Periglacial processes and featuresPresentation Transcript
Strange but true…The Odd Tale Of PeriglacialEnvironments
The key to understanding periglacial landforms lies in knowingall about the one thing these environments have that does notoccur anywhere else on Earth…….. permafrost
PingosAnd other odd mounds
Palsa, McMillan Pass, Yukon Territory.This peat palsa is exposed in a road cut along the Canol Highway in east-central Yukon. It is several metres in extent (notestanding figure in centre of feature). The growth of a lens of clear ice elevates roughly circular areas of peat bogs to formthese features in areas of extensive permafrost. The mountainous landscape in the background is part of the SelwynMountains
Rock burst, northern Manitoba.A rock burst occurs in permafrost terrain when hydraulic pressures, generated by the trapping of water in bedrockfissures during autumnal freezing, exceed the strength of the rock. This rock burst was a knob of Precambriangneiss that protruded above the till plains in the tundra of northernmost Manitoba
Patterned Ground Very very odd indeed…
Ice-wedge polygons in peatland, Hudson Bay Lowlands, Manitoba.Splendid examples of ice-wedge polygons, a form of patterned ground, are shown above. They occur in the permafrostpeatlands of the Hudson Bay Lowlands, which are composed mainly of dry sphagnum. Brown polygons mark the locationof massive ice wedges that extend from the surface down to 2 or 3 m
Easily seen from the air, this network of cracks spaced about 10 m apart is caused by wedges of ice which have grown inthe thick peat that covers the surface. They show that the mean annual air temperature is about 0 c and thus that thenorthern tip of newfoundland island lies in the zone of discontinuous permafrost.
Ice-wedge trough in peat near Steele Lake, Hudson Bay Lowlands, Manitoba.In this scene, a geologist is coring permafrost on the open tundra heath that is growing onsphagnum peat about 3 m thick. The deep gutters are the thawed tops of wedges of massive icethat have grown in the peat and form polygonal networks. The polygonal networks of ice wedgesare one class of patterned ground landforms
thermokarst Depressed & shallow
Ice-rich, frozen peatlands and thermokarst ponds, Seal River, Manitoba.This is an area of permafrost and ice-rich sphagnum peat. Numerous small ponds are thermokarst features that haveformed where ice in the peat has melted out. Drainage is generally poor in these flat peatlands, but in some cases, suchas in the bottom right of the photo, waterways serve as inter-connecting links between the ponds
Ice-wedge polygons and thermokarst ponds in peatlands, Hudson Bay Lowlands, Manitoba.This scene in the Hudson Bay Lowlands shows a mozaic of dry icy peat that is characterized by ice wedge polygons,together with shallow thermokarst ponds and lakes at sites where ice has melted out. Lakes vary in size from severalmetres to several hundred metres. This landscape suggests that ground ice in the peat is melting out faster than newground ice is forming
Ice-wedge polygons and thermokarst ponds in peatlands, Hudson Bay Lowlands, Manitoba.This is a scene from the Hudson Bay Lowlands, shows an exceptional overview of patterned ground north of treeline, in anarea covered with peatland permafrost. The "crackled" pattern is produced by ice wedges in the sphagnum peat. Shallowponds called thermokarst depressions, have formed at sites where ground ice has melted out
Circular thermokarst ponds in peatlands, Hudson Bay Lowlands, Manitoba.Large, circlular, shallow thermokarst ponds in peatlands of the Hudson Bay Lowlands are formed where the peat hasthawed recently and ground ice has melted out. Their circular shape is an equilibrium configuration resulting from theprogressive thawing and collapse of peat around a body of massive ice that was initially irregular in shape. The thermokarstponds range in diameter from tens of metres to about a kilometre, but they are usually only about 1 m deep. Extensiveareas of flat peatland terrain characterize areas of northeastern Manitoba that were covered by postglacial lakes and seas.
Degrading ice-wedge polygons and themokarst peatlands, Hudson Bay Lowlands, Manitoba.This is a typical peatland landscape in the Hudson Bay Lowlands, characterized by the presence of numerous, shallowthermokarst ponds. Cracks, shown in left foreground, mark the location of ice wedges that are presently melting out.These features form the outline of the ice-wedge polygons that are common in this region.
Thermokarst ponds and drunken forest, Churchill, Manitoba.Thawing of ice-rich permafrost causes subsidence of the land surface, creating ponds and causing trees to tilt, as shown in thispeatland terrain.
Solifluction features Slip & slide
Active layer detachment slides in a burn along theMackenzie River (1996).
Solifluction lobes and terraces, Lewis Hills.Elevations above about 500 m have a peculiar smooth terrain composed of a mixture of glacial debris and frost rubblethat is shaped into tongues and festoons by slow downslope movement due to frost action and soil creep. Caribouherds frequent this terrain to browse the fields of lichen blown clear of snow
The huge mass of debris on the summit is composed of glacial till and weathered rubble. Frostaction and soil creep are slowly shifting it downslope, producing a steep leading edge frombeneath which rivulets are seeping
People &permafrost A challenging combination
Row of thermosyphons installed in a frozen core dam at the Ekati Diamond Mine, NWT, to ensure the thermalintegrity of the structure. Thermosyphons are heat exchangers that extract heat from the ground in the winter usinga two-phase convective process and transfer it to the air above. They are in use throughout the North for a variety ofinfrastructures. In Yellowknife, for example, they have been used beneath paved roads and parking areas.
This cross-section, from asite located on a peatplateau south of FortSimpson, shows theincrease in thaw depth, andthe associated groundsurface settlement and pipesettlement that has occurred12 years followingconstruction.