Geology Reference
In-Depth Information
9.5. RAPID MASS WASTING
A number of high-magnitude, low-frequency, mass-wasting processes also fashion slopes
in periglacial environments. Their geomorphic signifi cance is best described in the context
of their magnitude and frequency.
9.5.1. Active-Layer-Detachment Slides
Localized and small-scale slope failures that are confi ned to the active layer are common
in regions underlain by perennially-frozen and ice-rich unconsolidated sediments. They
are usually restricted to middle or upper slopes, with the entrance and exit of the failure
plane well within the slope (Figure 9.13). These rapid mass movements are termed active-
layer- detach ment sl ides (Ha r r is a nd L ewkow icz , 19 9 3a, b, 2 0 0 0 ; L ewkow icz , 19 9 2) . Fa i lu re
involves the thawed or thawing active layer, together with any overlying vegetation mat,
becoming detached from underlying frozen material.
The cause of failure is associated with a reduction in shear strength through time. This
results from the progressive weathering of soil and a gradual increase in basal active-layer
ice content (see Chapters 5 and 7). Because many active-layer-detachment failures occur
in mid- and late summer, soil shearing during annual gelifl uction movement is often the
cause (Harris and Lewkowicz, 2000). The permafrost table acts as a lubricated slip plane
and controls the depth of the failure plane. Thus, active-layer detachments are usually
shallow, the slump scar or hollow often being no more than 1 m deep. In particularly fi ne-
grained sediments possessing high liquid limits, failure may take the form of mudfl ows.
Where icy sediments are exposed in the permafrost, regressive- thaw-slumping may follow
(see Chapter 8).
In terrain underlain by ice-rich and poorly-lithifi ed or unconsolidated sediments,
active-layer detachments are especially frequent (Chatwin and Rutter, 1978; French, 1988,
pp. 167-168; Harris and Lewkowicz, 1993a, b, 2000; Hodgson, 1982; Hughes et al., 1973;
Figure 9.13.
Active-layer detachment, Ellesmere Island, Arctic Canada. This slide occurred
approximately 24 hours prior to the photograph as a result of rapid thaw at the base of the active
layer caused by warm temperatures and nearly continuous sunshine. Photo taken August 2005.
Photograph and caption information are courtesy of Professor A. G. Lewkowicz.
