Kevin Hall
Geography Department, University of Natal, PO Box 375, Pietermaritzburg 3200, South Africa ABSTRACT
Rock moisture content was determined for rock samples on different aspects of rock out- crops on Livingston Island during a summer season. As a result of the dominant rain- bearing northerly winds the southern aspect usually has rock moisture levels lower than the northern. The southern aspect, however, experiences high rock moisture levels during periods of snowmelt; snow accumulates on the southern, lee-side of the rock outcrops.
Wetting and drying events are more frequent on the northern exposure, although not as common as at a site open through the full 360°, while the southern aspect tends to experi- ence continuous, low moisture levels with infrequent dry events. Contrary to earlier sugges- tions, freeze-thaw weathering does not appear to be a major factor during the summer.
Although rock moisture levels are conducive to freeze-thaw, rock temperatures rarely go below O°e. Rather, it appears that weathering due to wetting and drying may be more com- mon on the northern aspects than was previously thought while chemical weathering is active on southerly aspects. Rock moisture levels may support rock damage due to segregation ice during the winter freeze when the rate of freezing is slowed by the overlying snow cover.
RESUME
Le contenu en eau de roches a ete inesure sur des echantillons divers preleves pendant l'ete sur des affleurements de l'Ile Livingston. Comme les vents pluvieux dominants vien- nent du nord, les roches exposees au sud ont habituellement des teneurs en eau plus basses que celles exposees au nord. Sur les versants exposes au sud, des teneurs en humidite sont cependant elevees pendant les periodes de fonte de neige car la neige s'accumule sur les versants sud qui se trouvent sous le vent. Les phenomenes de sechage et d'humidification sont plus frequents sur les affleurements exposes au nord, tout en n'etant pas aussi nom- breux que sur les sites exposes
a
tous les vents (360°). Sur les affleurements exposes au sud, les niveaux d'humidite restent peu eleves et les asscchements sont rares.Contrairement
a
ce qui a ete suggere precedemment, l'alteration par gel-degel ne semble pas etre, l'ete, un facteur de desagregation principal. Quoique les niveaux d'humidite soient favorables aux actions de gel-degel, les temperatures des roches y descendent rare- ment sous O°e. Au contraire, il apparait que l'alteration due aux alternances sechage / humidification doit etre plus frequente sur les versants exposes au nord, tandis que l'altera- tion chimique serait surtout active sur les versants exposes au sud. Les niveaux d'humidite des roches peuvent engendrer une desagregation duea
la formation de glace de segregation pendant l'hiver parce que la vitesse de gel est ralentie par la couverture de neige.KEY WORDS: Maritime Antarctic 1045-6740/93/030245-09$09.50
©1993 by John Wiley& Sons, Ltd.
Rock moisture Cryogenic weathering
Received4November 1992 Accepred1April 1993
246 K. Hall INTRODUCTION
Quantitative data regarding weathering pro- cesses in the Antarctic are relatively rare.
Where they do exist, they focus primarily on the continent and the dry valleys of the McMurdo region in particular (e.g. Campbell and Claridge, 1988). The dry valleys are certainly very unusual from the point of view of weathering owing to their extreme aridity and cold. For this reason, they are doubly important in that they are per- ceived as an analogue for the hyper-arid inner planets of our solar system (Vishniac and Main- zer, 1973; Hall, 1989). However, the dry valley environment does not constitute the 'norm' for the Antarctic. Even within the various continen- tal oases significant differences in environmental conditions occur: note the comments of Pickard (1986) with respect to the summer wetness of the Vestfold Hills, a condition rarely experienced in the McMurdo region. However, above and beyond these differences on the continent there is the distinction between the continental and the maritime Antarctic. Although recognized as a biologically active zone, and of geological signi- ficance, little attention has been paid to the dis- tinctly different weathering regime experienced on the islands as compared with the continent proper.
Rock moisture is difficult to measure in the field (Thorn, 1988). As a consequence, field de- terminations of weathering processes and labora- tory simulations have both been put in question (McGreevy and Whalley, 1985; Thorn, 1992).
However, following the pioneering work of Ritchie and Davison (1968) some attempts have been made to monitor rock moisture content (e.g.
