Frost weathering and rock fall in an arctic environment, Longyearbyen, Svalbard

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1 Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN 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, temperature 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 Our understanding of cryogenic weathering and of the temperature conditions in which it occurs has recently made significant progress (Coutard & Francou 1989, Matsuoka 1991, André 1993, Ødegård et al. 1995, Lewkowicz 2001, Hall & André 2001). Nevertheless, all-year around studies with high-resolution monitoring are rare. Temperature variations of at least 2 C per minute have been identified as active decay agents (Hall & André 2001), but until now only the possible occurrence of such thermal shocks has been mentioned in arctic research (Ødegård et al. 1995, Lewkowicz 2001). Among the environmental factors, moisture content is considered to have a major control on frost shattering (Matsuoka 1991, Prick 1997). Nevertheless, rock moisture content is the least monitored parameter in field studies. Monitoring over a long period has rarely been achieved (Ritchie & Davison 1968, Hall 1988, Humlum 1992), and most of the data consists of periodic measurements (Harris & Prick 1997, Matsuoka 1991). The aim of the present study is to monitor rock temperature and moisture content and to assess the importance 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 relation to temperature and humidity data. Only very few studies have been carried out on rock fall and talus slopes on Svalbard (Rapp 1960, Åkerman 1984, André 1993). Field data for this research project will be collected for at least one and a half years. This paper presents only preliminary results, based on observations for rock fall occurrence from the summer and fall of 2001, and for rock temperature and moisture content observations from the summer of 2001 to spring INVESTIGATION AREA The field site is located 2 km NW of Longyearbyen ( N, E). The studied cliff is about 25 m high and faces a NNE direction. This rockwall is characterised by a tectonically undisturbed succession of subhorizontal sandstones and shales (Major & Nagy 1972). These rocks belong to the lower Cretaceous and are highly fractured. The sandstone porosity, measured by immersion under atmospheric pressure of non-fractured blocks, is about 5.2 to 5.6%. The meteorological station of Svalbard Airport is 3 km away from the study site. The mean annual air temperature there was 5.8 C during the period The precipitation measured at sea level is about 200 mm. Snow is the dominant type of precipitation. At sea level, a persistent snow cover is usually registered from late September to late May. The base of the studied cliff has been covered during the winter by a snow accumulation up to 8 m thick at some places. This is an area of continuous permafrost, reaching a maximum thickness of 450 m, but only ranging between m in coastal areas. 3 ROCK TEMPERATURE On the studied cliff, thermistors were used to measure the rock temperature from August 2001 in boreholes 1, 10 and 40 cm deep. The rock temperature is also monitored close to the surface in thin cracks, into which temperature probes are inserted. The temperature is monitored every 10 minutes at 10 and 40 cm deep, and every minute close to the surface, in order to assess the occurrence of thermal shocks (Hall & André 2001). The thermistors are manufactured by Geminidataloggers. 907

2 Their measuring range is from 40 C to 125 C, with a precision of 0.2 C. Their response time is 8 seconds in water and 30 seconds in air. The monitored temperatures (Fig. 1) show that the rockwall is experiencing numerous and sometimes considerable temperature fluctuations, even during the polar winter, and under a thick snow layer (see the period from Nov. 23 to Jan. 11 on the upper graph of Fig. 1). Nevertheless, the temperature close to the surface 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 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 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

3 and in the spring. During the polar winter, the temperature approaches zero degrees several times as a result of milder weather conditions. The freezing of the rock surface in the fall and its melting in the spring cause several freeze/thaw cycles to a depth of 1 cm. The amplitude of temperature variations decreases from the surface downwards due to thermal flow dampening. Even daily temperature fluctuations reach depths of 40 cm, but are very attenuated, with a delay of several hours. At 40 cm deep, the rock freezes once in the fall and remains frozen. These results are consistent with these reported in other works (Coutard & Francou 1989, Lewkowicz 2001). Matsuoka (1991) however, observed that whilst N-facing rockwalls on Svalbard underwent only one annual freeze/thaw cycle, SW-facing walls experienced up to 27 cycles. These sites were located at 300 to 500 m a.s.l., thereby explaining the colder conditions reported by Matsuoka, compared to those presented on Figure 1. The temperature data presented on Figure 1 reflects the whole range of possible thermal conditions, including summer sun shining on the rockwall. For example, when the sky is clear, like on August 28th and 29th, the sun shining directly on the rock in the early morning causes an abrupt temperature change as registered at a depth of 1 cm (Fig. 1). Such sharp rock temperature variations have been reported in other cold environments (Coutard & Francou 1989). Nevertheless, these short events cannot be considered as thermal shocks, as 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. rapid moisture content changes are directly correlated with weather fluctuations, rain and fog inducing an intense hydration. Conversely, snowfall does not necessarily induce a moisture uptake and weight losses can be observed when cold snow covers the samples. As already underlined, moisture content must be considerable during freezing for rocks to be damaged. We can expose the sandstone to high moisture content values (Fig. 2) and rockwall temperatures at that time (Fig. 1), and therefore display how often critical conditions are met from the weathering point of view. It is important to note that high moisture content is reached on very few occasions over the monitored period and does not necessarily coincide with freeze/thaw cycles, a relationship also confirmed by Ritchie & Davison (1968). We consider those degrees of saturation higher than 50% as potentially dangerous. This is a very low threshold, as the lowest critical degree of saturation generally lies around 60% (Prick 1997, 1999). These conditions are met on September 12th, 13th, 14th, 16th, 18th, 24th; October 22nd, 23rd, 24th; April 5th. Among these periods, 4 were prone to frost weathering, although we admit that this figure may have been greater as some high moisture levels occurring between measurements might have been missed. On September 19th, the rock temperature dropped below 0 C when the rock moisture content was probably still quite high. Late on September 24th, temperatures became negative and remained so for days. The high moisture content registered on October 22nd was due 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 seasonal distribution of rock moisture content variations. All the samples show simultaneous fluctuations of similar amplitude. Figure 2 shows that large and rapid variations of rock moisture content occur during the fall and spring, and that the winter is characterised by a progressive drying of the rock, a process also described by Humlum (1992). This is probably the result of sublimation in Svalbards dry climate. The Figure 2. Evolution of the moisture content (expressed in % of the total saturation measured by progressive immersion) of a parallelepipedic sandstone sample (74 cm 3 ) measured from September 10th 2001 to April 12th No measurements were carried out from November 22nd to December 28th and from January 1st to 9th. 909

4 to mild air temperatures, enabling small sized samples to reach the melting point and snow precipitation. The temperature of the superficial part of the rockwall did not rise above the melting point during the second half of October however. In this case, the temperature and humidity conditions were favourable for frost damage on the small size samples but not on the rockwall. On April 5th, the rockwall underwent a rapid freezing, reaching 16 C on April 6th. Frost weathering is therefore theoretically unable to occur frequently on Svalbard. When the necessary temperature and moisture conditions are met however, weathering action may be intense, because of the high moisture content present in the rock (e.g. September 19th and 24th), the quick cooling (April 5th) or the extended duration of freezing periods (October 22nd). On the basis of punctual evaluations of blocks moisture content in summer, Matsuoka (1991) concluded that moisture was insufficient on Svalbard for frost shattering to be effective. Our current results bring evidence that high moisture levels can be reached in the fall and in the spring. 5 WEATHERING PROCESSES Using as criteria their dynamic Young s modulus variations, a regular evaluation of rock weathering before cracking, weight loss or any other visible change, is made for rock pieces exposed to the natural environment. A non-destructive determination of the Young s modulus is carried out using a Grindosonic apparatus whose principle has been explained in former publications (Prick 1997, 1999). Five parallelepipedic samples of fresh local sandstone, with an average volume of 73.8 cm 3, were exposed on a natural ledge at the studied rockwall from September 3rd, 2001 to January 30th, When retrieved, the block surfaces looked unchanged, the angles and ridges were still as sharp. The sample dry weights and their dynamic Young s modulus had not changed significantly, as no decay had occurred during this period. This may mean that conditions favourable for frost weathering did not occur often enough to initiate rock decay and that a longer exposure time is required for weathering to occur. It may also mean that the critical degree of saturation of this sandstone is very high (e.g. more than 90%) and that the moisture content maxima represented on Figure 2 are actually not high enough to cause shattering. Alternatively, as previously outlined, the porosity of this sandstone is quite low. It is well known that rocks with a very low porosity are not frost sensitive, Lautridou and Ozouf (1982) propose a threshold value of 6% for limestones. According to this empirical rule, the local sandstone is not frost sensitive, at least when its porous media is taken into consideration. Nevertheless, considering the crack density of the outcropping sandstone beds, it is clear that determinant processes linked to water uptake and migration will take place in these cracks. Wedging is therefore the cryogenic weathering process most likely to occur on this type of rock outcrop (see also: Coutard & Francou 1989). Ice, filling rock macro-cracks, was observed during the winter and spring. This result simply reflects the fact that the technique used cannot give any indication about rocks whose decay would happen exclusively through the outcrop fractures. Using the Grindosonic implies rock samples with a perfect geometrical shape, and with no cracks. The Grindosonic measurements are therefore very helpful at detecting damage caused in the porous media by volume expansion or segregation ice formation. They fail however to identify those weathering processes acting within the cracks that separate the blocks in the field from which the shaped samples are taken. Two samples of 5 French limestones (Caen, Vilhonneur, Larrys, Charentenay, Sireuil), previously used in other weathering experimentation (Prick 1997, 1999), have been exposed for the same period at the same location as the local sandstone samples. The Caen limestone did not show any change in its modulus of elasticity. The four other limestones showed an average decrease in Young s dynamic modulus values by 1.8% (Larrys), 6.1% (Charentenay), 7.4% (Vilhonneur) and 11.1% (Sireuil) however. Previous research regarding the frost sensitivity of these five limestones in four different experimental conditions (Prick 1997, 1999), shows that only the Caen limestone was insensitive to any tested conditions. This observation on imported rocks has an important implication. It shows that the local environment at the studied site should be considered as aggressive from a weathering point of view, with the exception of those rocks that are not frost sensitive because of their remarkable physical characteristics (e.g. Caen limestone). The local sandstone, when not chemically weathered or fractured, is quite resistant to cryogenic weathering. Locally there is some evidence for chemical weathering on the studied rockwall. Firstly, iron oxidation can colour the rock surface and disintegrate the iron carbonate nodules included in the bedrock, leaving small circular depressions on clay-ironstone lenses. Secondly, salt outbursts can develop locally during dry summer periods. Although it is not clear whether this salt precipitation leads to physical or chemical weathering, as salt weathering can act both ways, it clearly causes induration and/or desquamation of the local rock surface. Chemical processes are considered increasingly likely to play a role in cold environment weathering (André 1993). The decay features described and the processes leading to their occurrence will need 910

5 further investigation. These processes are part of the so-called granular weathering, as they lead to the formation of very fine debris that will contribute to the fine fraction of the talus slope. 6 ROCK FALL Twelve squares of 50 by 50 cm were painted on sandstone and shale beds displaying various degrees of fracturing. The colour spray used was chosen to minimise alteration of the rock albedo, according to a wellknown technique (Matsuoka 1991). The decay of these squares was assessed simply by visual observation between July and October 2001 after which the squares disappeared under the snow accumulating at the base of the rockwall. Only 1 square showed no rock fall activity at all. 4 out of the 12 squares showed a visible loss of painted surface, with at least one small piece of rock falling off. Seven other squares lost tiny painted rock pieces that could be found on the ground, but were so small that they did not cause a visible loss on the painted surface. This technique has shown surprisingly quick and widespread debris release. Five sediment traps set at the base of the rockwall collected the falling rock debris. These were emptied about 4 times a week and the collected debris was sieved at 2 mm, dried and weighed. The largest rock falls occurred mainly in September and October (Fig. 3). The largest rock fall event monitored in this study happened on September 19th. This was not due to a rapid freezing rate, nor to high frost intensity, but as a result of the wet weather that prevailed during the preceding days (Fig. 2), wetting the cliff at depth prior to freezing. Hydraulic pressure exerted on the waterfilled cracks by a freezing front progressing slowly from the surface is the most likely process to have caused this large debris liberation, as the moderate freezing temperatures were unlikely to cause a freezing of all the water present in the crack system. More debris liberation took place in the following days, probably linked to the slow progression of the freezing front into the rock wall to depths greater than 10 cm, but less than 40 cm (Fig. 1). On September 24th, rock debris were probably liberated by the melting of part of the ice that had been cementing blocks together. September 19th and 24th have previously been defined as days theoretically prone to intense rock weathering. The next important debris liberation occurred between October 8th and 10th, due to a melting of the rockwall surface that led to the same kind of debris liberation that occurred on September 24th. The results presented here from Svalbard show a maximum in rock fall activity in the autumn. Important activity is also theoretically expected in the spring, when the rockwall thaws at depth from the surface Figure 3. Dry weight of the rock debris larger than 2 mm, collected on a 3.50 m long sediment trap set at the base of the studied rockwall. Results are presented for the period of July 16th to November 19th, 2001 and expressed in kg of debris for