Geology Reference
In-Depth Information
4.5.4. Discussion and Perspective
It is important to understand why coherent rock disintegrates under cold-climate condi-
tions, yet the preceding discussion presents at least two different mechanisms. To provide
a degree of closure, let us consider some simple fi eld observations undertaken on the
cracking of rocks and the inferences that can be made.
Table 4.4 summarizes the statistics associated with 200 glacial erratic pebbles that were
monitored by J. R. Mackay for signs of mechanical disintegration over a 19-year period
in the western Canadian Arctic (Mackay, 1999). All were exposed on the fl oor of a
recently-drained lake basin. The climate of the area is cold and relatively damp, being
located close to the Arctic coast (see Table 2.1 for Inuvik and Tuktoyaktuk, and Table 3.1
for freezing and thawing degree-days at Tuktoyaktuk). Prior to the period of observation,
many of the erratics had undergone thousands of freeze-thaw cycles when the rocks were
either exposed at the ground surface or buried in the active layer. Thus, the fi rst inference
is that rock does not experience regular disintegration on an annual basis because all of
the pebbles at the beginning of the observation period were unaltered. Second, during the
period of observation (1973-1994), only 10 rocks (i.e., 5%) were observed to experience
sub-aerial weathering. Therefore, cold-climate rock shattering is not an especially active
process. Third, Mackay identifi es three possible mechanisms: (i) traditional freeze-thaw
volume expansion (“hydro-fracturing”), (ii) ice segregation (the Hallet model), and (iii)
thermal stress. If ice segregation were the mechanism, fracture planes should be observed
as sub-parallel to the rock surface, i.e. the cooling surface. When the specifi c rocks were
examined, and using this criterion, a fi ne-grained granite pebble that had fractured indi-
cated hydro-fracturing or thermal stress rather than ice segregation as the cause (see
Mackay, 1999, fi gure 3). Likewise, a fi ne-grained granite pebble also indicated hydro-
fracturing or thermal stress as the cause (see Mackay, 1999, fi gure 4a, b). By contrast, a
coarse-grained granite pebble suggested ice segregation or hydro-fracturing rather than
thermal stress (see Mackay, 1999, fi gure 4c, d). Finally, three dolomite pebbles all cracked
along foliations approximately parallel to the ground surface, suggesting ice segregation
as the cause. In addition, eight rocks, four of which were dolomite, showed evidence of
what Mackay termed “explosive shattering,” and what others have termed “frost-bursting”
(Michaud et al., 1989), because rock fragments were scattered in the immediate vicinity.
Whether this explosive shattering was the result of ice segregation, hydro-fracturing,
or thermal shock is unknown. However, Mackay cites anecdotal evidence given by A. L.
Washburn (1969, pp. 32-33) of an occasion in mid-February 1933 at Letty Harbour
Table 4.4.
Summary of observations made by J. R. Mackay between 1974 and 1993 upon the
shattering of rocks on the fl oor of a drained lake, Mackenzie Delta region, Canada.
Rocks:
Igneous
Sedimentary
Metamorphic
Total
(Sandstones
dolomites)
Initial number (1974)
151
28
21
200
Final number (1993)
136
24
6
166
No. that shattered:
2
1 (sandstone)
1
10
6 (dolomite)
Source: Mackay (1999). Reproduced by permission of John Wiley & Sons Ltd.
