Geology Reference
In-Depth Information
Table 9.4 .
Instances of alpine debris slides and fl ows in Scandinavia and Svalbard, and
estimated indices of their denudation impact. From Rapp (1985).
Locality
Date
Catchment
Rock Type
Volume
Denudation
Area (km
2
)
of debris
Debris
Rock
Extrapolated
(m
3
)
(mm)
(mm)
rock
1
(mm)
Nissunvagge,
23.6.79
5.5
Amphibolite
85 000
15.5
10.8
54
N. Sweden
Tarfala,
6.7.72
11
Amphibolite
55 000
5
3.5
18
N. Sweden
Kärkevagge,
6.10.59
15
Mica-schist
4 600
0.3
0.2
1
N. Sweden
Longyearbyen, 11.7.72
4.5
Schist,
5 000
1
0.7
3.5
Svalbard
sandstone
Ulvadal,
26.6.60
7
Granite
3000 000
43
30.1
150
W. Nor way
1
Extrapolated rates of rock denudation are given in mm per 1000 years assuming that the recurrence
interval is 200 years.
intervals varying between 2 and 10 years, and, depending upon magnitude, can transport
anywhere between 50 and 200 m
2
of rock debris. A recent case study from Mont Albert in
the Gaspésie Mountains of eastern Québec, Canada, involved the use of dendrochronol-
ogy and air photographs to date slushfl ow events (Laroque et al., 2001). It was concluded
that slushfl ows occur at regular 20-30 year intervals and have contributed to the formation
of a tongue-shaped accumulation of 17 900 m
2
at the base of one small fi rst-order stream.
It is clear that slushfl ows and snow avalanches are geomorphic events that may under-
take considerable geomorphic activity. However, it is diffi cult to assess their importance
relative to debris fl ows. A general perspective is that debris-fl ow activity ranges from
events of major geomorphic importance, with recurrence intervals of several hundreds of
years, to smaller “spot” events occurring every 2-3 years within limited areas. As regards
avalanches, the clear relationship between winter snowfall and spring temperatures is
critical, and recent investigations suggest that avalanches are, in themselves, an important
denudation process in snowy mountainous regions. Slush avalanches appear confi ned to
smaller valleys where they occur in years of relatively rapid spring thaw. With all three
processes, frequency and magnitude concepts are fundamental to any interpretation of
their geomorphic signifi cance.
9.5.3. Rockfall
Where bedrock outcrops in the form of vertical or near-vertical free-faces, rockfalls
assume local importance and talus may accumulate below the free-face. Theoretically, if
the scree is not renewed, the free-face will be progressively eliminated by the accumula-
tion and upward growth of the talus. In many areas of Svalbard, northern Scandinavia,
and the eastern Canadian Arctic, extensive and imposing rock faces have been inherited
from previous glacial periods; in other areas, these slope forms occur in association with
specifi c geologic structures (intrusive sills, dykes), with sea cliffs, or with deeply-incised
stream valleys. The weathering and recession of these free-faces occurs primarily through
the melting of interstitial ice and the loosening of rock particles by frost wedging in the
