Kevin Hall and Alida Hall
Geography Department, University of Natal, Pietermaritzburg 3200, South Africa ABSTRACT
The heating of rock by insolation during subzero air temperatures may cause thermal stresses within that rock. The values ofI1tmay be such that fracturing due to thermal shock may occur.
The uneven heating of a rock body may cause buttressing of the heated faces such that thermal stresses are accentuated. Upon removal of the heat source, rapid cooling may occur and values of
!:J.t may be sufficient to cause thermal shock.
Replications of these thermal stresses may lead to fatigue and failure. The zone within which these stresses may occur is also one within which freeze-thaw can take place if water is present. It is suggested that thermally induced fracturing of rock in cold environments may be a significant but underrated process. Thermal results of laboratory simulation experiments are presented during which values of!:J.t = >500 QC/h occurred for short periods.
RESUME
L 'echauffement de roches par insolation pendant des periodes 011 la temperature de I'air est inferieureit0 QC peut causer des tensions dans les roches. Les valeurs de variations de temperature peuvent etre telles que des fracturations des roches par choc thermique peuvent se produire.
L 'echauffement inegal d 'une masse rocheuse peut provoquer des tensions des faces echauffees qui accentuent les stress thermiques. Lorsque la source de chaleur disparait, un refroidissement rapide peut survenir et les variations de temperatures peuvent etre suffisantes pour causer des chocs thermiques.
Des repetitions de ces efforts peuvent conduireit une fatigue des roches et
a
leur rupture. La zone dans laquelle ces efforts surviennent est aussi celle dans laquelle les alternances gel-degel peuvent prendre place si de I'eau est presente. Il est suggere que la fracturation des roches par fluctuations thermiques dans les regions froides peut etre un processus significatif quoique sous- estime. Des resultats d 'experiences simulant ces fluctuations thermiques sont presentees. Pendant ces experiences, les valeurs des fluctuations de la temperature ont ete egales ou superieuresa
500 QC par heure pour de courtes periodes.
KEY WORDS: Thermal shock Thermal fatigue Freeze-thaw Weathering Cold regions
INTRODUCTION
In recent years there has been a significant increase in detailed, quantitative studies of mechanical weathering processes (Yatsu, 1988). However, one aspect that has received little attention, particularly
1045-6740/91/020103-10$05.00
~)1991 by John Wiley&Sons, Ltd.
in cold regions, is that of thermal gradients and rate of change of temperature within rock. These factors greatly influence a number of mechanical weather- ing processes and, in some cases, certain theories rely upon specific rates. In the case of freeze-thaw weathering, the rate of change of temperature and
Received7January 1991 Accepled26February 1991
104 K.Hall and A. Hall
the temperature gradient within the rock are both of central importance.
Thomas (1938), from an early study of the effects of freezing on building materials, noted that it was the cooling rate in the early part of the freeze cycle that exerted greatest influence on rock damage.
This rate of cooling had far more effect than the actual temperature ultimately reached. Thomas argued that with a low rate of freezing, ice extrusion would occur and thus large pressures would not be achieved. Conversely, with more rapid temperature changes, less extrusion takes place and more un- frozen water would be available within the rock to generate high interstitial pressures. Newton's Law of Cooling states that the rate of heat loss from a warm body is proportional to the difference in temperature between that body and the surround- ing medium. Thus, with large temperature differ- ences between two mediums (i.e. the rock and the air), the temperature gradient within the rock can alsobeexpected to be great and the rate of temper- ature change, particularly in the outer part of the rock, to be very large. Thomas (1938, p. 72) also argued that the ratio between the exposed surface and the total volume of the specimen affects the temperature gradient.Ifthe temperature difference between the specimen and its surroundings is large, then large temperature gradients result, owing to the outer faces cooling more rapidly than the rock interior. One consequence of this is the increased likelihood of unfrozen water being trapped within the rock and high pressures being developed.
