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 and debris slopes in the Swiss 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 and soil movement up to meter scale, supply- ing boulders to rock and developing lobes with risers of 30 cm or higher. The growth and decay of , originating from long-term climatic change, lead to freeze-thaw activity reaching meter-to- decameter scale depths. Permafrost melting can trigger falls and debris flows in the thawing phase of mil- lennial freeze-thaw cycles.

Introduction thaw cycles, as well as of diurnal and annual freeze- thaw cycles. Millennial freeze-thaw cycles can also Freeze-thaw action induces both rock weathering and operate in the permafrost zone as a result of melting , destabilizing rock and debris slopes in and refreezing of the top and bottom of the permafrost high mountain regions. Two types of freeze-thaw body, although their effects would be less dramatic than cycles, diurnal and annual, are normally recognized in the transient permafrost zone. During the Little according to the period for the completion of one cycle. Age, a large part of the transient permafrost zone was In addition, recent global warming has highlighted a probably characterised by a freezing phase of a millen- third type, which has a much longer period. nial cycle. The 20th Century warming will have Corresponding to the growth and decay of permafrost, switched this zone into a thawing phase. this type of freeze-thaw is completed typically in many centuries or millennia (e.g., Haeberli, 1996) and here is The prediction of future geomorphic changes due to termed the millennial freeze-thaw cycle. The relation- global warming requires the distinction of effects due to ship between the freeze-thaw types and the magnitude and nature of resulting geomorphic processes, however, has been poorly understood because of the lack of long- term, continuous monitoring of processes and variables.

The periglacial belt in a mountain area is usually sub- divided into permafrost and seasonal frost zones, main- ly 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- 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 ranges of diurnal fluctuation in the rock surface tem- distinction is necessary, in particular, in the permafrost perature. As a result, diurnal freeze-thaw cycles take and transient permafrost zones where permafrost melt- place only in early autumn. In contrast, because of the ing is in progress and the three freeze-thaw types are lack of snowcover, the south-facing rockwall undergoes superimposed, causing slope instability. This report large diurnal fluctuation in the surface temperature aims at evaluating the effects of the three kinds of throughout the year. This thermal condition favours the freeze-thaw cycles on alpine slope instability, based on high frequency of diurnal freeze-thaw cycles during all studies of contemporary periglacial processes in the seasons except midsummer. Swiss Alps. Attention will be focused on the scales of geomorphic change caused by each freeze-thaw type. Temperature fluctuations across 0¡C, however, do not always indicate freeze-thaw alternations effective in The study area is located in the Upper Engadin, eas- rock breakage. An abundant moisture supply is neces- tern Switzerland. The lower limit of permafrost lies at sary for frost damage (e.g., Matsuoka, 1991; Prick, about 2400 m ASL on northern exposures, rising to 1997). Subzero temperatures, following the infiltration about 3000 m ASL on southern exposures. The of into the joints and pores in the bedrock, may periglacial belt, lying above the timberline at 2000 to cause effective freezing expansion. Consequently, the 2200 m ASL, includes both present-day permafrost and effective diurnal freeze-thaw cycles must be conside- non-permafrost areas. The most extensive periglacial rably fewer than those counted from the fluctuation in landscape is the talus-to-rock sequence, which rock temperature. develops on slopes covered by coarse debris. and solifluction features are also common, and Frost (or thaw) penetration in the bedrock is usually are characteristic of the slopes underlain by fine debris 30 cm or shallower during a diurnal freeze-thaw cycle (Matsuoka et al., 1997). (Figure 2). In response to the amount of the latent heat exchange, however, wet rocks favourable for freezing Diurnal freeze-thaw cycles expansion are subjected to much shallower freeze-thaw. Furthermore, frost damage can occur at depths cooled ROCK SLOPES to a few degrees below 0¡C (e.g., Matsuoka, 1994). Thus The magnitude and frequency of diurnal freeze-thaw diurnal is considered to be active with- cycles depend partly on the aspect of slopes. This ten- in 10 to 20 cm of the rock surface and able to produce dency is enhanced on steep rockwalls. Figure 2 displays rock debris up to cobble size. Controlled by spa- the contrast of rock surface temperatures between north cing, the size of the released rock debris can be smaller. and south-facing rockwalls (TFN and TFS sites). Both In fact, pebbles are the major components of are located at 2850 m ASL. Covered with thick snow below the south-facing rockwalls. Observations of sca- from winter to spring, the north-facing rockwall expe- ling from the painted bedrock also showed that a num- riences continuous subzero temperatures. Even during ber of rock fragments smaller than 5 cm were produced summer months, the minimal insolation leads to small 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 fluctua- tions are displayed by isotherms at 2¡C intervals.

