ABSTRACTS / RESUMES

COMITATO GLACIOLOGICO ITALlANO - TORINO 1997

Weathering rates of Tertiary sedimentary bedrock in Japan

IDep.rtment of CivilEn~ineerin~.Chuo Uni\·ersiIY. Kasug•.

Bunkvo·ku. Tokvo 112. r.p.n 'Inslitule ofG~sciencc:s.^ChuoU~iversiIY. Kasu~^•.

Bunkyo.ku. Tokyo 112. J.pan

The rates of weathering of bedrock have been considered to be one of the most important factors that influence the modes and rates of denudational processes such as erosion and mass movements. The weathering rates were investiga- ted in/the following way.

(1) The weathering rates were examined for twO kinds of the definition: first the rate (dZ/dt) ac which chickness of weathered zone of bedrock (2)increases with cime(I)and second the rate (dRldt) at '.l:hich strength of weathered materials ac a given depch (R) decreases with cime (I). (2)

The emergence age of marine-erosional terrace is assumed to be equal to che weathering cime(t)for the bedrock un- der the cerrace surface.(J)The u'eathered macerials except che soluble of bedrock under the terrace veneer are assu- med co have scarcelv been eroded awav. (4) The weathe- ring proftles were observed for the drilling cores obtained by the presem authors. (5) The drilling cores were exami·

ned in che laboratory for the changes in the needle pene- tration hardness with depth. (6) The weathering proftles were divided into four weathered zones according to the change in the mechanical property: i.e. highly weathered zone (H). moderacely (M), slightly(S) and faindy (F). (7) The thickness of weathered zones is defined as the depth from the bedrock surface co the weathering front of each of the four weathered zones.

The data were obtained for the bedrock of the marine ero·

sional·terraces in the Boso Peninsula, Japan. The terraces are divided into five levds. The bedrock in this area is the Pliocene marine sedimentary rocks of the interbedded mudstone, sandstone and conglomerate. The drilling cores were obtained at the 11 sites for sandstone and at the 13 si- tes for mudstone on the terrace surfaces with three diffe- rem ages.

Mode of deceleration in the weathering rates (dZ/dt) with weathering time(I) differs between mudstone and sandsto- ne. In the faindy weathered zone, mudstone is weathered faster than sandstone at the beginning of weathering. After about 400 years in the weathering rime. sandstone exceeds mudstone in the weathering rates.

The rates of decrease in mechanical properties (dRs/dt) with weathering rime (I) also differs between sandstone and mudstone. At the shallow zone, e.g. 3 cm and 10 cm deep, mudstone begins to be weathered earlier than sand- stone, but after the certain elapsed rime from the begin- ning of weathering, i.e. about 70 years for 3 cm deep and about 200 years for 10 cm deep, mudstonewill be excee- ded by sandstone in the races of decrease inRs• Ac the zo- ne deeper chan about 30 cm, however, sandstone starts co be weathered earlier and fascer than mudstone.

196

The impact of temperature record interval and sensor location on weathering inference in

periglacial environments

UniversilY of Northern BrilishColumbia.Geography Programme 3333 UniversilY Way. Prince Gc:orgc. B.C.. V2N 4Z9.Canada •

In many weathering srudies. particularly those in perigla.

cial regions, much emphasis is placed on the thermal Con.

ditions. The realityis that it is moisture, not temperarun:.

chat is the limiting facror. Nevertheless, with respecr to the ,thermal conditions, many deductions are based on air tem.

perarures which are. in reality, meaningless as a_surro~ate for rock conditions. TIUsuse of air temperarureshasresul- ted in subjective interpretations of weathering environ- ments/processes chat have served co reinforce. rather than question. che presumption of freeze-thau' weathering in periglacial environments. Further. almOSt all available rock temperarure data are inadequate for any meaningful de.

duction of process, particularly that of freeze-tha\\·.

Without information regarding the presence of water within the rock (including its amount. distribution and chemistry) together with that on the rate of fall of tempera- rure within the rock as well as the amplirudes of the freeze and thaw cycles, so it is impossible to assume the operation of freeze-thaw or to be able to deduce which freeze-tha\\' mechanism was active.

Detailed temperarure data, obtained at 30 second or one minute intervals, from recent srudies in Antarctica and the Canadian Rockies, show the imPOrtance of such high fre- quency data acquisition for the evaluation of weathering processes. Not only are such data necessary for the establi- shing of which freeze-thaw processisoperative(ifany) but they also show that mechanical processes other than that of freeze-thaw may be operative and, possibly, more im.

portant. Data analysis of freeze events allowed for the de- termination of the rate of fall of temperature and thus de- duction of possible freeze-thaw mechanism(s). More im.

portantly, the derailed data provide evidence for the opera- tion of thermal shock(at_~2 Comin-')as well as thermal Stress fatigue. The significant control of aspect on tempera- rure, a facror often ignored in weathering process interpre- tation, can also have an impacr on thermal stress fatigue.