TrenhaiJe and Mercan, 1984; Hall, 1986, 1991a;
Humlum, 1992). Nevertheless, empirical data are still extremely limited and most deductions as to the nature and rates of weathering in cold regions remain subjective and unverified (Hall, 1986, 1991b; Thorn, 1992). Following a pilot study undertaken in Alaska (Hall, 1991a) an attempt was made to obtain information on the spatial and temporal variability of rock moisture from a mari- time Antarctic site. Although significant in itself, the rock moisture data are of even greater value when combined with information pertaining to rock properties (Hall, 1993a) and rock tempera- tures (Hall, 1993b), as obtained here.
STUDY AREA
The Byers Peninsula, located at the western ex- tremity of Livingston Island (62°40'S, 61°00'W), constitutes the largest ice-free area (50 km2) in
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Rock Moisture Data and Implications for Weathering 247 the South Shetland Islands and is one of the
largest in lesser Antarctica (Figure 1). According to John and Sugden (1971) the mean annual tem- perature is -3°C and the annual precipitation is of the order of 100 to 150 cm water equivalent.
The study area consists of an extensive assem- blage of raised beach platforms at a number of levels, with the highest (54 m a.s.l.) dated at 9700 years BP, interspersed with upstanding volcanic plugs that frequently exhibit columnar jointing and a number of dykes and sills. The geology, mainly volcanics interbedded with conglomerates and sandstones, is well described by Hobbs (1968) and Smellie et al. (1980). During most years the mean daily temperature is above freezing from December to March and permafrost is believed to be present below an active layer of 0.3 to 0.7 m thickness (Thom, 1978). Snow accumulation is particularly pronounced on the lee side of ob- stacles, this normally being the south side as a result of dominant northerly winds. Extensive cloud cover in this region limits daily radiation receipt and this is particularly pronounced on the southern aspects of outcrops. Thus, snow accu- mulations in the lee of obstacles survive well into January and the next season's snowfall begins soon after the snow of the preceding winter has finally ablated.
METHODOLOGY
Fundamental to the study of weathering pro- cesses, be they mechanical, chemical or biological, is knowledge regarding rock moisture content and its variability both spatially and temporally. An initial study of rock moisture content on Signy Island (Hall, 1986) was unstructured and was thus unable to show variation of moisture with respect to both time and space, although the subsequent daily monitoring of a rock tablet for one year did provide some useful data for an open site (Hall, 1988). In the present study three sites were util- ized to monitor rock moisture content. The first, at the site of the field camp, was on a raised beach at c. 12 m a.s.1. and approximately 400 m from the sea. The second site was a dolerite dyke, with an east to west trend, a further 200 m inland at c. 25 m a.s.1. The third site was to the south of the first two and was on a small volcanic rock boss at c. 30 m a.s.1. and 500 m from the coast. At site 1 three blocks of rock from each of site 2 and site 3 were placed on the horizontal ground surface, open .through the whole 360°, next to an automatic
weather station. At site 2 three pieces of dyke rock were placed on each of the north and south faces of the dyke at a height of 1 m above the ground surface. At site 3, three pieces of local rock were positioned 1 m above the ground sur- face on each of the north, east, south and west faces of the rock boss. All of these rocks were weighed several times each day. Each rock sample had been previously oven-dried for 24 hours at 105°C and then weighed to get its dry weight. Each had also been saturated by immer- sion in water for 24 hours and weighed again to get its saturated weight. Thus it was possible to transform each daily weighing to a percentage saturation for that rock. By this method varia- tions in rock moisture content for sites of differing exposure were obtained for a period of 45 days during January and February.
At site 1 an automatic weather station was used to log incoming radiation, wind direction and speed, together with air, ground surface and 5 cm depth temperatures throughout the field season.
In addition, a Squirrel logger was also used at site 1 to monitor the upper surface and north-facing surface of a piece of dole rite positioned on the ground surface. At site 2 thermocouples were affixed to the rock surface and, via drill holes, at 1 cm depth on both the north and south faces;
temperatures were read at the same time as the rocks were weighed. At site 3 thermocouples were affixed to the rock surface on each of the cardinal faces and also read when the rocks there were weighed. This instrumentation therefore provided standard c1imatological data (from the automatic weather station at site 1) together with actual temperatures experienced by the rock at each of the rock moisture monitoring sites (Hall, 1993b).
