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

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 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°1338N, 15°3600E). 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 5.8°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 40°C to 125°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 weathering 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. No described by Humlum (1992). This is probably the measurements were carried out from November 22nd to result of sublimation in Svalbards dry climate. The December 28th and from January 1st to 9th.

909 to mild air temperatures, enabling small sized samples sensitive, at least when its porous media is taken to reach the melting point and snow precipitation. The into consideration. Nevertheless, considering the crack temperature of the superficial part of the rockwall did density of the outcropping sandstone beds, it is clear not rise above the melting point during the second half that determinant processes linked to water uptake and of October however. In this case, the temperature and migration will take place in these cracks. Wedging is humidity conditions were favourable for frost damage therefore the cryogenic weathering process most likely on the small size samples but not on the rockwall. On to occur on this type of rock outcrop (see also: Coutard April 5th, the rockwall underwent a rapid freezing, & Francou 1989). Ice, filling rock macro-cracks, was reaching 16°C on April 6th. observed during the winter and spring. This result sim- Frost weathering is therefore theoretically unable to ply reflects the fact that the technique used cannot give occur frequently on Svalbard. When the necessary any indication about rocks whose decay would happen temperature and moisture conditions are met however, exclusively through the outcrop fractures. Using the weathering action may be intense, because of the high Grindosonic implies rock samples with a perfect geo- moisture content present in the rock (e.g. September metrical shape, and with no cracks. The Grindosonic 19th and 24th), the quick cooling (April 5th) or the measurements are therefore very helpful at detecting extended duration of freezing periods (October 22nd). damage caused in the porous media by volume expan- On the basis of punctual evaluations of blocks mois- sion or segregation ice formation. They fail however to ture content in summer, Matsuoka (1991) concluded identify those weathering processes acting within the that moisture was insufficient on Svalbard for frost cracks that separate the blocks in the field from which shattering to be effective. Our current results bring the shaped samples are taken. evidence that high moisture levels can be reached in Two samples of 5 French limestones (Caen, the fall and in the spring. Vilhonneur, Larrys, Charentenay, Sireuil), previously used in other weathering experimentation (Prick 1997, 1999), have been exposed for the same period at the 5 WEATHERING PROCESSES same location as the local sandstone samples. The Caen limestone did not show any change in its modu- Using as criteria their dynamic Young’s modulus vari- lus of elasticity. The four other limestones showed ations, a regular evaluation of rock weathering before an average decrease in Young’s dynamic modulus cracking, weight loss or any other visible change, is values by 1.8% (Larrys), 6.1% (Charentenay), 7.4% made for rock pieces exposed to the natural environ- (Vilhonneur) and 11.1% (Sireuil) however. Previous ment. A non-destructive determination of the Young’s research regarding the frost sensitivity of these five modulus is carried out using a Grindosonic apparatus limestones in four different experimental conditions whose principle has been explained in former publica- (Prick 1997, 1999), shows that only the Caen limestone tions (Prick 1997, 1999). was insensitive to any tested conditions. This observa- Five parallelepipedic samples of fresh local sand- tion on imported rocks has an important implication. It stone, with an average volume of 73.8 cm3, were shows that the local environment at the studied site exposed on a natural ledge at the studied rockwall from should be considered as aggressive from a weathering September 3rd, 2001 to January 30th, 2002. When point of view, with the exception of those rocks that are retrieved, the block surfaces looked unchanged, the not frost sensitive because of their remarkable physical angles and ridges were still as sharp. The sample dry characteristics (e.g. Caen limestone). The local sand- weights and their dynamic Young’s modulus had not stone, when not chemically weathered or fractured, is changed significantly, as no decay had occurred during quite resistant to cryogenic weathering. this period. Locally there is some evidence for chemical weath- This may mean that conditions favourable for frost ering on the studied rockwall. Firstly, iron oxidation weathering did not occur often enough to initiate rock can colour the rock surface and disintegrate the iron decay and that a longer exposure time is required for carbonate nodules included in the bedrock, leaving weathering to occur. It may also mean that the critical small circular depressions on clay-ironstone lenses. degree of saturation of this sandstone is very high (e.g. Secondly, salt outbursts can develop locally during dry more than 90%) and that the moisture content maxima summer periods. Although it is not clear whether this represented on Figure 2 are actually not high enough to salt precipitation leads to physical or chemical weather- cause shattering. Alternatively, as previously outlined, ing, as salt weathering can act both ways, it clearly the porosity of this sandstone is quite low. It is well causes induration and/or desquamation of the local known that rocks with a very low porosity are not frost rock surface. Chemical processes are considered sensitive, Lautridou and Ozouf (1982) propose a increasingly likely to play a role in cold environment threshold value of 6% for limestones. According weathering (André 1993). The decay features described to this empirical rule, the local sandstone is not frost and the processes leading to their occurrence will need