712 The 7th International Permafrost Conference DEBRIS SLOPES mainly from diurnal freeze-thaw cycles. In fact, the Large parts of the debris slopes in the study area are sorting depth of the stripes is about 5 cm and the risers covered with snow for half of the year. Diurnal freeze- of the lobes are about 10 cm high, values similar to the thaw cycles are most frequent in early autumn and are depth of soil movement induced by diurnal frost heave prevented by the late-lying snowcover in spring activity. (Figure 3). Windy crest slopes lack snowcover and experience frequent freeze-thaw cycles in both autumn Annual freeze-thaw cycles and spring (Matsuoka et al., 1997).

ROCK SLOPES Debris slopes experience shallower freeze-thaw Regardless of the aspect and the presence of per- depths than rock slopes, because of the lower thermal mafrost, rock slopes in the periglacial belt are subjected conductivity and larger latent heat. Diurnal frost depth to deep seasonal freezing and thawing. Direct determi- is typically about 5 cm and rarely in excess of 15 cm nation of annual frost or thaw penetration is difficult. (Figure 3). usually accompanies diurnal Equations derived from the thermal conduction theory, freeze-thaw cycles. The heave amount depends upon however, permit us to estimate the depth using the the soil granulometry, but rarely exceeds 2 cm. Despite freezing or thawing index at the rock surface such small individual heaves, the cumulative amount is (Matsuoka, 1994). The modified Berggren equation considerable. Thin debris mantles and insignificant (Aldrich, 1956), one of the Stefan-type equations, was snowcover combine to make diurnal frost heaving pre- used for the calculation of the frost (or thaw) penetra- vail on crest slopes where small sorted stripes and lobes tion depth in the rockwalls. The , a dominate. These landforms are considered to originate 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 calcu- exchange and large cold air drainage through the open- lation includes assumptions of the vertical gradient of work clasts. Where a large part of the freeze-thaw layer the mean annual rock temperature being negligible and consists of fine debris, seasonal freezing is associated the freezing point at 0.0¡C. Such a simplification does with a large frost heave (5 cm or more). Formation of not seem to lower the accuracy of calculation signifi- ice lenses tends to be concentrated in the upper part of cantly (Matsuoka, 1994). This model was applied to the annual freeze-thaw layer, because the progressive TFN and TFS sites (Figure 2). The mean annual surface downward freezing may cause desiccation of the lower temperature was negative at both rockwalls, indicating part. the presence of permafrost. The maximum thaw depth in 1995 was computed to be 4.3 m at TFN site and 6.8 m Thawing of the heaved ground, often aided by at TFS site. These values are comparable with the com- snowmelt, raises the moisture content and mobility of puted frost depth in a seasonal frost environment of the the thawed soil, resulting in solifluction or small debris Japanese Alps (Matsuoka, 1994). Thus, the annual flows. In response to the locations of ice lenses, soil freeze-thaw depth in the alpine periglacial belt is typi- movements due to annual freeze-thaw cycles would cally 5±2 m, rarely reaching the decameter scale. occur mainly in the upper part of seasonal frost. In fact, many solifluction lobes in the study area have a riser 30 The annual freeze-thaw depth defines the boundary to 50 cm high. Solifluction lobes with similar riser to which frost weathering can operate annually. The heights seem to reflect the movement of soil mass some above calculation predicts that a rock mass up to about decimeters thick (e.g., Smith, 1987). Since the thickness 5 m thick is detachable from the rockwalls. The loca- of the mobile layer estimated from the riser height far tions at which frost damage happens, however, depend exceeds the diurnal freeze-thaw depths, these lobes are on several factors including the joint patterns, moisture considered to have developed as a result of repeated distribution, and temperature range at which ice segre- annual freeze-thaw cycles. In the permafrost zone, gation occurs. The concurrent monitoring of rock tem- upward freezing from the top of permafrost can pro- peratures and joint opening on the rockwall behind the duce ice-rich layers in the lower part of the Murt•l , the Upper Engadin, showed that (e.g., Mackay, 1981). This process could lead to deeper the largest opening at the rock surface occurred during soil movements near the active layer-permafrost inter- an early thawing period when meltwater infiltrated the face, although no data have yet been obtained in the joint and refroze (Matsuoka et al., 1997). Since only Alps. minor opening was recorded during seasonal freezing in winter, moisture supply is considered to play a major Millennial freeze-thaw cycles role in near-surface frost weathering. In permafrost areas, segregational freezing may lead to intensive frost Annual freezing and thawing are unlikely to reach damage at the base of active layer where moisture depths in excess of 5 m on debris slopes and 10 m on availability is high (Hallet et al., 1991), although this rock slopes, depths to which only permafrost can pene- idea has yet to be verified by field evidence. trate. Segregational freezing tends to produce ice rich- layers in the uppermost part of permafrost (e.g., Cheng, Frost damage associated with seasonal freezing is 1983) and possibly near the base of permafrost. Large- often followed by on thawing. For instance, in scale mass movements sometimes take place in the early June 1997, a block of rock about 100 m3 was transient permafrost zone. Some of these movements detached from the rockwall behind the Murt•l rock following abnormally warm summers are possibly glacier. The slip plane lay at about 2 m deep. This block associated with thawing of the top of permafrost. fall happened during a high temperature period after Thawing may also occur within or at the base of the snowmelt (Matsuoka, 1997). These conditions indicate permafrost body, causing much deeper changes. that rapid seasonal thawing triggered the block detach- ment. The block was broken into numerous boulders ROCK SLOPES and smaller debris, which were deposited on a There are a number of recent records of large-scale and a rock glacier. Despite low frequency, such a block cliff falls in the Alps. The starting zones of these falls fall would be an important source in terms of the debris were mainly located near the lower limit of permafrost. supply on screes and rock glaciers. For instance, in October 1988, a rock mass fell from the north-facing rockwall of Piz Morteratsch, the Upper DEBRIS SLOPES Engadin, the fragments being deposited on a glacier Seasonal freezing penetrates to about 2 m deep in (Haeberli et al., 1992). The detached rock mass had a debris slopes located just below the lower limit of per- volume of about 3¥105 m3 and thickness in excess of mafrost (Figure 3). Thaw penetration over the rock glac- the depth reached by annual freeze-thaw cycles. ier permafrost is slightly deeper, reaching 3 to 5 m (e.g., Coinciding with the period of the maximum seasonal Barsch, 1996), probably because of the small latent heat thawing, the cliff fall might have been triggered by the

