KEVIN HALL
Geography Department. University of Natal. Pietemuuitzburg 3200. South Africa
Received 30 September /984 Revised 24 January /985
ABSTRACT
Rock moisture content is a major control ofmechanical weathering, particularly freeze-thaw, and yet almost no data exist from field situations. This study presents moisture content values for rocks, taken from a variety of positions and conditions, in the maritime Antarctic. Additional information regarding the amount of water the rock could take up, as observed from laboratory experiments, is also presented. The results show that the approachesused in simulation experiments, particularly that of soaking a rock for 24 hours, may produce exaggerated results. It was found that the saturation coefficient (S-value) was a good indicator offrost susceptibility (based on water content) but that the derivation of that value may underestimate the potential of some rocks. The distribution of moisture within rocks is seen as an important, but unkown, factor. The results of these field moisture contents suggest that for simulations of freeze-thaw or hydration tobemeaningful then they should have rock water contentsbasedon field observations.
KEY WORDS Rock moisture content Rock properties Schist Freeze-thaw Hydration Signy Island Maritime Antarctic
INTRODUCTION
Central to studies of freeze-thaw weatheringis the presence of water within the rock which. upon freezing, causes damage to that rock. As fundamental as this may be there is, nevertheless, an enormous lack of data pertaining to rock water content under natural conditions; the work of Ritchie and Davison (1968) and Trenhaile and Mercan (1984) being the only ones currently available.Itis not just the 'presence' of water but the actual amount available, and its distribution within the rock. that is important This lack of field data from geomorphological studies, and even from such as engineering investigations of building materials (as noted by Litvan, 1980), has put the interpretation of field situations and the validity of laboratory simulation studies in some doubt (Trenhaile and Mercan, 1984). The present status of studies, both in the field and the laboratory, .and their implications has been dealt with in depth by McGreevy and Whalley (submitted) and sowillnot be repeated here. Suffice to say, the present position with respect to freeze-thaw and the role of rock moisture is one of 'fumbling in the dark' for, without actual values from rocks under field conditions, the applicability of simulation experiments and the consideration of weathering mechanisms is somewhat subjective.
As part of a long term, multidisciplinary approach to rock breakdown and the production of soil in the maritime Antarctic (Walton and Hall, submitted) a study was undertaken of mechanical weathering processes both in the field and in the laboratory. As part of that study, data on rock water content for the different local rock types under a variety of conditions were obtained as a control for laboratory experiments and as an aid to process investigation. In addition, physical properties of the same rocks were tested in the laboratory to find 'absolute' values for comparison with that found in the field.
0197-9337/86/020131-12$06.00
©
1986 by John Wiley& Sons, Ltd.132 K.HALL
The main rock involved in this study on Signy Island is quartz-micaschist. This is a rock, however, for which there appears to be a dearth of information pertaining to its physical properties, even wit~ respect to engineering aspects {Hall, in preparation). Regarding information on moisture content and lt~ controls, Goodman (1980)~itesthe permeability of schist as measured in the laboratory to be 10-8cm/s and10the field as 2 x 10-7cm/s, whilst 'fissured schist' was in the range of 1 x10-4to 3X 10-4cm/so Selby (1982) refers to the porosity of schist as being between (}001 and 1·00 per cent with a permeability of10-9 to lO-sm/day.
Fahey (1983) presents the greatest amount of available data (see Discussion) with respect to moisture conditions and freeze-thaw but sums up the whole situation (1983, p. 543) by saying 'Since this was a laboratory study, the experimental conditions are not particularly representative of actual periglacial environments'. The aim here is to presentboth laboratory and field data, to analyse these data and relate them to mechanical weathering processes and, particularly, to simulation studies of those processes.
STUDY AREA
The study was undertaken on Signy Island (Lat. 60043'S, Long. 45°38'W), one of the smaller islands in the South Orkney group (Figure 1), as part of the Fellfield Programme (Walton and Hall, submitted). Signy is approximately 6·4kmnorth to south, 4·8 km east. to west, with an area of 19·94km2and rising, at Tioga Hill, to a height of 279 m. Roughly one-third of the island is covered by an ice cap and many small areas are subject to long-term snow lay. A few streams, usually along the margins ofthe ice cover, flow for a short period during the summer months and a number of small lakes may be ice-free during this time. The island is an area of continuous permafrost with numerous periglacial features (Chambers, 1967).