A measure of weathering was obtained from both sites 2 and 3 as well as from a varietv ()f locations on the peninsula by means of a Sch~idt hammer, an indentor, by measuring weatherin~
rind thicknesses, and from the collection (,I weathered-free material from the rock fan:, (Hall, 1993a). In addition, analysis of interstttl.J1 rock water chemistry is being undertaken to CJIl1 some insight into the nature and concentratio'n III
salts present in the rock and their variabilit\ J'.1
function of aspect together with their po'~~lhk influence on both mechanical and chemK.J1 weathering processes. A number of generJI observations regarding periglacial processes were obtained as these give information both in their own right and indirectly with respect to the weathering regime of the island (Hall, 1992).
248 K. Hall
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Figure 2 Percentage saturation for the three samples(S I-S3) for the south side of the rock dyke.
49
49 45 41
53 drywt - 366.6g
41 28 37
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Julian Day (1991)
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Figure3 Three-term running mean for samplesSI-S3show- ing that the trends of the percentage saturation are the same.
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51 drywt=211.79 - 52 drywt - 256.199% volumetric expansion as water turns to ice).
While not negating the basic premise of White (i.e. the necessity of knowledge concerning rock moisture content) the above data indicate that the question is not as simple as it may first appear.
Clearly there is a rock moisture gradient, with the greatest amount of water near to the rock surface.
Thus, there can be high moisture levels in the outer part of a rock and a zone of saturation, or near-saturation, that would vary in thickness as a function of the amount of water made available.
For example, if sample SI (thickness = 50 mm) is saturated but not sample S3 (thickness
=
65 mm)then this implies that for S3 the outer 50 mm has a moisture content at or close to 100% and a lesser, unknown amount in the next 15 mm. As argued
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RESULTS AND DISCUSSION
For the monitoring of rock moisture content three samples were used at each site. This was partly as a safeguard against sample loss or damage (i.e. in such an instance there would not be total loss of data as would occur had only one sample been used) and partly as a means of establishing sample variability. However, it also provided an import- ant insight into the effect of sample mass upon considered percentage saturation and, via sample size, indirect information pertaining to the depth of bedrock wetting. As an example, the dry mass of samples SI to S3, from the south side of the dyke, were 211.7 g, 256.1 g and 366.6 g respect- ively. Consideration of Figure 2 indicates that if only a sample of mass SI had been used then this would have indicated a generally high level of saturation (>80%) while if S3 had been used then a lower level would have been obtained «70%).
While the values for S2 were notably lower than SI and marginally higher than S3, all three samples followed the same trends (Figure 3) with the degree of saturation varying as a function of rock mass. In other words, all three samples reflect the same changes in rock moisture content but differ in the degree of saturation.
With similar surface areas the thicknesses of the three samples SI to S3 (50 mm, 55 mm and 65 mm respectively) could be considered to crudely equate to a comparable depth of bedrock. Thus saturation of SI butnot S2 implies saturation to a depth of approximately 50 mm whilst saturation of sample S3 would imply saturation of bedrock to a depth of at least 65 mm. For instance, on Julianday 35, SI was saturated and so was S2 but not S3, thereby indicating sufficient moisture was available to saturate the rock to a depth of about 55 mm. Unfortunately, for the other test sites in this undertaking either the rock sample thick- nesses were very similar or samples became dam- aged and/or weathered such that these types of comparisons and deductions were not possible.
White (1976, p. 5) begged the question 'In how many mountain ranges or on how many arctic plains will bedrock fortuitously ever become
>50% water-saturated . . . and then undergo rapid freezing to crack the rock?' In essence, White argued that it was adsorbed water (hydra- tion) that was a major cause of rock breakdown in cold regions as the hydration mechanism was able to generate stresses sufficient to fracture the rock but did not require the high levels of rock mois- ture necessary for 'classic' freeze-thaw (i.e. the
Rock Moisture Data and Implications for Weathering 249
Figure 4 Actual values of percentage saturation for samples of similar size and mass for the north and south sides of the rock dyke.
and had temperatures markedly higher than the southern aspect, and only marginally below that of the northern aspect, the number of instances when this occurred were very few. The poor weather, with overcast skies and rain, that limited radiation to the eastern aspect often cleared by the late afternoon such that the western aspect received direct radiation on a number of oc- casions and was affected by the dry, northerly winds blowing past. In turn, this resulted in the western aspect having the (marginally) lowest saturation levels.
For the rock boss the sizes of the samples used equate to approximately the outer 60 mm of the bedrock. This then implies that during the record period the eastern aspect was always above 40%
saturated, the northern was mostly above 30%
and the south and west were, except for a few short periods prior to day 42, below 30% satu- rated. The north and south sides of the boss show reasonably similar patterns to that of the dyke, with comparable levels of saturation.
