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Frost Weathering and Rock Fall in an Arctic Environment, Longyearbyen, Svalbard

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Frost Weathering and Rock Fall in an Arctic Environment, Longyearbyen, Svalbard Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Frost weathering and rock fall in an arctic environment, Longyearbyen, Svalbard A. Prick The University Courses on Svalbard (U.N.I.S.), Longyearbyen, Norway ABSTRACT: A rockwall consisting of sandstone and shale is monitored on a year-around basis in order to improve understanding of the relationships between rock temperature, rock moisture content, weathering evolution and rock fall occurrence in a high latitude environment. Over the first 8 months of monitoring, tem- perature and moisture conditions prone to cause frost damage were met only rarely. No evidence of a frequent occurrence of thermal shocks was found. Cryogenic weathering is thought to act on this rockwall by wedging. Rock fall activity is evaluated using sediment traps and checking the decay evolution of painted squares on the rockwall. The largest rock fall events happened on days when conditions were met for important cryogenic weathering. 1 INTRODUCTION 2 INVESTIGATION AREA Our understanding of cryogenic weathering and of the The field site is located 2 km NW of Longyearbyen temperature conditions in which it occurs has recently (78°13Ј38ЈЈN, 15°36Ј00ЈЈE). The studied cliff is made significant progress (Coutard & Francou 1989, about 25 m high and faces a NNE direction. This Matsuoka 1991, André 1993, Ødegård et al. 1995, rockwall is characterised by a tectonically undis- Lewkowicz 2001, Hall & André 2001). Nevertheless, turbed succession of subhorizontal sandstones and all-year around studies with high-resolution monitor- shales (Major & Nagy 1972). These rocks belong to ing are rare. Temperature variations of at least 2°C per the lower Cretaceous and are highly fractured. The minute have been identified as active decay agents sandstone porosity, measured by immersion under (Hall & André 2001), but until now only the possible atmospheric pressure of non-fractured blocks, is occurrence of such thermal shocks has been men- about 5.2 to 5.6%. tioned in arctic research (Ødegård et al. 1995, The meteorological station of Svalbard Airport Lewkowicz 2001). Among the environmental factors, is 3 km away from the study site. The mean annual moisture content is considered to have a major control air temperature there was Ϫ5.8°C during the period on frost shattering (Matsuoka 1991, Prick 1997). 1975–2000. The precipitation measured at sea level is Nevertheless, rock moisture content is the least moni- about 200 mm. Snow is the dominant type of precipita- tored parameter in field studies. Monitoring over a tion. At sea level, a persistent snow cover is usually long period has rarely been achieved (Ritchie & registered from late September to late May. The base of Davison 1968, Hall 1988, Humlum 1992), and most the studied cliff has been covered during the winter of the data consists of periodic measurements (Harris 2001–2002 by a snow accumulation up to 8 m thick at & Prick 1997, Matsuoka 1991). some places. This is an area of continuous permafrost, The aim of the present study is to monitor rock tem- reaching a maximum thickness of 450 m, but only perature and moisture content and to assess the impor- ranging between 10–40 m in coastal areas. tance of frost action as a weathering agent in the arctic environment. Rock decay evolution and rock fall occurrence is studied on a rockwall and interpreted in 3 ROCK TEMPERATURE relation to temperature and humidity data. Only very few studies have been carried out on rock fall and On the studied cliff, thermistors were used to measure talus slopes on Svalbard (Rapp 1960, Åkerman 1984, the rock temperature from August 2001 in boreholes 1, André 1993). Field data for this research project will 10 and 40 cm deep. The rock temperature is also moni- be collected for at least one and a half years. This tored close to the surface in thin cracks, into which paper presents only preliminary results, based on temperature probes are inserted. The temperature is observations for rock fall occurrence from the sum- monitored every 10 minutes at 10 and 40 cm deep, and mer and fall of 2001, and for rock temperature and every minute close to the surface, in order to assess the moisture content observations from the summer of occurrence of thermal shocks (Hall & André 2001). The 2001 to spring 2002. thermistors are manufactured by Geminidataloggers. 