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3b. Periglacial landforms exist as a result of climate change before…
3b. Periglacial landforms exist as a result of climate change before and/or after glacial periods
Modifications of these landforms by processes associated with present and future climate changes
Patterned ground
As temperatures rose at the end of the periglacial period, patterned ground was often colonised by vegetation, making it hard to find and identify
Over time mass movement by creep also degrades the frost heaved domes, making the landform less obvious
Pattered ground around Leedon Tor, Dartmoor is now mainly covered by a layer of soil and grasses
Pingos
When temperatures rise the ice core thaws pingos collapse
The top of the dome collapses leaving a rampart surrounding a circular depression called an ognip
Llanberis in North Wales
Periglacial landforms
Pingos
Rounded ice-cored hills that can be as much as 90m in height and 800m in diameter
Open system
Form in valley bottoms
Water from the surrounding slopes collects under gravity, freezes and expands under artesian pressure
The overlying surface material is forced to dome upwards
Closed system
Develop beneath lake beds where the supply of water is from the immediate local area
As permafrost grows during cold periods, groundwater beneath the lake is trapped by the permafrost below and the frozen lake above
The saturated talik is compressed by the expanding ice around it and is under hydrostatic pressure
When the talik freezes it forces up the overlying sediment
Mackenzie delta of Canada
Patterned ground
Barrow Alaska. On slope angles of 3-50 degrees, the larger stones move greater distances downslope and the polygons become elongated into stone garlands
On slopes of 60 degrees and over, the polygons lose their shape and stone stripes develop
A result of frost heave
Periglacial landforms are a feature of current periglacial environments, but they are also fossil features, widespread in more temperate regions today (Lake District)
Climate change and the effects on geomorphic processes
Freeze thaw weathering is a dominant process in periglacial environments, due to seasonal fluctuations in temperature around freezing
Frost heave
As temperatures fall, water beneath the stones freezes and expands, pushing the stones upwards to the surface
Ground ice also pushes overlying, finer material upwards, producing a domed surface
Sub surface process that leads to a vertical sorting of material in the active layer.
Ground ice development
During summer melting periods, water percolates into the sub-surface geology where it accumulates below the water table
During the sub-zero winter months this water freezes and expands by 10% of its volume
As this expansion occurs, the ground surface is pushed upwards, as its unable to extend downwards into the permafrost below