Geology Reference
In-Depth Information
Another cause of thermokarst is associated with stream incision acting preferentially
along ice wedges causing the undermining and slumping of overlying material, tunneling,
and piping. Sometimes, lateral stream erosion may initiate thermokarst activity. For
example, retrogressive thaw slumps occur mainly upon steeper west- and southwest-facing
slopes of asymmetrical valleys on eastern Banks Island (French and Egginton, 1973). The
thermokarst activity appears to be triggered by the lateral migration of the stream towards
the base of the steeper slope.
A third cause of thermokarst is the destruction of surface vegetation and any organic
cover by fi re. For example, Table 8.1B demonstrates the increase in active-layer thickness
and the amount of near-surface permafrost that thawed following the 1968 forest fi re near
Inuvik, Canada. At some ridge sites, the active-layer thickness doubled in 20 years. As
explained in Chapter 4, lightning-induced forest fi res are characteristic of the boreal forest
and not uncommon on the tundra. Other local triggers for thermokarst include ice-push
and scour along coasts, cyclical changes in vegetation, slope instability, and deforestation
or disruption of the surface by human activity.
8.3. THAW-RELATED PROCESSES
Of the various processes associated with thermokarst, a basic distinction should be made
between those associated with subsidence and those associated with erosion.
8.3.1. Thermokarst Subsidence
Thermokarst subsidence is associated with a loss of water (excess ice) upon thawing and
its removal by either evaporation or drainage. Thermal melting depends upon heat con-
duction from, for example, a pool of water directly overlying icy soil, or through an inter-
vening layer of unfrozen soil. Therefore, quite unlike thermal erosion (see below), fl owing
water is not required. It follows that thermokarst subsidence can operate just as effi ciently
upon fl at and well-drained uplands as in poorly-drained valley bottoms. To illustrate,
simple thermokarst subsidence over a 5-year period at an experimental plot near Mayo,
Yukon Territory, amounted to 35cm. The active layer increased from 33cm to 90cm
(Figure 8.3) and total permafrost degradation was 92 cm, of which 35 cm resulted from
melt of excess ice and 57 cm was due to active-layer development.
8.3.2. Thermal Erosion
Thermal erosion refers to the complex of erosional processes that are associated with
running water acting upon ice-rich permafrost (Romanovskii, 1961; Shamanova, 1971).
Typically, thermal erosion results when surface runoff, from snowmelt, summer precipita-
tion, or thawing permafrost, becomes concentrated along ice wedges, causing preferential
thaws. It is sometimes referred to as “fl uvio-thermal” erosion. The gullies that result
(Figure 8.4) are often characterized by an inverted “T” cross-profi le because water fi rst
erodes vertically and then, as the bed becomes armored with transported sediment from
up-gully, erodes laterally to leave organic-mat overhangs. Slumping, piping, and the crea-
tion of small tunnels above and adjacent to the partially-eroded ice wedge are all common
(French, 1975b; Mackay, 1974c; Murton, 2001, pp. 185-186; Seppälä, 1997). Standing
water bodies may accumulate in the channel fl oor behind slumped masses to form “pool”
