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