GlOMOHPHOlOGY

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New insights into rock weathering from high-frequency rock temperature data: an Antarctic study of weathering by

thermal stress

Kevin Hall

^{a.' ,}

Marie-Fran<;oise Andre

^b

, Ceography Program. Ullir:ersiry ,,{Northern British Columbia. 3333 University Way, Prince George, BC, Canada V2N "Z9

bLaboralOire de Ceographie Physique, Unicersite Blaise Pascal, Up res-A 6042-CNRS. ./ rue Ledru. 63057 Clermont-Ferrand. France Accepted 17 May 2001

.<

23

2~ Abstract 25

26 A major limitation of many weathering studies has been the acquisition of rock temperature data at insufficiently frequent 27 intervals for the meaningful determination of the rate of change of temperature (0T/1). Equipment and/or logistical

28 constraints frequently facilitate temperature measurement at only hourly intervals or. at best. every 10 min. Such data are not

29 adequate for the determination of 0T/1 required for the evaluation of the freeze-thaw mechanism or thermal stress fatigue.

30 Recent undertakings at different sites in Antarctica (and at other cold-region locations) provide rock temperature

31 measurements at I-min intervals. which indicate that the perception of the weathering regime would be very different from

32 that generally assumed from longer-interval geomorphological data. These data clearly show that thermal stress fatigue and

33 thermal shock may be more active components of the Antarctic weathering regime than have generally been recognised: the

34 aridity of the study area limits the role of freeze-thaw weathering, Values of 0T/1 of ~2O( min-1 that suggest thermal

35 stress fatigue/shock is operative were recorded; observations of rock flaking are thought to reflect the impact of thermal

36 stress. Further, the data show that contrary to general perceptions. the southern aspect can. in summer. experience higher

37 rock surface temperatures than the north-facing exposure. An examination of rock fracture patterns found in the field shows

38 great similarity to fracture patterns developed in the laboratory as a direct result of thermal shock. The argument is made that

39 greater cognizance should be given to thermal effects. © 200 I Published by Elsevier Science B. V.

40

41 Keywords: Weathering; Rock temperatures: High-frequency measurements: Thennal stress: Thel1Tla1 shock: Antarctica 42

4445 46 47 48 49 50 51 52

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59

1.Introduction

In any consideration of rock weathering, there are three key factors, namely rock temperature, rock moisture and rock properties. These attributes, inde- pendently and in synergy, exert a major influence on the type, degree and rate of weathering that takes

• Corresponding author.

E-mail address:hall@unbc.ca (K. Hall).

61

place at any given site. For any meaningful investi- 62

gation of rock weathering, detailed data pertaining to 63

each of these attributes are required. Unfortunately. 64

consideration of much of the available literature 65

indicates that one or more of these key components 66

will usually be assumed (e.g. that water is both ⁶⁷ present and in sufficient quantities for freeze-thaw 68

weathering to occur under freezing conditions) and 69

that as a result, the nature of the weathering process 70

is inferred rather than proved. The same problem is

71

72 0169-555X/OI/$ -see front malleT © 2001 Published by Elsevier Science B.Y.

PII: S0169-555X(01)00101-5

K. Hnll. M.-F. ,I"dre / GI!O/l/orp/w/ogy00 (2001) 000-000 73

74 present with many laboratory simulations: tempera-

75 ture and moisture conditions rarely mimic natural

76 conditions simply because those attributes have never

77 been measured. Studies of weathering require that

78 the various attributes of the three key parameters be

79 measured in the field. Without such data, simulations

80 cannot be related back to any specific site and

81 knowledge of the weathering taking place at that site

82 is unfounded. Here, attributes of the rock tempera-

83 ture data that change the general perception of rock

84 weathering in cold regions are presented.

85 In any consideration of rock temperatures, there

86 are five main factors that must be recognised:

87

88 air temperatures are not a surrogate for rock

89 temperatures;

90 the minimum requirement is that rock surface

91 temperatures be measured;

92 rock temperature at various depths should be

93 measured;

94 the thermal gradient within the rock should be

95 calculated; and

96 values of the rate of change of temperature

97 (tJ.T/t) are required.

