What is a palsa?
A palsa is a peaty permafrost mound between 1m and 7m in height and less than 100m in diameter containing a core of alternating layers of segregated ice and peat or mineral soil material (ACGR, 1988)
Palsas
Where do Palsas form?
Usually occur in bogs and wetlands
Most documented palsas occur in Finland, Sweden, Norway, Iceland and Canada.
Palsas
What are the 3 hypotheses for Palsa genesis
Snow removal hypotheses
Vegetation change hypotheses
Buoyancy hypothesis
Palsas
What is the Snow Removal hypothesis?
Snow is a good insulator.
Snow removal leads to deep frost penetration.
Snow on the flanks of the palsa retard permafrost in the mire.
Palsas
What is the Vegetation change hypothesis?
Railton and Sparling 1973
An increase in albedo reduced summer heat influx preserving the frozen core
Some see this as a consequence rather than a cause
Palsas
What is the buoyancy hypothesis?
Nelson et al 1992
Parcels of frozen peat have a lower density and float
Peat dries out and insulates the core beneath.
Palsas
How do Palsas form?
The formation of a palsa begins when snow cover is locally so thin that winter frost penetrates sufficiently deeply to prevent summer heat from thawing it completely. The surface of the mire is then raised somewhat by frost processes.
During succeeding winters frost penetrates still deeper, the process of formation accelerates and the hump shows further heaving due to the freezing of pore water and ice segregation. As the surface rises, the wind becomes ever more effective in drying the surface peat and keeping it clear of snow.
Palsas
Case study: Palsas
Where?
Northern Quebec around the Boniface River
Circumpolar discontinuous zone
Mean annual air temperature -7oC
What?
Palsa- organic or mineral soil mounds with a permafrost core.
Diameter 20-70m…height 2-9m
Vegetated with lichens and dwarf shrubs, mosses some bare patches.
Uneven topography and variable depth of organic layer, snow depth and tree height.
Findings…
Snow cover had a lesser impact than might be expected because of? (time of snow melt, other more dominating factors such as vegetation)
Palsas
When does degredation begin to occur?
When the freezing of the palsa core reaches the till or silt layers at the base of the mire, the mature stage of palsa development begins. By this time the palsa stands well above the surface of the mire, displaying a relief of up to 7 m in western Finnish Lapland.
Degradation now starts, and peat blocks from the edges of the palsa collapse along open cracks into the pools which often surround the hummocks. During later stages, the vegetation may be removed so that the palsa surface is exposed to deflation and rain erosion.
Old palsas are partially destroyed by thermokarst, and become scarred by pits and collapse forms. Dead palsas are unfrozen remnants: either low (0.5 to 2 m high) circular rim ridges; or rounded open ponds and pond groups; or open peat surfaces without vegetation.
From such pools a new palsa may ultimately emerge after a renewed phase of peat formation, and the cycle of palsa development recommences from the beginning.
