periglacial landforms Flashcards
what is periglaciation?
cold environments in which ice is the dominant force but no glaciers are present
how are periglaciated areas defined more widely?
an area with:
-permafrost
-with some seasonal temp variation where the mean temp for at least some period in the summer rise above 0 degrees
-where freeze-thaw cycles dominate the landform creating processes
what % of periglaciated areas make up the world?
25%
what is permafrost?
the permanently frozen ground where soil temperatures have remained below 0 degrees for at least two years
what is continuous permafrost?
summers are so cold that there is only a very superficial surface melting of the ground
it has established to reach up to a depth of 1500 m
what is discontinuous permafrost?
found in slightly warmer areas
permanently frozen ground separated by unfrozen less cold areas
slightly warmer zones due to the proximity of surface water
what is the annual mean temperature of areas of discontinuous permafrost?
1 and -5 degrees
what is sporadic permafrost?
found when mean annual temperature is just below 0 degrees, and the summer temperatures reach several degrees above, bus isolated pockets of permanently frozen ground remain below the surface
what is the active layer?
summer temperatures sufficient to melt the surface layer of permafrost. This layer can be very mobile. It varies in thickness depending on the latitude and vegetation cover.
where can periglaciated environments be found?
in the northern hemisphere
above 66.5 degrees of latitude
what is the result of freeze thaw action in periglaciated areas?
screes develop at the foot of slopes as a result of frost shattering. On relatively flat areas, extensive spreads of angular boulders are left- knows as blockfield or felensmeer.
where does nivation take place in periglaciated areas?
beneath patches of snow in hollows, particularly on north-east facing slopes.
what happens during nivation in periglaciated areas and what does it form?
physical and chemical weathering operates under the snow, causing the underlying rick to disintegrate. As the snow melts in summer, the weathered particles are moved out downslope by the meltwater and SOLIFUCTION. Leads to formation of NIVATION HOLLOWS which can become CORRIES.
what is solifuction?
- when the active layer thaws in the summer, excessive lubrication reduces friction between soil particles
- this leads to solifluction sheets or lobes often forming terraces on the side of the valley
what is frost heave?
stones have a different specific heat capacity than finer sediment. stones under surface are pushed up through finer material which freezes and thaws until they eventually reach the surface where they fall down a mound forming patterns in the ground.
what is a pingo?
dome-shaped hill that is commonly up to 500m in diameter and up to 50m in height
where are pingos found?
common in periglacial areas that are characterised by permafrost and a seasonally changing active layer
what is at the core of a pingo?
an ice lens of varying size and the surface layer is made of soil often topped with vegetation
this surface can contain cracks as a result of ground swelling
where are closed systems pingos found?
typically to the Mackenzie River delta in Canada
closed system pingos stage one
-on low-lying areas where permafrost and continuous permafrost can be found
1. in this area of Canada there are hundreds of small lakes where water is trapped in small depressions during summer because the ground underneath is permanently frozen and effectively impermeable
2. over the winter, ground water underneath the lake sediments within the talk can be trapped by ice from the lake surface, as the lake freeze and advancing permafrost within the ground
close system pingos stage 2
- this decrease in temperature causes this groundwater to freeze into an ice lens, which grows over time as water freezes to the ice lens due to the increase in the hydrostatic presser
- this causes the sediment above to bulge upward into the characteristic pingo shape
what is hydrostatic pressure?
the pressure of fluid exerted by a confined fluid, such as water under or in a glacier
where are open system pingos formed?
occur in valley bottoms of discontinuous permafrost and talik where water form slopes collects under gravity and becomes trapped between the descending freezing plane is the active layer and the surrounding permafrost.
open system pingos formation.
water from slopes becomes trapped between the descending frozen layer and active layer and the surrounding permafrost. It freezes and expands under artesian pressure- promoting growth of an ice lens- pushes land above it as it expands. water underneath the permafrost can move through talik and the permafrost due to CAPILLARY ACTION and ARTESIAN PRESSURE causes water to migrates to lens and freezing pushing ground further upwards.
where are open system pingos common?
east greenland
what is capillary action?
movement of water through talik
what is artisian pressure?
draws moisture to the lens
what is a collapsed pingos called?
ognip
how can pingos rupture?
if they become too large and if the ground cracks it exposes the ice lens to the suns energy and atmosphere, this can cause them to melt then collapse
what happens after a pingo collapses?
it leaves a ruptured pingo with a depression and in the middle of that can fill with water and a lake, and a rampart around the edge that it price to the mass wasting process of solifluction
