Geography Reference
In-Depth Information
complex mechanisms including molecular attraction which is more
powerful than the forces holding the water within the pore spaces.
This then leaves relatively empty pore spaces near the segregated
ice which become filled by meltwater seeping into the soil from
above during the summer through suction processes, as described
in the section on soils in Chapter 3. This means the segregated ice
can slowly grow.
Frost cracking can occur by fracturing the ground as it contracts
at very cold temperatures. While there is expansion when water
freezes, as the temperature gets very low, ice and sediment contract
in volume. This is probably a major factor in the formation of
many polygonal crack features seen in the periglacial landscape.
Frost crack polygons are usually 5 to 30 metres across and develop
best when there is no insulating snow cover. These features cover
large areas in North America and Siberia. Water entering the crack
can later freeze and then these ice films expand over several years
of seasonal melting and freezing to become ice wedges . These
ground ice features can be 4 metres deep and 2 metres wide
forming a V-shaped ice wedge in the enlarged crack. Therefore,
vertical ice wedges in the sediment often accompany patterned
frost crack polygons on the surface.
In addition to crack polygons there are often regular geometric
patterns of stones or topography in periglacial areas. These can be
grouped into circles, nets, polygons, steps and stripes. These are
amazing features in the landscape and almost look like humans have
made patterns by sorting stones and vegetation into neat shapes.
Circles, nets and polygons are common on flat surfaces whereas
steps and stripes occur on slopes between 5° and 30°. On steeper
slopes, mass movement becomes a more dominant process destroy-
ing patterned features. Sorted stone circles typically have fine mater-
ial in the centre of an area of lowered relief and larger sediment
forms a higher perimeter. Sorted stripes look rather like a recently
ploughed field with ridges and furrows consisting of alternating
stripes of coarse and finer material. There are several mechanisms
hypothesised for these features linked to heave and thrust processes
and the possibility of mini convection circulation cells operating
within the sediment near the surface. In the daytime, especially in
summer, saturated soil near the surface warms while the soil below
remains frozen. Water at the surface (at say 1° or 2°C) is slightly
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