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PERMAFROST - Seventh International Conference (Proceedings),
Yellowknife (Canada), Collection Nordicana No 55, 1998
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THE ROLE OF DIURNAL, ANNUAL AND MILLENNIAL FREEZE-THAW
CYCLES IN CONTROLLING ALPINE SLOPE INSTABILITY
Norikazu Matsuoka1, Kazuomi Hirakawa2, Teiji Watanabe2, Wilfried Haeberli3 and Felix Keller4
1. Institute of Geoscience, University of Tsukuba, Ibaraki 305-8571, Japan,
e-mail: [email protected]
2. Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan
3. Department of Geography, University of Zurich, Zurich 8057, Switzerland
4. Academia Engiadina, Samedan 7503, Switzerland
Abstract
The instability of rock and debris slopes in the Swiss Alps was evaluated in light of the temporal and spatial
scales of freeze-thaw processes. Diurnal freezing and thawing penetrate to centimeter-to-decimeter scale
depths, producing rock debris mainly of pebble size or smaller on rock slopes and miniature patterned forms
on debris slopes. Annual freeze-thaw cycles result in weathering and soil movement up to meter scale, supplying boulders to rock glaciers and developing solifluction lobes with risers of 30 cm or higher. The growth and
decay of permafrost, originating from long-term climatic change, lead to freeze-thaw activity reaching meter-todecameter scale depths. Permafrost melting can trigger cliff falls and debris flows in the thawing phase of millennial freeze-thaw cycles.
Introduction
Freeze-thaw action induces both rock weathering and
mass wasting, destabilizing rock and debris slopes in
high mountain regions. Two types of freeze-thaw
cycles, diurnal and annual, are normally recognized
according to the period for the completion of one cycle.
In addition, recent global warming has highlighted a
third type, which has a much longer period.
Corresponding to the growth and decay of permafrost,
this type of freeze-thaw is completed typically in many
centuries or millennia (e.g., Haeberli, 1996) and here is
termed the millennial freeze-thaw cycle. The relationship between the freeze-thaw types and the magnitude
and nature of resulting geomorphic processes, however,
has been poorly understood because of the lack of longterm, continuous monitoring of processes and
variables.
The periglacial belt in a mountain area is usually subdivided into permafrost and seasonal frost zones, mainly in relation to elevation and aspect. Between the two
zones, a transient permafrost zone can be defined in
which permafrost has grown and decayed repeatedly in
response to climatic change during the Holocene
(Figure 1). The transient permafrost zone is, therefore,
characterized by the occurrence of millennial freeze-
thaw cycles, as well as of diurnal and annual freezethaw cycles. Millennial freeze-thaw cycles can also
operate in the permafrost zone as a result of melting
and refreezing of the top and bottom of the permafrost
body, although their effects would be less dramatic than
in the transient permafrost zone. During the Little Ice
Age, a large part of the transient permafrost zone was
probably characterised by a freezing phase of a millennial cycle. The 20th Century warming will have
switched this zone into a thawing phase.
The prediction of future geomorphic changes due to
global warming requires the distinction of effects due to
Figure 1. Altitudinal zonation of the periglacial belt in the Swiss Alps.
Norikazu Matsuoka, et al.
711
millennial cycles from those due to shorter cycles. The
distinction is necessary, in particular, in the permafrost
and transient permafrost zones where permafrost melting is in progress and the three freeze-thaw types are
superimposed, causing slope instability. This report
aims at evaluating the effects of the three kinds of
freeze-thaw cycles on alpine slope instability, based on
studies of contemporary periglacial processes in the
Swiss Alps. Attention will be focused on the scales of
geomorphic change caused by each freeze-thaw type.
The study area is located in the Upper Engadin, eastern Switzerland. The lower limit of permafrost lies at
about 2400 m ASL on northern exposures, rising to
about 3000 m ASL on southern exposures. The
periglacial belt, lying above the timberline at 2000 to
2200 m ASL, includes both present-day permafrost and
non-permafrost areas. The most extensive periglacial
landscape is the talus-to-rock glacier sequence, which
develops on slopes covered by coarse debris. Patterned
ground and solifluction features are also common, and
are characteristic of the slopes underlain by fine debris
(Matsuoka et al., 1997).
