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, 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
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 2002.
The field site is located 2 km NW of Longyearbyen
(78°1338N, 15°3600E). 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
1975–2000. 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
2001–2002 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 10–40 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.
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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 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.
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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
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.
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 cm3) measured from September 10th 2001 to April 12th 2002. No
measurements were carried out from November 22nd to
December 28th and from January 1st to 9th.
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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
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 cm3, were
exposed on a natural ledge at the studied rockwall from
September 3rd, 2001 to January 30th, 2002. 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
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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 m3.
(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.
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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 2001. 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-2000–00720.
Disclaimer: The European Commission is not
responsible for any views or results expressed.
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