Geology Reference
In-Depth Information
process and landform suggests that riser height of solifl uction lobes is indicative of
maximum depth of movement and prevailing freeze-thaw type (see Figure 9.9). For
example, in tropical high mountains, where diurnal frost is limited to shallow but relatively
rapid movements of the superfi cial 5-10 cm, thin stone-banked lobes are typical. In mid-
latitudes (i.e. areas of annual frost), where either deep seasonal frost or warm permafrost
occurs, solifl uction affects a thicker soil layer and produces medium-sized lobes, often
0.3-0.5 m high at the riser. In colder regions of either high-latitude or higher elevation,
the presence of permafrost results in two-sided freezing and plug-like fl ow that can affect
a soil mass 60 cm or more in thickness. This can also produce well-pronounced lobate
forms. Finally, in semi-arid polar deserts, thaw depths are shallow and movement is
restricted to snow banks and other sites of localized soil humidity. As a result, riser height
is often small, and solifl uction sheets, rather than lobes, are better developed.
The site-specifi c controls over solifl uction activity can be illustrated with two studies
from the upland terrain of the southern Yukon Territory, Canada. In the Ruby Range,
movement rates vary according to vegetation cover, aspect, and micro-climate (Price,
1973). Greatest rates (
2.4-2.7 cm year
−l
) occurs on north- and east-facing slopes, where
vegetation is poorly developed, while lower rates (
∼
1.6 cm/year) occur on south- and east-
facing slopes, where vegetation is well developed. Movement is least (
∼
0.7 cm/year) on
south- and west-facing slopes which, being exposed to prevailing winds, are drier than
those of other orientations. A second study examined lobe movement in the Kluane Range
(Kinnard and Lewkowicz, 2006). They observed that vegetated lobes progressively
increase in size downslope. This happens either by the merging of adjacent lobes to form
sheets or by an increase in riser height (Figure 9.12). It was inferred that lobe advance
leads to a progressive steepening of the riser and its eventual rupture and collapse.
Rebuilding of the lobe and the full cycle of development is estimated to take several
hundred years.
∼
Figure 9.12.
Diagram illustrating typical downslope trends in soil moisture, sediment-size com-
position, and morphology of solifl uction lobes, Kluane Range, Yukon Territory, Canada. The thick-
ness of the organic mat and thaw depths are not drawn to scale. From Hugenholtz and Lewkowicz
(2002) and Kinnard and Lewkowicz (2006). Reproduced by permission of John Wiley & Sons
Ltd.