Diurnal freeze-thaw cycles
ROCK SLOPES
The magnitude and frequency of diurnal freeze-thaw
cycles depend partly on the aspect of slopes. This tendency is enhanced on steep rockwalls. Figure 2 displays
the contrast of rock surface temperatures between north
and south-facing rockwalls (TFN and TFS sites). Both
are located at 2850 m ASL. Covered with thick snow
from winter to spring, the north-facing rockwall experiences continuous subzero temperatures. Even during
summer months, the minimal insolation leads to small
ranges of diurnal fluctuation in the rock surface temperature. As a result, diurnal freeze-thaw cycles take
place only in early autumn. In contrast, because of the
lack of snowcover, the south-facing rockwall undergoes
large diurnal fluctuation in the surface temperature
throughout the year. This thermal condition favours the
high frequency of diurnal freeze-thaw cycles during all
seasons except midsummer.
Temperature fluctuations across 0¡C, however, do not
always indicate freeze-thaw alternations effective in
rock breakage. An abundant moisture supply is necessary for frost damage (e.g., Matsuoka, 1991; Prick,
1997). Subzero temperatures, following the infiltration
of water into the joints and pores in the bedrock, may
cause effective freezing expansion. Consequently, the
effective diurnal freeze-thaw cycles must be considerably fewer than those counted from the fluctuation in
rock temperature.
Frost (or thaw) penetration in the bedrock is usually
30 cm or shallower during a diurnal freeze-thaw cycle
(Figure 2). In response to the amount of the latent heat
exchange, however, wet rocks favourable for freezing
expansion are subjected to much shallower freeze-thaw.
Furthermore, frost damage can occur at depths cooled
to a few degrees below 0¡C (e.g., Matsuoka, 1994). Thus
diurnal frost weathering is considered to be active within 10 to 20 cm of the rock surface and able to produce
rock debris up to cobble size. Controlled by joint spacing, the size of the released rock debris can be smaller.
In fact, pebbles are the major components of screes
below the south-facing rockwalls. Observations of scaling from the painted bedrock also showed that a number of rock fragments smaller than 5 cm were produced
every year.
Figure 2. Annual and short-term fluctuations in rock surface temperature at north-facing (TFN) and south-facing (TFS) rockwalls in 1995. Short-term fluctuations are displayed by isotherms at 2¡C intervals.
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The 7th International Permafrost Conference
DEBRIS SLOPES
Large parts of the debris slopes in the study area are
covered with snow for half of the year. Diurnal freezethaw cycles are most frequent in early autumn and are
prevented by the late-lying snowcover in spring
(Figure 3). Windy crest slopes lack snowcover and
experience frequent freeze-thaw cycles in both autumn
and spring (Matsuoka et al., 1997).
Debris slopes experience shallower freeze-thaw
depths than rock slopes, because of the lower thermal
conductivity and larger latent heat. Diurnal frost depth
is typically about 5 cm and rarely in excess of 15 cm
(Figure 3). Frost heaving usually accompanies diurnal
freeze-thaw cycles. The heave amount depends upon
the soil granulometry, but rarely exceeds 2 cm. Despite
such small individual heaves, the cumulative amount is
considerable. Thin debris mantles and insignificant
snowcover combine to make diurnal frost heaving prevail on crest slopes where small sorted stripes and lobes
dominate. These landforms are considered to originate
mainly from diurnal freeze-thaw cycles. In fact, the
sorting depth of the stripes is about 5 cm and the risers
of the lobes are about 10 cm high, values similar to the
depth of soil movement induced by diurnal frost heave
activity.
Annual freeze-thaw cycles
ROCK SLOPES
Regardless of the aspect and the presence of permafrost, rock slopes in the periglacial belt are subjected
to deep seasonal freezing and thawing. Direct determination of annual frost or thaw penetration is difficult.
Equations derived from the thermal conduction theory,
however, permit us to estimate the depth using the
freezing or thawing index at the rock surface
(Matsuoka, 1994). The modified Berggren equation
(Aldrich, 1956), one of the Stefan-type equations, was
used for the calculation of the frost (or thaw) penetration depth in the rockwalls. The thermal conductivity, a
parameter involved in this equation, was determined
Figure 3. Frost heave and ground temperatures at a solifluction lobe (1994-1996). The interval of the isotherms is 1¡C. The location of the experimental site is
indicated in Figure 4.
Norikazu Matsuoka, et al.