More recently, Michaud et al. (1989) suggest that 'frost bursting', the explosive failure of intact, massive rock, can occur when saturated rock is subject to intense, rapid freezing. The hydrostatic pressure developed in the pores and cracks of the rock generates strain energy which, if able to be stored by the rock, may ultimately be released in an explosive manner. According to Michaud et al., a fast rate of freezing helps increase the strength of the rock by sealing pores and fractures with ice, thereby transferring the mass into a continuous rock medium which is thus able to store strain energy. Bout (1982) also refers to 'thermal shock' associated with a
+
20°C to - 30 °C temperature change taking place within a 24 h period while Le Ber and Oter-Duthoit (1987) note the operation of thermal shock on saturated rocks at temperatures below -3°C.In other studies the rate of temperature change is often cited as significant. For instance, Battle (1960) suggested that this rate needed to be of the order of
~0.1°C/min for breakdown to occur, while Walder
and Hallet (1985) hypothesize that the rate should be very slow, preferably in the region of 0.025-0.1 cC/h. However, a rock does not experi- ence a singular rate of temperature change, but rather, at any given depth, the rate changes linearly with time (Walder and Hallet, 1985; Hall, 1988)-the so-called 'omega component' lag effect described by Jerwood et al. (1987). It is worth noting that Jerwood et al. (1987, p. 142) also state that: 'Rock freezing and thawing rates cannot be predicted directly from air cooling and warming rates. Rates of freezing and thawing are related to temperature differentials, and these in turn are dependent on the point at which freezing and thawing begin and end ... '. However, although many workers note the importance of the rate of temperature change upon the rock body, actual data are extremely rare (Table 1). Data which are missing relate to the change of temperature with depth (the temperature gradient) and its change through time together with the rate of temperature change at given depths and how this changes with time.
Yatsu (1988) shows that sudden increases of heating or cooling of a rock body create steep temperature gradients. Ifa high gradient is set up within a thin layer, then spalling can occur. Thus, with surface heating, the outer shell of the rock expands and tensile forces are created between it and the cooler, inner part of the rock (Bahr et al., 1986). With the removal of the heat source, the outer layer starts to contract but if the inner part is still warming owing to lag effects, then a zone of compressive stress will develop. When the tempera- ture change is ~2 °C/min, then the rock cracks, usually along grain boundaries (Richter and Sim- mons, 1974; Yatsu, 1988). In the case of anisotropic rock' .... if there is a mismatch in the thermoelas- tic behaviour of minerals across their grain boun- daries, internal thermal stresses may be generated when the rock is subjected to different tempera- tures, and the stresses thus induced may be large enough to promote the formation of new cracks' (Yatsu, 1988, p. 132).
Thus, within what is generally considered to be the freeze-thaw process per se, it is possible that there is a synergistic component of thermal fatigue.
It follows that considerations regarding the rate of temperature change and the thermal gradient with- in the rock need to be related not only to their effect on the freeze-thaw mechanism but also to their role in thermal stress fatigue. While this is not the case in many environments, two situations merit atten- tion. The first is high-altitude and/or high-latitude
Thermal Gradients and Rock Weathering 105 Table 1 Rates of change of temperature as found by different authors.
Author Location Rate (OC/h)* Depth (cm) Other details
Michaud et al. (1989) Canada 0.5 5 Cooling rate past O°C
Hare (1985) Canada 0.7-2.01 5
Thorn (1975) USA 0.26°C/d I
Whalley et al. (1984) Karakoram \.9 SE-facing rock surface
2.5 W-facing rock
surface Under clear 1.99 0.3 cm wide crack conditions 1.5 0.5 cm wide crack
1.3 JOcm cavity
1.0 SE-facing rock
surface
0.7 W-facing rock
surface
0.6 0.3 cm wide crack Cloudy conditions 0.5 0.5 cm wide crack
0.5 JOcm cavity
4.6 Basalt surface
1.4 Basalt 5 cm depth 2.7 Sandstone surface
" " 2.4 Sandstone 5 cm depth
Myagkov (1973) Antarctica 0.8°C/min Surface 15-20 °C/h -1
Van Autenboer (1964) Antarctica 16.3 Surface 49°C in 3 h
Jonsson (1985) Antarctica c.15 2-3 In crack
• Unless stated otherwise.
locations where large radiation inputs can occur during times of subzero temperatures, thereby creating steep temperature gradients, and which, when that heat source is removed, cause very rapid changes in temperature within the outer layer of the rock. The second is with respect to freeze-thaw simulations where high rates of heating and cooling are employed. In both cases the possibility arises that rock breakdown is directly related to thermal change, rather than to the freezing and thawing of interstitial water.