98

99 All too frequently, air temperatures are used to

100 exemplify the weathering regime. Unfortunately,

101 such undertakings are meaningless (Thorn et aI.,

102 1999) for they take no account of rock warming by

103 the sun (even at times when air temperatures may be

104 severely sUb-zero), rock to rock variations resulting

105 from albedo affects (Kelly and Zumberge, 1961;

106 Andre, 1993), or situations where the rock may in

107 fact not experience any thermal fluctuations because

108 it is insulated by a thick. snow cover. Even when

109 rock temperatures are recorded, this rarely includes

110 sufficient information to evaluate thermal gradients.

111 Equally, for a variety of reasons, collection of rock

112 temperature data is rarely, if ever, at a frequency

113 sufficient for any meaningful evaluation of the rate

114 of change of temperature (tJ.T/t). Both of these

115 attributes, thermal gradient and t::..T

/t,

are important

116 for evaluating the weathering process(es). Indeed,

117 measurements oft::..T

/t

are imperative for any under-

118 standing of the freeze-thaw mechanism as the vari-

119 ous mechanisms have a variety of controJling rates

120 which constrain whether they can operate, e.g. 0.1 °C min- I as suggested by Battle (1960). Beyond the

121

simplistic assumption of freeze-thaw (Hall. 1995), 122

other weathering processes, notably thermal stress 123

fatigue and thermal shock. are constrained by (among ¹²⁴ others) the rate of change of temperature. It is the ¹²⁵ measurement and importance of t::..T/r that will be 126

discussed here. 127

Paradoxically, in so-called "cold climates." it may 128

well be heat which is a major factor with respect to 129

geomorphic processes. The very name "cold c1i- 130

mates" implies, almost exclusively, the role of 'cold.' ¹³¹ As a result. most discussions of cold region weather- 132

ing tend towards consideration of only mechanical 133

processes. and usually, within those arguments, the 134

dominance of freeze-thaw weathering (see Hall, 135

1995, 1999 for discussions). However, such an ap- ¹³⁶ proach negates the impact of the summer, short as it 137

may be at high latitudes, plus the influence of rock 138

warming within the otherwise cold environmeni. Re- 139

cent studies (e.g. Balke et aI., 1991) have shown that 140

in cold regions, it is not the temperature that is the ¹⁴¹ limiting factor for chemical weathering, even in the 142

Antarctic, but rather it is the availability of water. ¹⁴³ Thus, the measurement of rock temperature, and its 144

temporal and spatial variability, is critical for the 145

understanding of rock weathering. Central to any 146

measurement of rock temperatures must be the eval- 147

uation of t::..T

/t.

Not only is this critical to under- 148

standing any freeze-thaw activity, but can also itself 149

be a cause of weathering-by thermal stress and 150

thermal shock. Thermal stress fatigue and/or ther- 151

mal shock are usuaJly considered under the synonym 15:

"insolation weathering." Unfortunately, this term 153

generates wrong perceptions. First, insolation does 154

not "weather" - i t is only one of the driving forces 155

for the thermal changes that actually cause the 156

weathering. This is very important indeed, for a 157

number of studies have, for example, shown that ¹⁵⁸ sub-surface rock temperature fluctuations are driven 159

by variations in wind speed even where air tempera- 160

ture and radiation input are held constant (e.g. ¹⁶¹ Nienow, 1987). Second, the role of 'insolation' seems ¹⁶² more appropriate to "hot environments" than to 163

"cold" ones-as shown by any comparison of "Hot 164

Desert" geomorphology texts (e.g. Abrahams and 165

Parsons, 1994) with "Cold Environment" oeomor-_c 166

phoJogy texts (e.g. French, 1996). Thirdly, the gen- 167

eral perception of the role and significance of "inso- 168

lation weathering" has been severely reduced, at

K. /lall. M.-F.,llIdr,; / G"fllllorp!III!lJgy00('200/) 000-000

239 2]1 228

240

20 24"

23S 229

235

237 236 232 233 234 230 227

:::.