713
from temperature curves at different depths. The calculation includes assumptions of the vertical gradient of
the mean annual rock temperature being negligible and
the freezing point at 0.0¡C. Such a simplification does
not seem to lower the accuracy of calculation significantly (Matsuoka, 1994). This model was applied to
TFN and TFS sites (Figure 2). The mean annual surface
temperature was negative at both rockwalls, indicating
the presence of permafrost. The maximum thaw depth
in 1995 was computed to be 4.3 m at TFN site and 6.8 m
at TFS site. These values are comparable with the computed frost depth in a seasonal frost environment of the
Japanese Alps (Matsuoka, 1994). Thus, the annual
freeze-thaw depth in the alpine periglacial belt is typically 5±2 m, rarely reaching the decameter scale.
The annual freeze-thaw depth defines the boundary
to which frost weathering can operate annually. The
above calculation predicts that a rock mass up to about
5 m thick is detachable from the rockwalls. The locations at which frost damage happens, however, depend
on several factors including the joint patterns, moisture
distribution, and temperature range at which ice segregation occurs. The concurrent monitoring of rock temperatures and joint opening on the rockwall behind the
Murt•l rock glacier, the Upper Engadin, showed that
the largest opening at the rock surface occurred during
an early thawing period when meltwater infiltrated the
joint and refroze (Matsuoka et al., 1997). Since only
minor opening was recorded during seasonal freezing
in winter, moisture supply is considered to play a major
role in near-surface frost weathering. In permafrost
areas, segregational freezing may lead to intensive frost
damage at the base of active layer where moisture
availability is high (Hallet et al., 1991), although this
idea has yet to be verified by field evidence.
Frost damage associated with seasonal freezing is
often followed by rockfalls on thawing. For instance, in
early June 1997, a block of rock about 100 m 3 was
detached from the rockwall behind the Murt•l rock
glacier. The slip plane lay at about 2 m deep. This block
fall happened during a high temperature period after
snowmelt (Matsuoka, 1997). These conditions indicate
that rapid seasonal thawing triggered the block detachment. The block was broken into numerous boulders
and smaller debris, which were deposited on a scree
and a rock glacier. Despite low frequency, such a block
fall would be an important source in terms of the debris
supply on screes and rock glaciers.
DEBRIS SLOPES
Seasonal freezing penetrates to about 2 m deep in
debris slopes located just below the lower limit of permafrost (Figure 3). Thaw penetration over the rock glacier permafrost is slightly deeper, reaching 3 to 5 m (e.g.,
Barsch, 1996), probably because of the small latent heat
714
exchange and large cold air drainage through the openwork clasts. Where a large part of the freeze-thaw layer
consists of fine debris, seasonal freezing is associated
with a large frost heave (5 cm or more). Formation of
ice lenses tends to be concentrated in the upper part of
the annual freeze-thaw layer, because the progressive
downward freezing may cause desiccation of the lower
part.
Thawing of the heaved ground, often aided by
snowmelt, raises the moisture content and mobility of
the thawed soil, resulting in solifluction or small debris
flows. In response to the locations of ice lenses, soil
movements due to annual freeze-thaw cycles would
occur mainly in the upper part of seasonal frost. In fact,
many solifluction lobes in the study area have a riser 30
to 50 cm high. Solifluction lobes with similar riser
heights seem to reflect the movement of soil mass some
decimeters thick (e.g., Smith, 1987). Since the thickness
of the mobile layer estimated from the riser height far
exceeds the diurnal freeze-thaw depths, these lobes are
considered to have developed as a result of repeated
annual freeze-thaw cycles. In the permafrost zone,
upward freezing from the top of permafrost can produce ice-rich layers in the lower part of the active layer
(e.g., Mackay, 1981). This process could lead to deeper
soil movements near the active layer-permafrost interface, although no data have yet been obtained in the
Alps.
Millennial freeze-thaw cycles
Annual freezing and thawing are unlikely to reach
depths in excess of 5 m on debris slopes and 10 m on
rock slopes, depths to which only permafrost can penetrate. Segregational freezing tends to produce ice richlayers in the uppermost part of permafrost (e.g., Cheng,
1983) and possibly near the base of permafrost. Largescale mass movements sometimes take place in the
transient permafrost zone. Some of these movements
following abnormally warm summers are possibly
associated with thawing of the top of permafrost.
