Geography Reference
In-Depth Information
more dense than the thawing water at 0°C below (water is densest
at 4°C). Thus the more dense water sinks and forces the less dense
water upwards. Therefore, a small convection cell can form (think
of the ocean or atmospheric circulation cells described in Chapter
1). It is thought that the edges of the convection cell are associated
with the surface sorted features as soil particles may move with the
water. However, the exact processes are not yet clear.
At a larger scale,
pingos
can form. These are ice-cored mounds
up to 60 metres high and 500 metres in length. The mounds
contain some segregated ice and a lens of massive ice. The top of
the mound often becomes cracked as the ice core within the pingo
grows larger, forcing the ground surface upwards. Hydrostatic
pingos are caused by the doming of frozen ground as a result of the
freezing of water and the growth of permafrost beneath a former
lake or other water body. The features are usually isolated land-
forms predominantly in areas of low relief. Pingos that form over
drained lakes are usually circular in shape, whereas pingos over old
river channels may be linear in form. Hydraulic pingos form most
commonly in discontinuous permafrost regions at the foot of slopes
and are usually circular or elliptical in shape, and result from the
inflow and freezing of groundwater seeping from upslope.
If segregated ice melts at any point then there can be a large
volume of excess water and subsidence. This can lead to a terrain
which is almost impassable, especially in summer, with lots of
depressions, many of which are filled with water. This terrain, con-
sisting of small irregularly shaped thaw lakes and depressions, is
called
thermokarst
.
Periglacial mass movement on slopes can result in landforms
such as
protalus ramparts
which are linear mounds of coarse sed-
iment that form a small distance from the base of a slope. When a
rockfall occurs boulders may slide across snow at the foot of the
slope coming to rest just beyond the edge of the snow.
Ploughing
boulders
move slowly downslope, leaving a depression on the
hillslope indicating its path and forming a small bulge of sediment
downslope of the boulder. The movement of the boulders, typic-
ally a few millimetres per year, is thought to occur because of the
different thermal conditions beneath the boulder compared with its
surroundings. Larger forms of ploughing boulders occur when a
whole mass of rock and sediment moves downslope, a little like a