226 220 221

225 224 223 222

:~:

emerges from behind a peak. A whole range of 218

differential stress fields result from these temperature changes (see Hall, 1999), and it is these which offer an explanation for the flaking observed on the rocks at the study site. Indeed, Dragovich (1967, p. 801).

in a discussion regarding flaking, cites the sugges- tions by Kvelberg and Popoff (1937) and Cailleux (1953) that" ... the surface-rock layer ... is affected by cool air which descends rapidly over it. This abrupt lowering of temperature forces the rock sur- face to contract and buckle outward from the under- lying rock, thereby causing flakes to develop."

Recognizing from above, the frequency of tem- perature measurement then becomes the key issue.

The obtainment of high frequency temperature data has hitherto been limited by a number of factors. At the conceptual level, the need to record at intervals of I min has not arisen. Assuming freeze-thaw weathering, most studies have required some mea- surement of the amplitude and length of freeze and/or thaw; in the field, 6.T

/t

has not been seen as significant in this regard. This conceptual component has been compounded by practical and logistical constraints. The availability of sensors that can re- solve temperatures. and loggers that can record at such intervals, are reasonably recent innovations.

Even then, logger memory size and battery mainte- nance in cold environments, exert extreme limita- tions on the logistics required for such undertakings;

battery changing and memory downloads become frequent, perhaps daily. Under such practical and philosophical constraints, it has hitherto been deemed expedient to worry more about freeze and thaw durations and amplitudes to the detriment of any consideration of 6.T/ t. Interestingly, it is from bio- logical studies that the best high-frequency data come (e.g. Kappen et aI., 1981; McKay and Friedmann.

1985; Friedmann et aI., 1987; Nienow, 1987). al- -"

though more recently, a number of geomorphologicJI :~,

studies have collected I-min records for short pen- ods of time (e.g. Warke et aI., 1996). In AntarctIC and Canadian studies, Hall (1997, 1998, 1999) hJ~

collected temperature records at 2-min, I-min and 30-s intervals for periods ranging from 1 week to:: .'.

months and, more recently (Hall, unpublished) for J __ . period of 1 year. In this paper, recent data (Hall.

1999) collected from the Antarctic will be used to show the value of I-min temperature data. and to least in geomorphology. by on-going reference to the

studies of Blackwelder (J933) and Griggs (J936).

The unquestioning acceptance of these studies (see Oilier. 1984 for arguments about this) has led to many geomorphologists discounting the possible role of thermal stresses in the breakdown of rock (see Hall. 1999 for a wider discussion, including the impact of differential variations in 6.L/oC as a function of crystal axes). Once recourse is made to engineering and ceramics studies, where thermal stress and thermal shock are central to many investi- gations, the potential of the role of thermal variations in causing rock disintegration becomes evident. Thus, if the negative attributes of the three discussion points above can be overcome, the impact of 6.T

/t

upon weathering in cold environments can be con- sidered.

The role of 6.T/t is even more signi ficant when it is realised that it is 'temperature independent: i.e.

that the rate of change of temperature that might effect damage can be anywhere on the temperature scale. Studies have shown (e.g. Richter and Sim- mons, 1974; Yatsu, 1988) that the threshold value for thermal shock approximates to a rate of tempera- ture change of 2 °C min -1. Values equal to or greater than this cause the rock to try and adjust at a rate that is greater than its ability to deform plasti- cally and so the rock fails. That value can, however, be anywhere on the temperature scale: from

+

32 to

+

34, from - 15 to - 17°C; it is not constrained to freezing temperatures or to positive temperatures, and does not require the presence of water. Once this is accepted, then it can be seen that 'cold environ- ments' may well be ideal locations for such events.

A typical scenario, particularly for the Antarctic, would envisage rock exposed to sub-zero air temper- atures (perhaps as low as - 30°C) but being heated by· incoming radiation on a clear day. That source of incoming energy is then "switched off' by cloud covering the sun or the sun moving behind a peak.

At that point, the temperature differential between the outer layers of the rock and the air is very large (e.g.