The vegetation is typical of the maritime Antarctic, comprising cushion mosses, fructicose lichens, and occasional outcrops of the grass Deschampsia antarctica (Smith, 1972~Geologically, the island comprises metamorphosed sediments, mainly quartz-micaschist with smaller areas of amphibolites, marbles, and quartzites (Mathews and Mating, 1967). There is a typical cold, oceanic climate with a mean monthly temperature ofc. - 4°C but the summer three months have means slightly above freezing (Watson, 1975). Rain predominates in January and February but the rest of the average (}4 m y-l precipitation is in the form of snow. Sunshine levels are low(i= c.
It
h day -1),whilst average wind speeds are in the region of 26 km h -1 (Watson, 1975). For specific details of climate, particularly microclimate, see Walton (1982) and CoUins et aJ.(1975). Sea ice surrounds the island during winter.
On the island there are three major reference sites: Factory Bluffs, Moraine Valley, and Jane Peak Col (Figure 1). Details of these sites are given in Walton and Hall (submitted) but in this study sample collection was not limited to the reference sites but encompassed the whole island.
APPROACH
Rock samples were collected from outcrops at the three study sites (Figure 1), from a variety oflocations all over the island. The samples were also obtained under a variety of conditions, i.e. from a rivulet after a prolonged period of melt, under a new snow cover, under the same cover at a later stage and then during melt, from non-snow covered locations, and even from 19 m depth on the seabed. In short, an attempt was made to sample the range of rock types, in a variety of physiographic positions, under a mixture of climatic conditions.
Small rock samples collected in the field were immediately weighed (using a portable electronic balance) to obtain their wet weight. Larger samples were sealed in plasticbagsand returned to the island's laboratory where they were subject to the point load test to assess their compressive and shear strengths at field water content (Hall, in preparation). The larger fractured rock pieces were then weighed to obtain their wet weight and, together with the small, ready-weighed field samples, then placed in an oven set at 105°C. The samples were then weighed again after 24 hours and then after 48 hours. As some rocks showed a continued, albeit very small, decrease in weight between the two weighings the 48 hour valuewasthen taken. The difference between the wet and dry weight was then expressed as a percentage. A large number of these rocks were then retained for more detailed analysis of their physical properties.
For the retained samples the following physical attributes were-aSCertained, -folloWing the procedures
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PACIFIC
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ROCK MOISTIJRE CONTENT
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FigureI. Signy Island and its location
134 K.HALL
suggested by Cooke (1979): porosity, water absorption capacity, saturation coefficient, microporosity, bulk:
density, and microporosity expressed as a per cent of total bulk volume. Where possible a notewasmade of minor lithologic differences within a rock type, i.e. in the quartz-micaschist whether there were pods of quartz, thick or thin bands of quartz, almost total absence of quartz, etc.
In addition, these same rock properties were obtained for 85 rock tablets cut from blocks of indigenous rock obtained from the field the preceding year. These rock tablets were returned to the field and a number will be withdrawn each year and their properties recalculated in an attempt to get some idea of rates of weathering. A comparison of the cut rock and field rock data also proved informative.
Thus it was possible to obtain information pertaining to the amount of water found in a rock under a variety of field conditions and the properties, related to water content, of that same rock. It was not possible to state the distribution of the water within the rock; however, a subjective estimation was attempted.
RESULTS
A total of 155 rocks was collected from the field and their water content ascertained. The mean water content for the total sample was (}53 per cent, by weight (5= (}40) but this could be further divided for the quartz-micaschists (abbreviate to 'qms') as a product of smalllithologic variations (TableI~The data in Table I suggest that, with respect to water content: qUartzite<quartz<qms with thin quartz bands< qms with thick quartz bands <qms with pods of quartz<qms with very little quartz.
TableI. Average water content of rocks. subdivided by lithologic variations. collected inthe field
X
Lithology n* (~J s
Quartzite 3 (}14 (}16
Quartz 11 (}30 (}20
Qms withthinbands quartz 22 (}32 (}30
Qms with thick bands quartz 7 (}46 (}34
Qms with pods of quartz 11 (}52 (}45
Qmst with very little quartz 12 1-17 (}34
• The residue oftheISSrocks sampled wereeit~eruncertain as to category they fell in or were not adequately described at time of collection to allow division.
t Qms=quanz-micaschist.