Thawing may also occur within or at the base of the
permafrost body, causing much deeper changes.
ROCK SLOPES
There are a number of recent records of large-scale
cliff falls in the Alps. The starting zones of these falls
were mainly located near the lower limit of permafrost.
For instance, in October 1988, a rock mass fell from the
north-facing rockwall of Piz Morteratsch, the Upper
Engadin, the fragments being deposited on a glacier
(Haeberli et al., 1992). The detached rock mass had a
volume of about 3¥105 m3 and thickness in excess of
the depth reached by annual freeze-thaw cycles.
Coinciding with the period of the maximum seasonal
thawing, the cliff fall might have been triggered by the
The 7th International Permafrost Conference
Figure 4. Permafrost distribution and landforms in the transient permafrost zone (the Trais Fluors region, Upper Engadin).
partial melting of permafrost and/or the penetration of
meltwater.
Other recent cliff falls (or rockslides) possibly associated with permafrost melting in the Alps include the Val
Pola landslide in 1987 (Dramis et al., 1995), Randa rockslide in 1991 (Schindler et al., 1993) and Zuetribistock
rockslide in 1996. The volume of the Val Pola landslide
is two orders of magnitude larger than the cliff fall at
Piz Morteratsch. Intense precipitation is considered to
have triggered this slide. However, the presence of icecemented rock blocks among the landslide debris indicates that permafrost melting possibly enhanced the
mobility of the rock mass prior to the heavy rain
(Dramis et al., 1995).
DEBRIS SLOPES
Figure 4 displays complicated landforms developed
near the lower limit of permafrost. The BTS values indicate that permafrost underlies the rock glacier (probably inactive), while permafrost is rare in the east-facing
debris slope. Lying near the borderline, however, the
latter slope can be subject to permafrost growth with
minimum cooling. The climatic fluctuation during the
Holocene would have allowed this debris slope to experience millennial freeze-thaw cycles. The major processes modifying the debris slope are debris flows and
solifluction which resulted respectively in alluvial
cones and numerous lobes. Upslope of a large alluvial
cone, lies a landslide scar, 150 m long, 100 m wide and 5
m deep. The total volume of the debris flow deposit in
the alluvial cone is on the order of 104 m3. The size of
the landslide is likely to exceed that originating from
the annual freeze-thaw action. Millennial freeze-thaw
cycles may have intensified weathering of the porous
calcareous rocks and destabilized the debris layer.
Zimmerman and Haeberli (1992) reported that many
large-scale debris flows originated recently near the
lower limit of permafrost. Permafrost melting appears
to have triggered directly or affected indirectly part of
these debris flows. Debris flows, possibly related to
recent permafrost melting, are also observed on the
frontal and side slopes of a number of rock glaciers
(e.g., Haeberli et al., 1993).
Summary and conclusions
Alpine slopes are subjected to three kinds of freezethaw cycles which may be completed in a day, a year or
a century. These freeze-thaw cycles influence the slope
stability over different temporal and spatial scales
(Table 1). Diurnal frost weathering is significant where
rainfall or snowmelt supplies abundant moisture to
rock surface, producing rock debris mainly of pebble
size or smaller. Regardless of the aspect and elevation,
Norikazu Matsuoka, et al.
715
Table 1. The role of freeze-thaw cycles in controlling slope instability in the Swiss Alps
most of the debris slopes experience high frequencies of
diurnal freeze-thaw cycles accompanied by frost heave
of up to 2 cm high and creep of the uppermost soil shallower than 15 cm. Such a shallow movement prevails
on slopes with a thin debris mantle, resulting in small
lobes and stripes. The annual freeze-thaw depth in
rockwalls is calculated to be about 5 m, which delimits
the maximum size of rock debris produced by frost
weathering. The annual freeze-thaw activity reaches
depths of 2 m or slightly more in debris slopes. The
associated soil movement eventually develops solifluction lobes with a riser of 30 cm or higher. The growth
and decay of permafrost, originating from long-term
climatic change, cause freeze-thaw action reaching
meter-to-decameter scale depths. Despite extremely low
frequency, segregational freezing lasting many centuries or millennia may permit the accumulation of icerich layers near the top and bottom of the permafrost
body. Permafrost melting can trigger cliff falls and
debris flows in the thawing phase of millennial freezethaw cycles.
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The 7th International Permafrost Conference
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