714 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 cones and numerous lobes. Upslope of a large alluvial meltwater. cone, lies a scar, 150 m long, 100 m wide and 5 m deep. The total volume of the debris flow deposit in Other recent cliff falls (or rockslides) possibly associa- the alluvial cone is on the order of 104 m3. The size of ted with permafrost melting in the Alps include the Val the landslide is likely to exceed that originating from Pola landslide in 1987 (Dramis et al., 1995), Randa rock- the annual freeze-thaw action. Millennial freeze-thaw slide in 1991 (Schindler et al., 1993) and Zuetribistock cycles may have intensified weathering of the porous rockslide in 1996. The volume of the Val Pola landslide calcareous rocks and destabilized the debris layer. is two orders of magnitude larger than the cliff fall at Piz Morteratsch. Intense is considered to Zimmerman and Haeberli (1992) reported that many have triggered this slide. However, the presence of ice- large-scale debris flows originated recently near the cemented rock blocks among the landslide debris indi- lower limit of permafrost. Permafrost melting appears cates that permafrost melting possibly enhanced the to have triggered directly or affected indirectly part of mobility of the rock mass prior to the heavy rain these debris flows. Debris flows, possibly related to (Dramis et al., 1995). recent permafrost melting, are also observed on the frontal and side slopes of a number of rock glaciers DEBRIS SLOPES (e.g., Haeberli et al., 1993). Figure 4 displays complicated landforms developed near the lower limit of permafrost. The BTS values indi- Summary and conclusions cate that permafrost underlies the rock glacier (proba- bly inactive), while permafrost is rare in the east-facing Alpine slopes are subjected to three kinds of freeze- debris slope. Lying near the borderline, however, the thaw cycles which may be completed in a day, a year or latter slope can be subject to permafrost growth with a century. These freeze-thaw cycles influence the slope minimum cooling. The climatic fluctuation during the stability over different temporal and spatial scales Holocene would have allowed this debris slope to expe- (Table 1). Diurnal frost weathering is significant where rience millennial freeze-thaw cycles. The major proces- rainfall or snowmelt supplies abundant moisture to ses modifying the debris slope are debris flows and rock surface, producing rock debris mainly of pebble solifluction which resulted respectively in alluvial 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 climatic change, cause freeze-thaw action reaching diurnal freeze-thaw cycles accompanied by frost heave meter-to-decameter scale depths. Despite extremely low of up to 2 cm high and creep of the uppermost soil shal- frequency, segregational freezing lasting many cen- lower than 15 cm. Such a shallow movement prevails turies or millennia may permit the accumulation of ice- on slopes with a thin debris mantle, resulting in small rich layers near the top and bottom of the permafrost lobes and stripes. The annual freeze-thaw depth in body. Permafrost melting can trigger cliff falls and rockwalls is calculated to be about 5 m, which delimits debris flows in the thawing phase of millennial freeze- the maximum size of rock debris produced by frost thaw cycles. weathering. The annual freeze-thaw activity reaches depths of 2 m or slightly more in debris slopes. The associated soil movement eventually develops solifluc- tion lobes with a riser of 30 cm or higher. The growth and decay of permafrost, originating from long-term

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