910 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 min- imise alteration of the rock albedo, according to a well- known 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 Figure 3. Dry weight of the rock debris larger than 2 mm, small that they did not cause a visible loss on the collected on a 3.50 m long sediment trap set at the base of painted surface. This technique has shown surprisingly the studied rockwall. Results are presented for the period of quick and widespread debris release. July 16th to November 19th, 2001 and expressed in kg of Five sediment traps set at the base of the rockwall debris for a one-meter long section of the 25 m high cliff. collected the falling rock debris. These were emptied For the days on which the debris are not collected, the weight value of the next rock collection is spread over the about 4 times a week and the collected debris was period elapsed since the last collection. The arrow indicates sieved at 2 mm, dried and weighed. The largest rock that on September 19th, only a part of the debris is repre- falls occurred mainly in September and October sented on this graph. After the occurrence of the largest (Fig. 3). The largest rock fall event monitored in this experienced rock fall event in this study, only the fine debris study happened on September 19th. This was not due was brought back to the laboratory. This was sieved at 2 mm, to a rapid freezing rate, nor to high frost intensity, but dried and weighed (72.4 kg). The largest blocks were left in as a result of the wet weather that prevailed during the the field, their volume estimated to be about 5 m3. preceding days (Fig. 2), wetting the cliff at depth prior to freezing. Hydraulic pressure exerted on the water- (Ødegård et al. 1995). The occurrence of freeze/thaw filled cracks by a freezing front progressing slowly cycles when rock moisture content is high, such as on from the surface is the most likely process to have April 5th 2002, indicates that conditions for rock decay caused this large debris liberation, as the moderate and important rock fall activity are met in the spring. freezing temperatures were unlikely to cause a freez- Further fieldwork will hopefully provide more data ing of all the water present in the crack system. More about spring and summer rock fall occurrence. 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 7 CONCLUSIONS 24th, rock debris were probably liberated by the melting of part of the ice that had been cementing blocks Despite the fact that this study does not yet present a together. September 19th and 24th have previously one-year data set, the following general conclusions been defined as days theoretically prone to intense can be drawn: rock weathering. The next important debris liberation 1. Rockwall temperature shows frequent fluctuations, occurred between October 8th and 10th, due to a melt- even during the polar night. Freeze/thaw cycles ing of the rockwall surface that led to the same kind of were nevertheless registered at depths of less than debris liberation that occurred on September 24th. 1 cm only during the autumn and spring. At 40 cm, The results presented here from Svalbard show a the rock freezes in the fall and remains frozen. maximum in rock fall activity in the autumn. Important 2. No evidence of thermal stress fatigue was found. activity is also theoretically expected in the spring, Only 3 thermal shocks were registered over the when the rockwall thaws at depth from the surface whole monitored period.