+

10 °C rock to - 30°C air) and so, following Newton's Law of Cooling, the rate of change of temperature (6.T/t) will be very high, potentially 2: 2 °C min -1. The same happens, but in the oppo- site temperature direction, when the sun then hits the cold rock once the cloud has passed or the sun

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4 K.lIall, M.·F. tllld,,; / Gt'OIIIvrp/w/ogy00(2001) 000-000

265 280

266 help justify the argument regarding the role of ther- were collected by an "ACR Systems" 'SmartReader ²⁸¹

267 mal stresses in cold environments. logger that has a resolution better than 0.1 °C and ²⁸² can store 32,768 readings; accuracy of the thermis- ²⁸³

268 tors was

±

0.2 °C. Additional data on humidity at the ²⁸⁴

269 2. Study area and procedures rock/air interface were also collected at I-min inter- ²⁹⁵

270 vals but are not discussed here. ²⁸⁶

271 Data were collected from the top horizontal sur-

272 face of a rock outcrop as well as from the north, ²⁸⁷

273 south, east and west aspects at (Fig. 1) the British 3. Results and discussion ²⁸⁸

274 Antarctic Survey Rothera base (67°34' S, 68°07' W). 289

275 The site (Fig. 2) comprises a medium-grained gran- The data set for part of 11 th December 1999 are 290 276 odiorite at an elevation of approximately 30 m a.s.L ideal for showing the value of 1-min data (Fig. 3). 291 277 Thermistors were located on the rock surface and Part 1 of Fig. 3 shows the complete data set for 292 278 data were collected every minute; a number of ther- December 11, 1999 while part 2 shows a 2-h record 293 279 mistors were also located in cracks, lichen cover, for the five sample locations. Part 3 is the detail for 294

fine debris, etc. but are not discussed here. Data the horizontal surface for that 2-h period. It is this

295 296

Fig. I. Location map to show the position of the study site at Rothera Scientific Station.

299 Fig. 2. Photograph of the rock outcrop where the sensors were 300 located. The four cardinal directions are marked and a rucksack is

shown for scale.

301

302 that will be used here to indicate the value of the

303 I-min data. Fig. 3, part 3, shows that over the 2-h

304 period, there were a number of fluctuations. Specific

305 among the many fluctuations is that from 0718 to

306 0721 h, during which there was a marked drop in

307 temperature (from 5.4 to - 0.3 QC), which will be

308 discussed shortly. Fig. 4, part 1, shows the detail of

309 temperatures that would be shown for that same 2-h

310 period if temperatures had been recorded every 10

311 min; such a rate being reasonably high in most

312 published studies. Evaluation of that record (Fig. 4,

313 part 1) would indicate the highest rate of change of

314 temperature to be 6.3 °C/I0min for the rising limb

315 from 0630 to 0640 h.If, however, these data are then

316 placed on top of the actual information (Fig. 4, part

317 2), it can be seen what an a~stract of reality the

318 10-min data represent. Many fluctuations are missed

319 by the 10-min record. For example, during the first

320 hour (0600-0700 h), the 10-min data show a rise to

321 4°C and then a decline to 2.2 °C while, for that same

322 period, the I-min data show nine rises and falls.

323 Equally, the 0700-0800-h record for the lO-min data

324 shows a fall in temperature while the I-min informa-

325 tion indicates eight rises and falls. During 0700-0800

326 h of falling limb for the lO-min data, there was, in

327 reality, a significant peak to the highest temperature

328 in this record period followed by a rapid fall. Clearly,

329 the lO-min data are inadequate for any meaningful

330 determination of thermal variation at the rock sur- face.

297 298

t"..

_~::

.,.~(~

""'.0.5m lOng rucksaCK for scale

The details of the temperatures during the period 0712-0722 h on December I I are shown in Fig. 5.