For the rocks collected in the field, data relating to the quartz-micaschist are presented in Table 11, where the 13 environmental/physiographic situations from which they were obtained is also shown. The average moisture content for each of the13groups is shown in Table III which also indicates that the samples can be subdivided into three groups. The first group (*1 in Table Ill) reflects those in a wet situation for a 'prolonged' (> 24 hours) period of time, the second (*2) those in a wet but well drained environment, and the third (*3) a sunny or water deficient situation. Thus the rock moisture content can be seen to vary both within and between samples. For comparison Table VII shows the moisture content for six sets of samples obtained during spring as the snow cover begins to ablate.
In Table IV the physical properties, pertinent to moisture content, of some of the rocks noted in Table III are presented. It is noticeable that in the section regarding clasts 'in rain for c. 12 hours and under snow 24 hours' there is a preponderance of clasts whose field water content is greater than that found in the laboratory experiments. Of the 11 samples for which there are data, nine exhibit a larger moisture percentage than was obtained by immersing the same rocks in water for 24 hours. The nine have a range of moisture contents between 14·14 per cent and 51·43 per cent greater than those found in the laboratory(x= 29·91 per cent,
5 = 13-29).Of the five samples from 'under> 24 hours snow', two exhibit a similar status. Conversely, the bulk
ROCK MOISTURE CONTENT 135 TableH. Moisture content obtained in the field at a variety of environmental locations
Moisture content
.x
dry rock Range of(YO> weight rock wts
Conditions Rock type
x
s (g) (g) nAfter 24 hours snow:
179·9
blown-free patterned Quartz (}28
ground site
After 24 hours snow:
(}16 137·0 37·7 to 16
blown-free patterned Qms (}43
ground site 309·0
After > 24 hours snow:
from under snow Quartz (}44 67-3
After >24 hours snow: 23-5 to
from under snow Qms 1.0 0.38 145·3 345-0 10
After 24 hours snow: 51·7 to
from faces, partly Qms (}411 (}27 425·7 758-0 5
under snow
227·7 to From a rock face
during snowmelt Qms (}56 (}25 368·0 585·0 6
From ground surface
(}12 164·3
melting out from Quartz snow
From ground surface
(}53 (}25 167-4 28·8 to
melting out from Qms
snow 398-0 13
From an area of wet, 140-3 to
soned stripes Quartz (}30 (}03 208·8 277·3 2
From an area of wet, 37·9 to
soned stripes Qms (}52 (}43 148·9 284-0 7
From a rock outcrop, 45·5 to
after snowmelt Qms (}41 (}05 385·3 848-0 5
From base of cliff 265-6 to
below an overhang Qms (}32 (}11 458·1 687·0 6
From 19 m depth on 1534 to
seabed Qms (}97 (}54 2245·7 2903 6
From a rivulet 155·7 to
running for > 36 hours Qms 1-19 (}45 236·0 271·2 4
From ground in rain 82·5 to
c. 12 hours then under Qms 1·27 (}42 156·3 267·1 4
snow >24 hours From ground in rain
c. 12 hours then under 64·8 to
snow >24 hours Quartz (}61 (}Q 73-3 81·7 2
From face of weathered 192·1 to
boulder in sun Qms (}29 (}25 255·6 298·6 3
Blocks of scree in sun Quartz (}15 66·1 1
Blocks of scree in sun 13·1 to
Qms (}18 (}25 2(}5 27·9 2
of the samples show a laboratory~erivedmoisture content greater than that found in the field; in fact 61·1 per cent show laboratory values larger than those that were found in the field. Both situations, where laboratory results are either greater or lesser than field values, have serious implications for simulation studies.
In addition to the actual moisture values, Table IV also shows the 'Saturation Coefficient' or ·S-value'.
