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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, supply- ing 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-to- decameter scale depths. Permafrost melting can trigger cliff 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 mass wasting, 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 Ice 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 water 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 glacier sequence, which rock temperature. develops on slopes covered by coarse debris. Patterned ground 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 frost weathering 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 joint 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 screes 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). Frost heaving 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 thermal conductivity, 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.

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