The importance of these data are that during the drop in temperature between 0718 and 0719 h, ~Tit was 3.2 °C min- I and the following minute showed a

~Tlt value of 1.6° min-^I; 4.8 °C over 2 min. The value of 3.2 °C min^{- I} exceeds the theoretical thresh- old for thermal shock as does the composite for 2 min, 0718-0720 h Thus, not only would the 10-min record have completely missed all the fluctuations that actually took place, but it would have shown no indication whatsoever of the high t1 Tit value that may be significant for rock failure. Another example of just such an event is that for the eastern aspect on 11th of December between 0852 and 0854 h when successive values of 6.Tit were 2.48 and 2.87 °C

min- I , respectively. Again. longer interval data

measurement would have failed to resolve these events. For comparison, consider the recent data presented by French and Guglielmin (1999, Table 2b, p. 334). Here, hourly data are used to determine the number of crossings of various thermal thresh- olds (0, - 2 and - 4°C) which are then applied to an evaluation of weathering, particularly in relation to freeze-thaw weathering.Ifthe data presented here for a 10-min record frequency can generalise, and thereby eliminate major variations to the extent shown in Fig. 5, then the question must arise as to the meaningfulness of an hourly evaluation. In fair- ness, French and Guglielmin (1999, p. 335) note that the aridity of the region would limit any freeze-thaw weathering. However, the evaluation of any cycling based on hourly data must be fraught with over generalization.

Significantly, the high magnitude thermal events were not found (within this data set) on all aspects.

The eastern and horizontal surfaces cited above were the only ones that experienced events with a 6.T

It

of ~2 °C min -^1. On the western aspect, the greatest values recorded were 1.6 °C min^{- I} during a 3-min period (1348-1351 h on December 11) when tem- peratures changed by 4°C; values of 1 °C min-^I were fairly common. On the southern aspect, the most common 6.T

It

value was of the order 0.4 °C min-I,with 0.6 °C min-I being the highest recorded.

The high southern value was at the same time (0852 h) as that recorded for the northern aspect when a 6.Tit rate of 1.82 °C min-^I was recorded, the

332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 36:

363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378

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Fig. 3. Detail of the temperature data for the four cardinal directions plus the horizontal surface for the I I th of December, 1999. The temperature data for the 2-h period, 0600-0800h. are then shown together with those for the horizontal surface during that 2-h penod 382

383 highest for this aspect. The more common

t:.TIt

rate

384 for the northern aspect was in the order of 0.7-0.8

385°C min-¹. Thus, it would appear that the high

t:.Tlt

386 values may not be found simultaneously on all as-

387 pects and that aspect preferences may occur depend- ing on the time of year and the local climatic condi-

tions. Indeed, Hall 0998, Fig. 2) showed distinct ) ! ...

aspect differences at the Rothera site, with the north- !~:

em aspect having the lowest recorded temperature ^39:

and the western aspect the highest for that record 39:

period. Data for the 11 December 1999, from this 393

present study, show a different aspect distribution:

K. lIall. M.-F.11/1(1,,; / Gelll/lllrplw/II!JY00(200J)000-000 7

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394 395

396 Fig. 4. The temperature curve for the 0600-0800-h period. shown in Fig. 3. based upon IO-min temperature data together with that same curve superimposedODthe I-min data recorded over the same period.

397 408

398 the southern aspect had the lowest temperature as

399 well as the highest. The aspect influence is clearly

400 more complex than has simplistically been presented

401 here; not the least being the need for daily compar-

402 isons over a longer time period to help filter out

403 day-to-day variations from those that might be sea-

404 sonal (Hall, in preparation). However, it would ap-

405 pear that with respect to _~T

/t

values, there is an

406 aspect influence, and that this may vary through the

407 spring to autumn period in terms of which aspect experiences the greatest thermal variations.

Elsewhere in the Antarctic, a number of studies 409

provide information in support of weathering by 410

thermal stress fatigue and/or shock. Gunn and War- 411

ren (1962, p. 60) state, with respect to the weather- 412

ing of fine-grained rocks in Victoria Land, "Al- 413

though experimental evidence suggests that the forces 414

exerted by rapid temperature change are too small to 415

fracture rocks ... it is difficult to find an alternative 416

explanation for some ... features." In support of this 417

argument, they note that temperature ranges as large 418

as 60°C may occur. Myagkov (1973), also in Victo-