911 3. Rock moisture content shows large and rapid vari- André, M.F. 1993. Les versants du Spitsberg: approche ations caused by weather conditions. Rock samples géographique des paysages polaires. Nancy: Presses tend to dry slowly during the winter season, proba- Universitaires de Nancy. bly because of sublimation. High moisture content Coutard, J.P. & Francou, B. 1989. Rock temperature mea- occurs rarely, and only in the fall and spring. surements in two alpine environments: implications for frost shattering. Arctic and Alpine Research 21 (4): 4. Conditions favorable for cryogenic weathering, 399–416. such as freezing of the rock when its moisture con- Hall, K. 1988. Daily monitoring of a rock tablet at a mari- tent is high, were probably met on very few days time Antarctic site: moisture and weathering results. (at least 4) over this 8 months period. When these British Antarctic Survey Bulletin 79: 17–25. conditions are met however, frost action may be Hall, K. & André, M.F. 2001. New insights into rock weath- very aggressive because of the high rock moisture ering from high-frequency rock temperature data: an content and the quick cooling or extended duration Antarctic study of weathering by thermal stress. of freezing periods. This frost efficiency is shown Geomorphology 41 (1): 3–35. by the decrease in modulus of elasticity from 4 of 5 Harris, S.A. & Prick, A. 1997. The periglacial environment of Plateau Mountain : an overview of current periglacial porous limestones exposed at the study site. research. Polar Geography 21 (2): 113–136. 5. A similar exposure did not cause any decrease in Humlum, O. 1992. Observations on rock moisture avail- the modulus of elasticity of 5 sandstone samples. ability in gneiss and basalt under natural, Arctic con- Frost shattering does not act through the porous ditions. Geografiska Annaler 74 A (2–3): 197–205. media in this low porosity sandstone, but by wedg- Lautridou, J.P. & Ozouf, J.Cl. 1982. Experimental frost ing of its well-developed crack system. shattering : 15 years of research at the Centre de 6. Rock fall occurrence showed a very irregular distri- Géomorphologie du CNRS. Progress in Physical bution between July and November 2001. Most Geography 6 (2): 215–232. rock falls took place in September and October. The Lewkowicz, A.G. 2001. Temperature regime of a small sand- largest occurred on days when the conditions were stone tor, latitude 80°N, Ellesmere Island, Nunavut, Canada. Permafrost and Periglacial Processes 12: particularly favourable for cryogenic weathering. 351–366. Major, H. & Nagy, J. 1972. Geology of the Adventdalen map area. Norsk Polarinstitutt Skrifter 138. ACKNOWLEDGEMENTS Matsuoka, N. 1991. A model of the rate of frost shattering: application to field data from Japan, Svalbard and Prof. Ole Humlum is warmly thanked for his support, Antarctica. Permafrost and Periglacial Processes comments and advice. Special thanks are due to Prof. 2 (4): 271–281. A. Ozer and other members of the Dep. of Geography Ødegård, R.S., Etzelmüller, B., Vatne, G. & Sollid, J.L. at the University of Liège for the use the Grindosonic 1995. Near-surface spring temperatures in an Arctic apparatus, and to Prof. E. Poty and his team for their coastal cliff: possible implications for rock breakdown. collaboration in the sawing of limestone samples. This In: O. Slaymaker (ed.), Steepland Geomorphology: research has been supported by a Marie Curie Fellow- 89–102. Chichester: Wiley. ship of the European Community program “Improving Prick, A. 1997. Critical degree of saturation as a threshold moisture level in frost weathering of limestones. the Human Research Potential” under the contract Permafrost and Periglacial Processes 8 (1): 91–99. number HPMF-CT-2000–00720. Prick, A. 1999. Etude dilatométrique de la cryoclastie et de Disclaimer: The European Commission is not l’haloclastie. Mémoire de la Classe des Sciences, responsible for any views or results expressed. Tome XIX. Bruxelles : Ed. Académie Royale de Belgique. Rapp, A. 1960. Talus slopes and mountain walls at Tem- REFERENCES pelfjorden, Spitsbergen. Norsk Polarinstitutt Skrifter 119: 96 p. Åkerman, H.J. 1984. Notes on talus morphology and Ritchie, T. & Davison, J.I. 1968. Moisture content and processes in Spitsbergen. Geografiska Annaler 66 A (4): freeze-thaw cycles of masonry materials. Journal of 267–284. materials, JMLSA 3 (3): 658–671.

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