Geology Reference
In-Depth Information
6
Surface Features of Permafrost
Permafrost gives rise to a number of unique landforms. Some result from the growth
of discrete ice bodies within permafrost, some refl ect the unusual groundwater
hydrology that characterizes permafrost terrain, some refl ect the thermal properties
of earth material when subject to freezing, and some refl ect the ability of warm, ice-rich
permafrost to creep and deform under its own weight. The most widespread perma-
frost-related landforms are the polygons associated with thermal-contraction cracking
of the ground. A number of small-scale features are characteristic of the active layer;
some are diagnostic of permafrost terrain while others occur in environments of both
seasonal and perennial frost. Landforms associated with the thaw of ice-rich permafrost
are discussed in Chapter 8.
6.1. INTRODUCTION
In broad terms, permafrost-related landforms can be divided into those associated with
either the growth (aggradation) or thaw (degradation) of permafrost. Those associated
with the thaw, subsidence, and erosion of ice-rich permafrost are examined in Chapter 8,
after discussion of ground ice in Chapter 7. In this chapter, we consider landforms associ-
ated with permafrost aggradation.
6.2. THERMAL-CONTRACTION-CRACK POLYGONS
Thermal-contraction-crack polygons are the most widespread, most visible, and most
characteristic feature of permafrost terrain (Figure 6.1). They are variously referred to as
tundra polygons, frost-fi ssure polygons, ice-wedge polygons, sand-wedge polygons, or
“Taimyr” polygons.
6.2.1. Coeffi cients of Thermal Expansion and Contraction
It is well known that different rocks, being composed of different minerals, possess dif-
ferent coeffi cients of expansion and contraction, and that these are temperature depend-
ent. This has been discussed earlier in the context of frost shattering of rock (Chapter 4).
It is also well known that the lowering of temperature of ice-rich frozen soil can lead to
thermal contraction of the ground and the formation of fi ssures. These develop because
pure ice has a coeffi cient of linear expansion of 52.7
10
1
at 0 °C and only 50.5
10
1
at
×
×
−
30 °C. It is generally assumed that the rates of expansion and contraction of ice-rich
unconsolidated sediments are probably little different to those of pure ice. Thermal-
contraction cracking also occurs in bedrock. Here, the coeffi cients of linear and
