Page 295 - Fundamentals of Geomorphology

278 PROCESS AND FORM

the altitudinal and latitudinal tree-lines: polar deserts layers that have stayed frozen between two successive
and semi-deserts, the High Arctic and ice-free areas of winters to frozen ground hundreds of metres thick and
Antarctica, tundra zones, boreal forest zones, and high thousands of years old. It develops where the depth
alpine periglacial zones, which extend in mid-latitudes of winter freezing is greater than the depth of sum-
and even low latitudes.The largest alpine periglacial zone mer thawing, so creating a zone of permanently frozen
is the Qinghai–Xizang (Tibet) Plateau of China. ground. Continuous and discontinuous permafrost
Periglacial environments characteristically experience zones are recognized (Figure 11.1). Some authors have
intense frosts during winter months and snow-free subdivided the zone of discontinuous permafrost into
ground during summer months. Four distinct climates two, three, or four subzones. In North America, a
produce such conditions – polar lowlands, subpolar low- tripartite sequence of widespread permafrost, sporadic
lands, mid-latitude lowlands, and highlands (Washburn permafrost, and isolated patches of permafrost is typical;
1979, 7–8): in Russia, massive island permafrost, islands permafrost,
and sporadic permafrost zones are common sequence
1 Polar lowland climates have a mean temperature of (Heginbottom 2002). A suprapermafrost layer, which
the coldest month less than 3 C. They are associated is the ground that lies above the permafrost table, tops
◦
with zones occupied by ice caps, bare rock surfaces, all types of permafrost. It consists of an active layer and
and tundra vegetation. an unfrozen layer or talik. The active layer is that part of
2 Subpolar lowland climates also have a mean tem- the suprapermafrost that melts during the day (in tem-
perature of the coldest month less than 3 C, but the perate and tropical regions) or during the spring thaw
◦
temperature of the warmest month exceeds 10 C. In (in high latitudes) (Figure 11.2). The depth of the active
◦
the Northern Hemisphere, the 10 C isotherm for the layer varies from about 10 cm to 15 m. In the con-
◦
warmest month sits roughly at the latitudinal tree- tinuous permafrost zone, the active layer usually sits
line, and subpolar lowland climates are associated directly upon the permafrost table. In the discontin-
with the northern boreal forests. uous permafrost zone, the active layer may not reach
3 Mid-latitude lowland climates have a mean tem- the permafrost table and the permafrost itself consists
perature of the coldest month less than 3 C, but the of patches of ice. Lying within, below, or sometimes
◦
mean temperature is more than 10 C for at least four above the permafrost are taliks, which are unfrozen areas
◦
months of the year. of irregular shapes. In the discontinuous permafrost,
4 Highland climates are cold owing to high elevation. chimney-like taliks may puncture the frozen ground.
They vary considerably over short distances owing Closed taliks are completely engulfed by frozen ground,
to aspect. Daily temperature changes tend to be while open taliks are connected with the active layer.
great. Open taliks normally occur near lakes and other bodies
of standing water, which provide a source of heat. Closed
taliks result from lake drainage, past climates, and other
Permafrost
reasons.
Continuous and discontinuous zones of permanently As well as occurring in Arctic and Antarctic regions
frozen ground, which is known as permafrost, currently (polar or latitudinal permafrost), permafrost also
underlie some 25 per cent of the Earth’s land surface. occurs in the alpine zone (mountain permafrost), on
Permafrost may be defined as soil or rock that remains some plateaux (plateau permafrost), and under some
frozen for two or more consecutive years. It is not the seas (marine permafrost) (Figure 11.1).
same as frozen ground, as depressed freezing points allow
some materials to stay unfrozen below 0 C and con- Ground ice
◦
siderable amounts of liquid water may exist in frozen
ground. Permafrost underlies large areas of the Northern Ground ice is ice in frozen ground. It has a fundamental
Hemisphere Arctic and subarctic. It ranges from thin influence upon periglacial geomorphology, affecting