As part of the British Antarctic Survey' Fellfield Programme' a range of simulations were under- taken based upon temperature and moisture condi- tions monitored in the field (Hall, 1986a. 1988).
Large blocks of rock were heated by infrared lamps during subzero temperatures to simulate Antarctic conditions. The results of these simulations, in terms of rates of temperature change and thermal gradients, are presented here.
METHODOLOGY
By use of a computer-controlled climatic simula- tion cabinet (Hall et al., 1989) temperature condi-
tions similar to those experienced on Signy Island (Maritime Antarctic) were replicated. Typical rock moisture content and chemistry are already known (Hall, 1986a;Hall et al., 1986). In the early experi- ments these conditions were replicated and air- based freeze-thaw cycles were used (Hall, 1988).
Subsequently, two rock types (details given in Hall, 1988) were subject to heating by variable control infrared lamps once the samples had attained a temperature of c. - 19°C. The cabinet air tempera- ture was maintained within the range - 19 cC to - 10 QC (some warming occurred during use of the lamps). Some warming cycles were short and in- tense; others of longer duration and less intense.
Heating by the lamps was removed by turning them off. In this way an attempt was made to simulate various forms of heating by the sun during periods of subzero air temperatures. The heat source, being suddenly removed, simulated the case of a cloud covering the sun or the rock going into shadow. During these simulations temperatures of the air. the rock surface, and at depths of 2.8cm and 3.3 cm (sample 1) and of 2.8 cm and 3.1 cm (sample 2) were measured every minute. In these experiments the rocks were dry so as to exclude any
RESULTS
Table 2 Examples of rates of change of temperature measured during the - 20 cC freeze cycle.
106 K. Hall and A. Hall
complications that the presence of water could induce.
Freeze-thaw simulations utilized cycles of - 3°C (at 4.2°C/h), -6°C (at 3°C/h) and -20°C (at 1 °C/h and 3°C/h). Data obtained during~ulti~le replication of these cycles on quartz-mlcaschlst show rates (Table 2) that cause no thermal stresses because they are much too small (within the range 0.2-3.0°C/h); details regarding these experiments and their results are given in Hall (1986a, 1988). It has been shown (Hall, 1988) that rock temperature decreases linearly with time (e.g.r
= -
0.98 for the - 6 °C cycle) and that the rate of temperature change also varies in a linear fashion (r=
0.92).The implications of these rates with respect to the freeze-thaw mechanism are also discussed in Hall (1988). Suffice it here to state that within the simulated air temperaturefreeze-thaw cycling used in these experiments, the monitored thermal gradi-
ents and rate of change of temperature were not sufficient to cause thermal stress fatigue. In addi- tions, during none of the freeze cycles were condi- tions conducive to those required by Thomas (1938) or Michaud et al. (1989) to produce any form of 'frost bursting'. Ifanything, during some freeze cycles the attributes were more akin to those suggested by the hypothetical model of Walder and Hallet (1985).
Examples of the sort of rates of temperature change measured when infrared heating was turned on and off are given in Tables 3 and 4. As expected, the rate of change was greatest at the rock surface and decreased with depth. The rate of change also decreases linearly with time in both cases (r= 0.99 andr
=
1.0, respectively). However, rates are extre- mely high/orshort periods a/time,with values far in excess of the 2 °C/min (120 °C/h) deemed sufficient for thermal fatigue by Yatsu (1988) and others.Using linear regression, it appears that rates of 2 cC/min operate to a depth of c. 2.2 cm. Thus, the outer 2 cm shell of the rock experiences alternating tensile and compressive stresses associated with the heating and cooling phase, respectively. This situ- ation is highly likely to produce thermal stress fatigue.