There are major thermal differences between aspeccs_(~18 CO) and these not only have implications for process diffe- rentiation but also for the implementation of buttressing that can enhance the role of thermal strcss fatigue. It is suggested that in many periglacial environments processes other than freeze-thaw (e.g. thermal stress and/or wetting and drying) are more active and imPOrtant in sediment and landform development . Data to indicate the impacr of re- cord interval and aspect will be presented together with examples of its importance for process understanding. The need for these type of data to replace the qualitative pre- sumptions of cold (and other) region weathering will be emphasized..

The Impact of Temperature Record Interval and Sensor Location on Weathering Inference In Periglacial Environments.

KeVln Hall. Geography Programme. UnIVersity of Northern British ColumDla. 3333 University Way. Pnnce George. B.C.. Canada V2N 4Z9

Determination of weathering in penglacial environments is dependent (primarily) on rock properties, rock mOisture content and the thermal conditions to which the rock is subjected.

In/onnat/on presented here re/etessolelyto the thennal conditions.

For any realistic understanding of rock weathering it is necessary to monitor the actual rock temperatures (i.e. air temperatures arenota surrogate for rock temperatures)

Data shown here IndIcaterheinfluence of aspect and record Interval on the interpretatIon of the weathering environment.

and

The Canadian Rockies Alexander Island

(Antarctica)

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Background Information Panel2

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Influence of record interval

_ Panel 3= Influence of aspect Panel 4

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Influence of rate of change of temperature

( Deduction of weathering regime is often based on temperature information. For example, frequent crossings of zero degrees is perceived as conducivetofreeze-thaw weathering· often, though, without information on rock moisture contentl The perception of the thermal oscillations are frequentlybasedon temperature measurements at intervals of one hour or, at best. 15 minutes. Examples here show the difference in what the rock actually experiences as a function of the recortl interval ( ",'"'~"").

Aspect' also pleys a significant role. Not only are there mar1<ed aspect differences in weathering regime that can influence the formattion of landforms and/or sediments but lacketrecognition of aspect in placing of monitoring sensors can give a biased view of the weathering regime (Panel 3).

The rate of change of temperature is extremely important. Any deduction of the freeze·lhaw process is dependent upon the rate of change of temperalure. Equally, however, I the rate of change of temperature can. in its own right. cause weathering. thermal stress fatigue or thermal shock. The ability to determine thermal effects is greatly dependan

\.. on recortl interval. Lack of high freguencv data has produced a bias view of rock thermal conditions in cold regions ( Panei 4).

" Based on data collected at 30 second intervals, it was possible to generate a series of graphs to depict what would have been found had the recortl intervals been 1,5,10, 15.2030 and 60 minutes respectively ( ). These graphs were generated for the north·facing and south-facing exposures of a rock outcrop in the Canadian Rockies for the same day and time period ( thus aspect differences are also evident). Anhough these are summer data (and hence not applicable to freeze-lhaw) the impact of recortl interval in showing an ever-increasing number 01 thermal oscillations is obvious. A mar1<ed difference can even be seen between the 5 minute recortl and the one minute recortl. Consider the different perceptions of weathering environmentgeneratedbyeech graph. A one minute recortl interval is considered optimal. This recortl interval '- is also deemed important (see Panel4)for determination of actual weathering processes. Itis argued that recortl intervals gretaer than 1 minute are of limned value.

Aspect also plays an important role (Panel 3). Whilst it is obvious that there are large thermal differences as a function of aspect, data are rarely provided to show this and sometimes a single orientation is used to depict a weathering regime. Data from Antarctica are presented 10 indicate the role of aspect inciuding, based on this summer information, how the 24 hours of daylight in high latitudes can impact on what would normally be the shadow side of outcrops.

r The rate of change of temperature is extremely important for deducing the treeze·thaw process and for consideration of thermal stress fatigue or shock. Unless data are collected at one minute intervals it is not possible to evaluate this (Panel 4). As water was not present during these studies and these are summer data no consideration was given to freeze-thaw. Rather. it is the role of thermal stresses in cold environments that were considered the significant factor. Various studies have indicated that a rate of change of temperature greater or equal to 2 degrees C per minute is the threshold for thermal shock. Rates if change lower than this threshold can still induce fatigue, particularly if changes are frequent and of large amplitUde. The graphs, from both Canada and Antarctica, show examples of thermal events that meet or exceed the threshold '- as well as vanous examples of large amplitude variability. Without data at one minute intervals this role of thermal stress/shock would not have been evident.

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Data from Alexander Island (Antarctica)

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1640 hrs 26 January to 1632 hrs 27 January 199 Data at 1 minute intelVals

DETAILS FROM CANADIAN STUDIES

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DETAILS FROM ANTARCTIC STUDIES

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Permafrost Periglac. Process.9: 47-55 (1998)