Geology Reference
In-Depth Information
1978). The moisture contents of such sediments are close to their liquid limits so the muds
liquefy and fl ow readily. Where internal stresses cannot be relieved, the mud bursts
through the semi-rigid surface layer to create the mudboil. The process occurs primarily
in late summer when the active-layer thickness is greatest, and is often associated with
excess pore-water pressures created by summer rain and/or thawing of the ice-rich layer
(the so-called “transient” layer; see Chapter 5) at the top of permafrost. Sometimes, mud-
bursts produce small-scale mudfl ows which extend down slope for several meters.
Polygonal patterned ground can be either small (diameter
>
1.0 m) or large (diameter
<
1.0-2.0 m). The larger forms result from thermal-contraction cracking and have been
discussed earlier. The smaller forms constitute nets that are delineated by furrows and
narrow cracks to give a hummocky micro-relief of between 0.1 m and 0.3 m (Figure 6.22B).
Many are as small as 5 cm but most are 20-50 cm in diameter. Terms such as “tundra
Figure 6.23. Examples of sorted and non-sorted stripes, Arctic Canada. (A) Oblique air view of
non-sorted stripes, eastern Banks Island. (B) Sorted stripes formed in shale, Eastern Prince Patrick
Island.
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