Hirschwald (1912) and Thomas (1938) have shown that a rock with an S-value of(}8 or greater indicates that rock tobefrost susceptible. Others, notably Kreuger (1923) and Tourenq (1970) have suggested the S-value shouldbeset at 0·85 whilst MacInnes and Beaudoin (1968) suggest 0·9 (for a discussion see McGreevy and
136 K.HALL
Table Ill. Average water contents of 13 different environments with statistical comparisons
Environment
oXwater content
(:YJ s
Snow-free patterned ground site After 24 hours snow from under snow After 24 hours snow,partlysnow covered From rock face during snowmelt
From sloping ground surface during snowmelt
From area of wet, sorted stripes From rock outcrop after snowmelt From overhang at cliff base From 19 m depth on seabed From rivulet running for >36 hours From ground in c. 12 hours rain,>24 hours snow
From face of boulder in sun From blocks of scree in sun
(}42 (}95 (}48 (}56 (}50 (}47 (}41 (}32 (}97 1-19
1'05 (}29 (}17
(}16 (}40 (}27 (}25 (}26 1{38 (}05 (}11 (}54
(}45 (}47 (}2S (}18
.1 Krusltall-Wallis test indicates these four tobe of the same population.
• 1 Kruskall-WalIis test indicates thes six tobe of the same population.
• 3 Krusltall-Wallis test indicates these three tobe of the same population.
Whalley, submitted). For a value of0-8 then 30-2per cent of the rocks tested here can be considered frost susceptible, at0.85 then only18.6per cent and at0.9a bare7per cent. However, this S-value does not take cognisance of the distribution of the water within the rock (see below), a factor of great importance. For those rocks with S-values~ 0·8 there is no apparent lithologic subdivision within the samples tested.
For the 54 rock tablets cut from quartz-micaschist that were placed in the field, therewas a noticeable difference in some of the measured physical properties (Table V). Whilst porosity values remained relatively similar those of the Saturation Coefficient and the Water Absorption Coefficient were much lower than was found for the uncut, field samples. This too may have serious implications for laboratory simulations and their applicability to field situations. An attempt is being made to investigate this by means of scanning electron microscope analysis of cut sections.
Assuming the amount of water taken up by the rock under vacuum must approximate to filling all the readily available space in the rock, then it is possible to compare this with the actual amount of water found under field conditions to find the degree of available space unused (Table VI). This still does not take cognisance ofwhere the water is in the rock, and it may well be (see below) that some has concentrated in a peripheral zone whilst in others it is disseminated. However, Table VI shows that, in response to White's(1976)question re how many rocks have> 50per cent water content, in this instance40·4per cent do, with17·0per cent being>90per cent full. Conversely59·6per cent have less than50per cent moisture content with27·7per cent exhibiting>70per cent of available space unfilled.
Table VI also shows the calculated S-value for46ofthe presented samples. Ofthese,13 (28'3per cent) appear to have, when compared with actual field moisture content, an S-value which is too low whilst four(8'7 per cent) have an S-value apparently too high. The bulk of the samples(63'0per cent) have an S-value correctly reflecting the percentage of unused space and there is a reasonable correlation (r= -0-60)indicating this (wherex= percentage unused space andy= S-value). Those samples for which the S-value is too high may show a better correlation if sampled at another time, for the data reflect solely the conditions prevailing at the time of collection. Those where the S-value is too low are thought to be reflecting the use, in the derivative equation for the S-value, of the value of rock saturated in water for24hours which, as has been noted, may well beless than that found in the field if water is available for a longer period.