In the outer shell of the rock there is a very steep temperature gradient (x = 7.5°C/cm for the outer 3.3 cm). Within this shell, freezing and thawing can also take place if water is present. From the avail- able data it would seem that it is approximately the outer 1 cm of the rock within which interstitial water can thaw and then be subject to very sudden freezing. Because the zone within which thermal stress f'atigue can operate encompasses that within which freeze-thaw can also occur, it may be extre- mely difficult to discern the role played by either in rock breakdown.
Two further factors may operate to aid weather- ing via thermally-induced stresses. First, the rock is not heated equally, and, second, parts of the rock may be actively warming while others are cooling.
In the first instance, as replicated in the simulations, certain faces receive heat while others are still in shadow (Figure 1). Thus, the temperatures pre- sented in Figure 1 are with respect to the heated surface. However, those surfaces not receiving direct incoming radiation will not expand and will act to constrain the lateral expansion of those which are heated. In other words, a form of 'but- tressing , (Folk et aI., 1982) takes place. With respect to the second factor, the temperature data show (Figure 2) that the interior of the rock con- tinues to experience warming when the outer part
Temperature Time Rate
(cC) (decimal h) (CC/h)
0 to -1 0.417 2.4
0 to -3 1.084 2.8
-1 to -3 0.667 3.0
-3 to -6 1.268 2.4
-6 to -10 1.680 2.4
-10 to -15 1.935 2.6
-15 to -19.6 15.726 0.29
0 to -1 0.416 2.4
0 to -3 1.182 2.5
-1 to -3 0.766 2.6
-3 to -6 1.232 2.4
-6 to -10 1.619 2.5
-10 to -15 1.927 2.6
-15 to -19.2 15.065 0.28
0 to -I 0.416 2.4
0 to -3 1.165 2.6
-I to -3 0.749 2.7
-3 to -6 1.222 2.5
-6 to -10 1.646 2.4
-10 to -15 1.927 2.6
-IS to -19.4 15.065 0.29 Location
3.1 cm depth Surface
2.8 cm depth
Thermal Gradients and Rock Weathering 107 Table 3 Rates of change of temperature experienced when infrared lamps turned on, with
initial air/rock temperature at -19°C.
Location Temperature(0C) Time taken (decimal h) t..t (0C) Rate(oC/h)
Surface
from -19.1
to -18.1 0.017 1.0 58.8
-9.6 0.017 8.5 500.0
-2.5 0.013 7.1 546.2
+2.3 0.019 4.8 252.6
+4.0 0.015 1.5 100.0
2.8 cm depth
from -18.6
to -18.4 0.017 0.2 11.8
-17.3 0.017 0.7 41.2
-16.3 0.013 1.0 76.9
-15.3 0.019 1.0 52.6
-14.5 0.016 0.8 50.0
-13.9 0.015 0.6 40.0
-13.3 0.019 0.6 31.6
-13.2 0.017 0.1 58.8
-12.9 0.016 0.3 18.7
3.1 cm depth
from -18.8
to -18.7 0.017 0.1 5.9
-17.6 0.017 0.9 52.2
-16.7 0.013 0.9 69.2
-15.6 0.019 1.1 57.9
-15.0 0.016 0.6 37.5
-14.3 0.015 0.7 46.7
-13.7 0.019 0.6 31.6
-13.3 0.017 0.4 23.5
-13.3 0.016 0 0
cools following the heat source removal. This cre- ates the situation whereby the inner body is at- tempting to expand while the outer part is contracting. The result is a zone of compressive stress which, because it is not uniform, may cause shearing.
