- Permafrost - Hydrolaccolith
Permafrost defines rock or soil in which the temperature remains
at or below 0°C for at least two consecutive years (Everdingen 2002). The
term is also used to describe perennially frozen ground. It is the result of
continuously cold climate as it occurred in the Pleistocene epoch in our latitudes
or like it occurs today in higher latitudes in Canada or Russia. Massive ice
may form either as buried surface ice (glacier) or as intrasedimental ice (intrusive
ice). The main factor of distribution and thickness is the climate but permafrost
also depends on vegetation, snow cover, drainage and soil type. Usually a seasonally
thawing layer overlies the permafrost. In most areas characteristic geomorphologic
features like patterned ground, palsas (ice- cored peat mounds), ice lenses
and wedges, frost shattered rock, shallow drained lakes containing taliks (unfrozen
areas), and pingos (symmetrical hills in a flat landscape) are visible. The
thickness of permafrost ranges from a few meters up to 1500 meters in Siberia.
Mostly it occurs in sediments but there are also some examples of permafrost
in fractured rock (Gascoyne 2000).
Frozen ground may contain small amounts of liquid water, which is adsorbed onto
the surface of soil or sediment particles and is liquid even when the temperature
is well below the freezing point. In non-saline soils the amount of unfrozen
water tends to be low, especially when the grain size is large. Saline soils
normally occur in costal areas. The particle size is smaller there. When permafrost
develops in saline soils the ice crystal grows away from the particle into the
pore space and rejects dissolved salts into the remaining unfrozen liquid. The
soil containing these higher concentrated liquids is called cryopeg. The more
saline it becomes, the more the freezing point of the brine decreases. Marine
salinity of permafrost is in Russia primarily located at the cost of the Arctic
Sea. The other type of saline permafrost is the continental type. In Russia
it is located along the mid-course of the river Lena and Aldan and in the west
of Lake Baikal. This type is generated in areas where high summer temperatures
and a negative balance of soil moisture promote salt accumulation in the soils
and underlying strata. The formation of brines is also possible in such non-saline
soils if there is a supply of mineralised water. When the soil freezes, its
volume expands up to 9% and results in the expulsion of pore water that may
accumulate in aquifers or lakes (Gascoyne 2000).
Periglacial environments can also show chemical precipitation due to salt rejection.
They are formed because of solubility constraints at lower temperatures. CO2
release or uptake may play a roll when water freezes. The precipitates - for
instance Fe, SiO2 or CaCO3 - have been formed on sediment
grains during frost and ice formation. Precipitates like calcite can be identified
as cryogenic because of their unusual isotopic composition (e.g. 13C
up to +17 promille due to non- equilibrium conditions; Gascoyne 2000).
Permafrost is present in all countries around the Arctic regions like Russia
or Canada. Approximately half of the territories of Russia are perennially frozen.
The following map gives an overview of the permafrost zones in Eurasia.
[Permafrost zones of Eurasia (S.L. Smith, Geological Survey of Canada, 2001)]
There are three types of permafrost zones: continuous, discontinuous
and sporadic. Continuous permafrost is described for a geographical region where
more than 90 % of the ground is underlain by permafrost. Discontinuous permafrost
occurs in a special region, where 30 to 90 percent of the lands surface are
frozen ground. If permafrost underlies 5 to 30 percent of the exposed land surface
it is called sporadic. Individual areas of frozen ground are surrounded by completely
unfrozen ground (Everdingen 2002).
The following sketch gives a rough overview of the distribution of permafrost
zones in the area of lake Baikal.
[Distribution of permafrost zones around Lake Baikal]
The red point refers to a hydrolaccolith
that was visited during the excursion.