160
Climatic Geomorphology
3.3. Mass movements
These types of processes correspond to the movement of detrital material on slopes due
to gravity. In periglacial environments these processes reach a great importance due to
the high water content of the active layer that contributes to the reduction of the shear
strength of the wet material. The permafrost behaves as an impervious material so that its
top acts as a hydrostatically loaded surface that favours the displacement of the overlying
active layer (French, 1996). The different types of mass movements act with variable
intensity depending on the climatic, topographic and lithological conditions.
In steep glaciated mountains, sea cliffs, and fluvial scarps the water percolates through
the rocks and the frost-induced expansion may favour the generation of rock-falls. These
slope movements are particularly common during the melting period and undercutting
may enhance their activity. The result is the formation of talus or scree accumulations at
the foot of the rock cliffs. The scarp retreat rates calculated with different methods give
values of 0.3 to 0.6 mrn/yr (Andr6, 1993), well below those estimated for temperate and
semiarid areas.
Solifluction (Andersson, 1906) is one of the most widespread processes in periglacial
areas. It is a slow downslope flow of water-saturated detrital material. Although this
type of movement can occur under a variety of climatic conditions, it is especially
effective in periglacial regions where the term gelifluction is used (Baulig, 1957). Frostcreep may act in association with this process. It involves the dilation of the soil
perpendicular to the surface during freezing and vertical settlement with the thawing
(Washburn, 1967) (Figure 7.13). Three different types of movements are distinguished.
The potential frost-creep (PFC) is that related to the frost-induced heave of the soil (P1
to P2). This expansion of the soil is perpendicular to the slope, which constitutes the
cooling surface (Taber, 1929). A gelifluction component (G) may also act and the
retrogressive movement (R) is opposite to the former ones and is related to the
cohesion among particles (Davison, 1889). During a frost-thaw cycle the trajectory of
the particles would be from P~ to P4. These movements act jointly but the resulting
landforms are designated with the term gelifluction because this is generally the
dominant process.
Frost-creep increases with the number of freeze-thaw cycles, slope angle, moisture
content of the soil, and decreases with depth (Washburn, 1979). Gelifluction may be
active with slopes from 1 ~ Its activity starts with the thawing, reaches its maximum
intensity in summer and decreases in autumn, due to the progressive water loss from the
ground by evaporation and interstitial flow. Furthermore, gelifluction activity also
increases with slope angle and water content of the soil and decreases progressively with
depth. Experimental studies indicate that this process is commonly restricted to the upper
50 cm of the active layer (French, 1996). Laboratory simulation experiments indicate
that gelifluction occurs only during thaw consolidation of the upper parts of the soil
profile, while thawing of the deeper layer provokes little downslope displacement
(Harris and Davies, 2000). Other experiments carried out by these researchers (Harris
et al., 2003) revealed that gelifluction is not a time-dependent viscosity-controlled flow,
but it presents elasto-plastic behaviour. Gelifluction is strongly controlled by the grain
size and texture of the soil. The coarse-grained and highly porous soils favour the
drainage of the ground, whereas the fine-grained and low permeability soils become
Climatic Geomorphology
3.3. Mass movements
These types of processes correspond to the movement of detrital material on slopes due
to gravity. In periglacial environments these processes reach a great importance due to
the high water content of the active layer that contributes to the reduction of the shear
strength of the wet material. The permafrost behaves as an impervious material so that its
top acts as a hydrostatically loaded surface that favours the displacement of the overlying
active layer (French, 1996). The different types of mass movements act with variable
intensity depending on the climatic, topographic and lithological conditions.
In steep glaciated mountains, sea cliffs, and fluvial scarps the water percolates through
the rocks and the frost-induced expansion may favour the generation of rock-falls. These
slope movements are particularly common during the melting period and undercutting
may enhance their activity. The result is the formation of talus or scree accumulations at
the foot of the rock cliffs. The scarp retreat rates calculated with different methods give
values of 0.3 to 0.6 mrn/yr (Andr6, 1993), well below those estimated for temperate and
semiarid areas.
Solifluction (Andersson, 1906) is one of the most widespread processes in periglacial
areas. It is a slow downslope flow of water-saturated detrital material. Although this
type of movement can occur under a variety of climatic conditions, it is especially
effective in periglacial regions where the term gelifluction is used (Baulig, 1957). Frostcreep may act in association with this process. It involves the dilation of the soil
perpendicular to the surface during freezing and vertical settlement with the thawing
(Washburn, 1967) (Figure 7.13). Three different types of movements are distinguished.
The potential frost-creep (PFC) is that related to the frost-induced heave of the soil (P1
to P2). This expansion of the soil is perpendicular to the slope, which constitutes the
cooling surface (Taber, 1929). A gelifluction component (G) may also act and the
retrogressive movement (R) is opposite to the former ones and is related to the
cohesion among particles (Davison, 1889). During a frost-thaw cycle the trajectory of
the particles would be from P~ to P4. These movements act jointly but the resulting
landforms are designated with the term gelifluction because this is generally the
dominant process.
Frost-creep increases with the number of freeze-thaw cycles, slope angle, moisture
content of the soil, and decreases with depth (Washburn, 1979). Gelifluction may be
active with slopes from 1 ~ Its activity starts with the thawing, reaches its maximum
intensity in summer and decreases in autumn, due to the progressive water loss from the
ground by evaporation and interstitial flow. Furthermore, gelifluction activity also
increases with slope angle and water content of the soil and decreases progressively with
depth. Experimental studies indicate that this process is commonly restricted to the upper
50 cm of the active layer (French, 1996). Laboratory simulation experiments indicate
that gelifluction occurs only during thaw consolidation of the upper parts of the soil
profile, while thawing of the deeper layer provokes little downslope displacement
(Harris and Davies, 2000). Other experiments carried out by these researchers (Harris
et al., 2003) revealed that gelifluction is not a time-dependent viscosity-controlled flow,
but it presents elasto-plastic behaviour. Gelifluction is strongly controlled by the grain
size and texture of the soil. The coarse-grained and highly porous soils favour the
drainage of the ground, whereas the fine-grained and low permeability soils become