Michaudet al. (1989) describe frost bursting as a result of strain energy developed within a saturated rock, subject to rapid freezing. The hydrostatic pressure of the unfrozen water develops in the pores and cracks of the rock body. Both Le Ber and Oter- Dethoit (1987) and Michaud et al. (1989) specify that the rock must be saturated for frost bursting to occur. The presence of saturated rock during winter in Antarctica, on other than perhaps a wave-cut platform or beach, must be questioned (Hall, 1986b; Trenhaile and Mercan, 1984). Intrinsically, there does not appear to be a problem with the
theory; rather, it is the practicality of saturating the rock which is in question. However, if the rate of temperature change is rapid and the rock is able to constrain th~eenergy, there is no reason why strain energy should not develop, even in the total ab- sence of water during either the heating or cooling events. During either phase, the 'rock mass is neither expanding nor contracting in a uniform fashion; rather, the outer part experiences the greatest change, while with increasing depth the amount of attempted change decreases (Hockman and Kessler, 1950). This can be modelled as a series of zones each experiencing different amounts of change (Figure 3). Ifthe rock can withstand this without shear, then strain energy builds up only to be released catastrophically, in the manner de- scribed by Michaud et al. (1989), along a pre- existing line (or lines) of relative weakness.
dientS an
\ Ora 1'herrna
130
108 K. Hall and A. Hall
Temperature (0C)
Time taken (decimal h) dt (cC) +9.2
+2.4 0.017
-5.2 6.8
0.018 -8.9 7.6
0.016 -10.7 3.7
0.015 -12.1 1.8
0.020 -13.3 1.4
0.031 -14.1 1.2
0.034 -15.1 0.8
0.049 -16.0 1.0
0.069 -17.1 0.9
0.148 -18.0 1.1
0.500 -19.0 1.0
2.599
1.0 -13.3
-14.0
0.017
0.6 35..
-15.0
0.217
1.0 4.t
-16.0
0.215
1.0 4.7
-17.0
0.317
1.0 3.2
-18.0
0.667
1.0 1.5
-19.0
2.698
1.0 0.4
-15.5 -15.0
0.151
0.5 3.3
-16.0
0.627
1.0 1.6
-17.0
0.303
1.0 3.3
-18.0
0.565
1.0 1.8
-19.0
1.335
1.0 0.8
Table 4 Rates of change of temperature experienced when infrared lamps Location
2.8 cm depth from to
3.3 cm depth from to
Surface from to
oft,T crack
\ 541
de~c
t,T
th2 ca
it' c
Michaud et al. (1989) cite rates of temperature change of the order of 0.5 cC/h. These are far below that required for thermal stress fatigue (2cC/min:
Yatsu, 1988). However, these authors were at- tempting tD develop a hypothesis with respect to frost action in a saturaTed rock. In this Study, the rocks were not saturated but rates of temperaure change were. for short periods of time, in excess (by as much as a factor of4)of that considered to be the threshold for normal strain fatigue. That the rocks used in the simulation experiments did not shatter does not nega te the hypothesis of strain energy;
multiple replications are needed to weaken the rock sUfficiently until it Ultimately fails
catastrophi,..~"
when no longer able to constrain
rh~
.In the Dry Valleys of ,L Antarctica anri :
fr"""
elUded) and experience very low air temperaI but with strong radiation inputs. Although weathering is probably operative in these locati(
no signs could be found. It was noticeable, partit larly in the Andes, that the cracking pattern w.
often polygonal, somev.'hat akin to frost crackingi permafrost regions, Qualitatively, these observa tions suggest that many of these rocks shatter bv thermal stress fatigue associated with rapid
temn~-'
ature Changes. Certainly, the crack _ served are similar to thnc,..
(1986. Figure .., \ durpri '
110 K. Hall and A. Hall
Zone 01 attempted change (rapidly diminIshIng Inwards)
AIR -19·C
----
-,-,
Slram energy developed ~'---'~-::
Unaffected Inlerlor
l
Zone of allempled change where~rapid coolingISlaking place
Figure 3 A simple block model. showing the development of tensile stresses during the heating phase and the compressive stresses during cooling.
continued heating while the outer shell is cooling generate a zone of compressive stress. Because these forces are not uniform. they might cause shear stress.
In summary. the main results of these simulation ex periments indica te:
(i) Freeze-thaw cycles induced by changes in air temperature are insufficient to cause thermal stress fatigue.
(ii) The concept of frost bursting as proposed by Michaudet al.(1989) appears viable
if
the rock is closeto saturated.Figure 4 A large rock block at 4300 m a.s.l. in the Andes. showing fracture patterns considered to result from thermal stress fatigue.