Geology Reference
In-Depth Information
was undertaken in Kärkevagge, a deeply-eroded glacial valley in northern Lapland (Rapp,
1960a) (see Chapter 4, pp. 68-69). A specifi c conclusion was that, next to solution, rapid
mass movements (debris fl ows and slides, slush avalanches, rockfalls) were the most
important in terms of denudation (Table 9.3). However, as mentioned in Chapter 4, it is
unwise to assume that the activity measured in the Kärkevagge is typical of all mountain-
ous, periglacial environments. Much of the movement reported in 1960 resulted from a
series of extreme events that occurred the previous year and which were regarded as “a
centennial or probably even millennium maximum” (Rapp, 1960a, p. 185). It is important,
therefore, to consider the frequency and magnitude of occurrence of episodic events when
assessing their overall importance. For example, assuming a recurrence interval of 200
years for such major events, the rates of rock denudation from several documented debris
slides and fl ows which occurred in Scandinavia and Spitsbergen over the previous three
decades can be extrapolated (Table 9.4). This analysis demonstrates that the activity
recorded at Kärkevagge is at the low end of the high magnitude scale, and that denudation
accomplished by such activity is highly variable and largely dependent upon lithology and
antecedent conditions.
In some instances, avalanche activity may be more important than debris-fl ow activity.
For example, long-term studies by in the Canadian Rockies (Luckman, 1977, 1978) indicate
mean avalanche debris-accumulation rates of 5 mm/year. In a second study, in northwest
Spitsbergen (André, 1993, 1995b), the amount of debris deposited at the foot of avalanche
paths was correlated with rockwall surfaces. Average denudation rates were obtained of
0.007 mm/year in massive gneiss and 0.08 mm/year in fractured mica-schist (Table 9.5).
These values are considerably higher than comparable values from Kärkevagge.
Slushfl ows are a particular type of snow avalanche in which water-saturated snow
masses are mobilized when rainfall, snowmelt, or a combination of the two increases the
water content of the snow. When instability is reached, the pack is mobilized. They are
widespread in arctic and sub-arctic environments (André, 1995b; Elder and Kattelman,
1993; Laroque et al., 2001; Nyberg, 1985; Onesti, 1985; Onesti and Hestness, 1989)
and often occur on relatively gentle slopes. Slush avalanches probably have recurrence
Table 9. 3.
Ranked list of major slope process at Kärkevagge, Swedish Lapland, 1952-1960.
Process
Tonnes/
Denudation
Mass transfers
Remarks
km/year
(mm/year)
(tonne-metres)
1. Transport of solutes by
26
0.010
136 500
-
running water
2. Debris slides and fl ows
49.4
0.019
96 300
Extreme event,
October 1959
3. Slush avalanches, rock
14
0.005
20 000
Extreme events, 1956
debris transport
and 1958
4. Rockfalls
8.7
0.003
19 600
Seasonal events of high
frequency
5. Solifl uction
5.4
0.002
5 300-19 800
Based on 9 km of
solifl ucted slope
length; material
density of 1.8
6. Talus creep
1.5
0.001
2 700-4700
Based on 6 km of talus
slope length; material
density of 1.8
Source: Rapp (1960a).