The data from the open site reflect a combina- tion of frequent moisture inputs and rapid drying tl:1at result in a highly fluctuating graph (Figure 6).
Being exposed through the full 3600 the rocks freely received any and all precipitation, resulting in the high moisture contents. Equally, drying occurred due to the non-precipitation-bearing winds of any direction together with heating by incoming radiation at any time during daylight hours. Thus the rocks show both rapid wetting and rapid drying. Figure 6 also indicates a general decrease in moisture content from Julian day 7
1-
North side of dyke ._- South side of dyke 49 4114 20
16 23 33
Julian Day(1991) 10
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above, the 65 mm of sample S3 would crudely equate to the outer 65 mm of bedrock and so reflects the moisture gradient found there. Thus, while a large block of rock is not, as a whole,
>50% saturated, the outer shell of that rock may well be. High rock moisture levels are also re- quired by the ice segregation model of Hallet (1983) and so, again, knowledge of the moisture gradient is important. Although, following the question of White, freezing did not occur during the record period when rock moisture levels were high, it must be recognized that it is a possibility within this region.
Using clasts of similar mass and surface dimen- sions for the north and south sides of the dolerite dyke (Figure 4) it can be seen that the south side has a markedly lower degree of saturation when compared with the north. The lowest levels attained on the north side were of the order of 36% saturation, but most were above 40%, while on the south side levels below 20% saturation were attained and, except for the latter part of the record, most values were below 40%. As meteorological data indicate the dominant, rain- bearing winds to be from the north, it is not sur- prising that the northern aspect of the dyke had the higher degree of saturation. The south face exhibited high levels of rock saturation when the snow that accumulated on this lee side started to melt. Such a situation is shown by sample SI for Julian days 43 to 49 where rock samples posi- tioned prior to snowfall and buried by the snow indicate high moisture contents as the snow melted. The high rock moisture levels are only found in the outer part of the rock, there not being sufficient water available to penetrate to any substantial depth owing to the snow losing contact with the rock as thaw progresses. Thus, the northern aspect experienced frequent high levels of wetting as a consequence of the northerly, rain-bearing winds while the southern aspect re- ceived substantial wetting mainly as a result of snowmelt.
The data from the rock boss (Figure 5) still indicate the northern aspect to be wetter than the southern but the highest saturation levels occur on the eastern aspect. That the east experienced the highest moisture levels was not due to it re- ceiving more precipitation but was rather a func- tion of fewer drying events. The eastern aspect was little affected by dry northerly or southerly winds and received little in the way of direct radiation to facilitate evaporation. Although on clear days the eastern sidewaswarmed by the sun
250 K. Hall
100 80
60 Percentage Saturation
44 ., 38 37 39 30 32 26 26
Julian Day
Figure 5 Percentage saturation for samples of similar size and mass for the four aspects of the rock boss.
through to day 38 followed by, from day 40, mois- ture content peaks. This reflects a diminution in the amount of rain from the record start through to day 38 (i.e. 7 February) followed by an in- crease in snowfall that resulted, owing to its melt- ing, in short, sharp peaks of high rock sample moisture levels. These data, although of great value, suggest that care should be taken in using data from samples in such an exposed situation
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Percentage saturation for a rock sample at the open
(as was obtained in the earlier study of Hall, 1988) and then extrapolating the results to any particular site where aspect exerts control.
With respect to weathering, the rock moisture data help explain the cause of rock breakdown for this area and also show the importance of aspect upon weathering in general. Throughout the re- cord period temperatures rarely fell below
ooe,
either in the air or at the rock surface. In fact, there were only five events when rock surface temperatures went below O°C and the lowest value recorded was -1.3
cc.
Only near the end of the study period were low temperatures starting to occur but they were in conjunction with snow- falls that covered much of the rock surfaces. At the start of the study period a large percentage (c. 60%) of the area was still snow covered.Thus, many of the upstanding rock outcrops, like the dyke and the rock boss used in this study, are insulated from air temperature fluctuations for a substantial part of the year by the protective snow cover. Only those rocks that become exposed in early spring or remain uncovered into late autumn are subject to freezing temperatures other than that of the annual freeze. On the Byers Peninsula it would appear that it is not the absence of water that is the constraint upon effective freeze-thaw action but rather that, owing to the insulating snow cover, the rock is not exposed to conducive