Table IV. Properties relating to moisture content for a number of rocks from a variety of environmental locations Water
absorption %H20
Porosity coefficient after %Field
Location Rock (%) (%) S-value 24 hours H2O
Scree Quartz 1·36 (}81 (}60 (}31 (}15
Scree Fine-grained qms 1·99 1-42 (}71 (}54 (}15
Under snow Qms 2095 HO (}9O (}98 (}91
>24 hours
Under snow Qms 3-34 2·76 (}83 1-04 H3
>24 hours
Under snow Qms 1·56 1'19 (}76 (}43 (}35
> 24 hours
Under snow Qms HO (}97 (}88 (}36 (}45
> 24 hours
Under snow Qms (}60 (}41 (}69 (}16 (}1O
>24 hours In rain 12 hours
and under snow Quartz 1·14 (}76 (}67 (}29 (}15
24 hours In rain 12 hours
and under snow Fractured quartz 1'59 1·27 (}80 (}49 (}61
24 hours Inrain 12 hours
and under snow Fine banded qms 1'70 1·43 (}84 (}53 (}63
24 hours Inrain 12 hours
and under snow Small pods qms 1·90 1·42 (}75 (}54 (}72
24 hours Inrain 12 hours
and under snow Quartz 1-38 (}79 (}57 (}31 (}46
24 hours In rain 12 hours
and under snow Qms large %Si02 H3 (}68 (}60 (}25 (}37
24 hours In rain 12 hours
and under snow Small pods, qms 2'59 1·83 (}71 Q-68 1-40
24 hours Inrain 12 hours
and under snow Qms,high %Si02 1·87 1·58 (}85 (}58 (}21
24 hours In rain 12 hours
and under snow Qms, low %Si02 2'72 2..04 (}75 (}78 1·1
24 hours In rain 12 hours
and under snow Qms, low %Si02 2'56 2'20 (}86 (}85 (}99
24 hours Inrain 12 hours
and under snow Qms, low %Si02 2·78 2-55 (}92 (}95 1-86
24 hours From ground in
Quartzite 2'19 1·50
sunny weather (}69 0.57 (}31
Vertical cliff Qms 2'59 1'18 (}45 (}41 (}32
Vertical cliff Qms 1·69 H8 (}70 (}45 (}32
From ground Qms, small pods 2-46 1-48 (}60 (}54 (}13
From ground Fine banded qms 1·22 (}96 (}79 (}36 (}27
From ground Qms, large pods 1·27 (}85 (}67 (}32 (}11
From ground Qms, small %Si02 (}55 (}41 (}75 (}15 (}15
From ground Qms, big pods 1'51 l-Q6 (}70 (}39 (}19
138
Table IV.(Coned.)
K.HALL
Water
absorption %H2O
Porosity coefficient after %Field
Location Rock (%) (%) S-value 24hours H2O
From ground Qms, small pods 2·28 1·99 0-88 0-72 0-39
From ground Qms, high %Si02 1·27 1-09 0-86 0-4 0-31
From ground Qms, thick bands 1'22 0-81 0-67 0-29 0-07
From ground Qms, thin bands Hl2 0-76 0-75 0-29 0-19
From ground Qms. large pods 1·37 1-14 0-83 0-42 0-15
From ground Qms, small bands 0-74 0-44 0-60 0-17 0-23
From ground Qms, folded 2-43 1·97 0-81 0-71 0-13
From ground Qms 1·98 1-32 0-67 0-48- 0-68
Rock face Qms 4-65 3-45 0-74 0-39 0-17
Rock face Qms 1·34 H>6 0-79 0-39 0-17
TableV. Properties of the cut rock tablets
Rock Qms Qms Qms
Porosity (~J
1·84 Hl7 2·29
Water absorption coefficient
(~J 0-53 0-58 0-92
Saturation coefficient
0-27 0-31 0-41 Qms=quartz-micaschist.
DISCUSSION
The most significant finding is that of the existence of rocks, in a subaerial environment, with > 50 per cent moisture content (Table VI) within a climate where they are subject to freezing. Thus. in response to the question of White (1976, p. 5) as to '...willbedrock ever become water-saturated from melting snow or rain and then undergo rapid freezing to crack the rock', it can be said that in the maritime Antarctic this may be the case. Of the samples obtained, 40 per cent indicated moisture contents in excess of 50 per cent (with 17 per cent of them above 90 per cent saturation) near to an area shown by Walton (1982, Table 11) to be subject to freeze-thaw during the sampling period. Recent detailed measurements at one of the reference sites have shown that short-term freeze-thaw cycles are·a typical feature of summer conditions (Walton, personal communication); thus frost shattering must be considered a potentially active mechanism. It may even be that, in the light of the test procedures and the rocks themselves, an even greater percentage of the rocks were prone to frost damage.
If the data indicate< 50 per cent saturation then this is with respect to the whole rock and does not consider the actual distribution of that moisture within the rock. If the moisture is evenly distributed within the sample then that value is meaningful but what if there is a concentration in the peripheral zone? Laboratory testing of samples indicated that those from the field had higher S-values and water absorption capacities than did cut blocks of the same material. Considering the schistose nature of the rock it could be envisaged that weathering prises the laminae apart at the margins and that this accounts for the higher values found for the field samples. This in turn would tend to suggest that moisture would be more readily available at the periphery and that breakdown would take place here and gradually work inwards. Unfortunately, no means ofaccurately assessing water distribution within the rock, in the field, is known to the author and so the specifics of water location within the rock remain unknown.