This layer shows the extent of permafrost across Alaska, classified
into seven categories.
The ground at and near the surface that goes through an annual
freeze–thaw cycle is called the "active layer" and is
from a few inches to several feet thick. Permafrost lies below the
active layer–depth of summer thaw–and extends from a few
feet thick in the south to over 1,000 feet in the Arctic. Depths of
permafrost vary even within short distances in the discontinuous and
sporadic permafrost zones and depend upon exposure, ground cover, soil
characteristics, and other factors.
Taliks
In some areas, permafrost exists at depth, well below the active
layer, with a thawed zone in between known as a talik layer. In areas
of continuous permafrost, talik often occurs beneath lakes and rivers.
Ongoing climate change is causing a new type of talik not associated
with water bodies. This "supra-permafrost talik" occurs
because ground that thawed in the summer does not completely refreeze
in the winter. Talik can be further categorized as "open",
"closed", or "through" based on its position
relative to areas of permafrost.
In areas of shallow permafrost, surface saturation coupled with
limited potential rooting depth yields characteristic ecosystem types
such as wetlands, tundra, and black spruce forest. As ice–rich
permafrost thaws, thermokarst results. Thermokarst landscapes include
numerous hollows, hummocks, and small lakes. In some areas, climate
change is accelerating the development of thermokarst.
Permafrost and engineering
Permafrost is a critical consideration in the engineering and design
of structures and engineering works such as pipelines, roads, and
railroads. Particularly in fine–grained soils, the frozen ground
forms an extremely strong and stable foundation material if it is kept
in the frozen state. However, if the permafrost is allowed to thaw,
the soil becomes extremely weak and foundation failures are very
common.
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