907 Their measuring range is from Ϫ40°C to ϩ125°C, with considerable temperature fluctuations, even during a precision of 0.2°C. Their response time is 8 seconds in the polar winter, and under a thick snow layer (see the water and 30 seconds in air. period from Nov. 23 to Jan. 11 on the upper graph of The monitored temperatures (Fig. 1) show that the Fig. 1). Nevertheless, the temperature close to the sur- rockwall is experiencing numerous and sometimes face crossed the zero degree threshold only in the fall Figure 1. Upper graph: Evolution of rock temperature in the studied rockwall, measured every 10 minutes at 10 and 40 cm deep and every minute at 1 cm deep, from August 20th 2001 to January 11th 2002. The rock beds supporting these three probes have been covered by a thick snow layer after November 23rd; Lower graph: Rock temperature measured every minute in a crack on a wind-blown portion of the rockwall, from January 18th to April 13th 2002. As demonstrated in this study, the discrepancy between the temperature in a crack and the temperature in a 1 cm deep borehole is not larger that the one induced by microclimatic differences between different portions of the same rockwall, only a few meters apart. 908 and in the spring. During the polar winter, the temper- rapid moisture content changes are directly correlated ature approaches zero degrees several times as a result with weather fluctuations, rain and fog inducing an of milder weather conditions. The freezing of the rock intense hydration. Conversely, snowfall does not neces- surface in the fall and its melting in the spring cause sarily induce a moisture uptake and weight losses can several freeze/thaw cycles to a depth of 1 cm. The be observed when cold snow covers the samples. amplitude of temperature variations decreases from As already underlined, moisture content must be the surface downwards due to thermal flow dampen- considerable during freezing for rocks to be damaged. ing. Even daily temperature fluctuations reach depths We can expose the sandstone to high moisture content of 40 cm, but are very attenuated, with a delay of sev- values (Fig. 2) and rockwall temperatures at that time eral hours. At 40 cm deep, the rock freezes once in the (Fig. 1), and therefore display how often “critical” con- fall and remains frozen. ditions are met from the weathering point of view. It is These results are consistent with these reported in important to note that high moisture content is reached other works (Coutard & Francou 1989, Lewkowicz on very few occasions over the monitored period and 2001). Matsuoka (1991) however, observed that whilst does not necessarily coincide with freeze/thaw cycles, N-facing rockwalls on Svalbard underwent only one a relationship also confirmed by Ritchie & Davison annual freeze/thaw cycle, SW-facing walls experienced (1968). We consider those degrees of saturation higher up to 27 cycles. These sites were located at 300 to than 50% as potentially dangerous. This is a very low 500 m a.s.l., thereby explaining the colder conditions threshold, as the lowest critical degree of saturation reported by Matsuoka, compared to those presented generally lies around 60% (Prick 1997, 1999). These on Figure 1. conditions are met on September 12th, 13th, 14th, The temperature data presented on Figure 1 reflects 16th, 18th, 24th; October 22nd, 23rd, 24th; April 5th. the whole range of possible thermal conditions, includ- Among these periods, 4 were prone to frost weather- ing summer sun shining on the rockwall. For example, ing, although we admit that this figure may have been when the sky is clear, like on August 28th and 29th, the greater as some high moisture levels occurring sun shining directly on the rock in the early morning between measurements might have been missed. On causes an abrupt temperature change as registered at September 19th, the rock temperature dropped below a depth of 1 cm (Fig. 1). Such sharp rock temperature 0°C when the rock moisture content was probably still variations have been reported in other cold environ- quite high. Late on September 24th, temperatures ments (Coutard & Francou 1989). Nevertheless, these became negative and remained so for days. The high short events cannot be considered as thermal shocks, as moisture content registered on October 22nd was due they did not reach a temperature variation rate of 2°C/minute. From August 20th, 2001 to April 13th, 2002, only 3 thermal shocks were observed. All of them occurred at very low temperatures between March 7th and 15th and were not connected to an insolation effect. Being the only evidence we have up to now of the occurrence of thermal shocks, we cannot yet conclude that these processes cause any stress in this dry polar environment. 4 ROCK MOISTURE Two shaped and 5 unshaped sandstone blocks are exposed to atmospheric conditions outside the UNIS building (1400 m away from the studied rockwall). They are weighed daily, at roughly the same time, in order to provide data about the amplitude and sea- sonal distribution of rock moisture content variations. All the samples show simultaneous fluctuations of similar amplitude. Figure 2 shows that large and rapid Figure 2. Evolution of the moisture content (expressed in variations of rock moisture content occur during the % of the total saturation measured by progressive immer- fall and spring, and that the winter is characterised sion) of a parallelepipedic sandstone sample (74 cm3) mea- by a progressive drying of the rock, a process also sured from September 10th 2001 to April 12th 2002.
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