a one-meter long section of the 25 m high cliff. For the days on which the debris are not collected, the weight value of the next rock collection is spread over the period elapsed since the last collection. The arrow indicates that on September 19th, only a part of the debris is represented on this graph. After the occurrence of the largest experienced rock fall event in this study, only the fine debris was brought back to the laboratory. This was sieved at 2 mm, dried and weighed (72.4 kg). The largest blocks were left in the field, their volume estimated to be about 5 m 3. (Ødegård et al. 1995). The occurrence of freeze/thaw cycles when rock moisture content is high, such as on April 5th 2002, indicates that conditions for rock decay and important rock fall activity are met in the spring. Further fieldwork will hopefully provide more data about spring and summer rock fall occurrence. 7 CONCLUSIONS Despite the fact that this study does not yet present a one-year data set, the following general conclusions can be drawn: 1. Rockwall temperature shows frequent fluctuations, even during the polar night. Freeze/thaw cycles were nevertheless registered at depths of less than 1 cm only during the autumn and spring. At 40 cm, the rock freezes in the fall and remains frozen. 2. No evidence of thermal stress fatigue was found. Only 3 thermal shocks were registered over the whole monitored period. 911

6 3. Rock moisture content shows large and rapid variations caused by weather conditions. Rock samples tend to dry slowly during the winter season, probably because of sublimation. High moisture content occurs rarely, and only in the fall and spring. 4. Conditions favorable for cryogenic weathering, such as freezing of the rock when its moisture content is high, were probably met on very few days (at least 4) over this 8 months period. When these conditions are met however, frost action may be very aggressive because of the high rock moisture content and the quick cooling or extended duration of freezing periods. This frost efficiency is shown by the decrease in modulus of elasticity from 4 of 5 porous limestones exposed at the study site. 5. A similar exposure did not cause any decrease in the modulus of elasticity of 5 sandstone samples. Frost shattering does not act through the porous media in this low porosity sandstone, but by wedging of its well-developed crack system. 6. Rock fall occurrence showed a very irregular distribution between July and November Most rock falls took place in September and October. The largest occurred on days when the conditions were particularly favourable for cryogenic weathering. ACKNOWLEDGEMENTS Prof. Ole Humlum is warmly thanked for his support, comments and advice. Special thanks are due to Prof. A. Ozer and other members of the Dep. of Geography at the University of Liège for the use the Grindosonic apparatus, and to Prof. E. Poty and his team for their collaboration in the sawing of limestone samples. This research has been supported by a Marie Curie Fellowship of the European Community program Improving the Human Research Potential under the contract number HPMF-CT Disclaimer: The European Commission is not responsible for any views or results expressed. REFERENCES Åkerman, H.J Notes on talus morphology and processes in Spitsbergen. Geografiska Annaler 66 A (4): André, M.F Les versants du Spitsberg: approche géographique des paysages polaires. Nancy: Presses Universitaires de Nancy. Coutard, J.P. & Francou, B Rock temperature measurements in two alpine environments: implications for frost shattering. Arctic and Alpine Research 21 (4): Hall, K Daily monitoring of a rock tablet at a maritime Antarctic site: moisture and weathering results. British Antarctic Survey Bulletin 79: Hall, K. & André, M.F New insights into rock weathering from high-frequency rock temperature data: an Antarctic study of weathering by thermal stress. Geomorphology 41 (1): Harris, S.A. & Prick, A The periglacial environment of Plateau Mountain : an overview of current periglacial research. Polar Geography 21 (2): Humlum, O Observations on rock moisture availability in gneiss and basalt under natural, Arctic conditions. Geografiska Annaler 74 A (2 3): Lautridou, J.P. & Ozouf, J.Cl Experimental frost shattering : 15 years of research at the Centre de Géomorphologie du CNRS. Progress in Physical Geography 6 (2): Lewkowicz, A.G Temperature regime of a small sandstone tor, latitude 80 N, Ellesmere Island, Nunavut, Canada. Permafrost and Periglacial Processes 12: Major, H. & Nagy, J Geology of the Adventdalen map area. Norsk Polarinstitutt Skrifter 138. Matsuoka, N A model of the rate of frost shattering: application to field data from Japan, Svalbard and Antarctica. Permafrost and Periglacial Processes 2 (4): Ødegård, R.S., Etzelmüller, B., Vatne, G. & Sollid, J.L Near-surface spring temperatures in an Arctic coastal cliff: possible implications for rock breakdown. In: O. Slaymaker (ed.), Steepland Geomorphology: Chichester: Wiley. Prick, A Critical degree of saturation as a threshold moisture level in frost weathering of limestones. Permafrost and Periglacial Processes 8 (1): Prick, A Etude dilatométrique de la cryoclastie et de l haloclastie. Mémoire de la Classe des Sciences, Tome XIX. Bruxelles : Ed. Académie Royale de Belgique. Rapp, A Talus slopes and mountain walls at Tempelfjorden, Spitsbergen. Norsk Polarinstitutt Skrifter 119: 96 p. Ritchie, T. & Davison, J.I Moisture content and freeze-thaw cycles of masonry materials. Journal of materials, JMLSA 3 (3):