Geology Reference
In-Depth Information
(B)
ml/g
1.015
1.5
1.0
1.010
(A)
0.5
1.005
30
granite
1.000
25
0.0
20
0.5
0.995
15
10
1.0
0.990
5
1.5
0.985
0
10
-7
100
200
300
400
500
600
700
¥
-10
0
10
20
30
40
50
60
70
'Spalling tendency'
index
Temperature (°C)
Figure
4.8.
The nature of insolation (spalling) weathering. (A) Graph showing a theoretical
spalling-tendency index (
x
axis) versus number of quenches (
y
axis). See Rice (1976, p. 61) for details
of the spalling-tendency index. (B) Temperature-volume-pressure diagram for water (from Inter-
national Critical Tables) superimposed on a plot of the average temperature-induced expansion of
granite. From Winkler (1977).
coeffi cient, and the modulus of elasticity. Therefore, weathering by thermal shock demands
two requirements. First, the rock must experience temperature contrasts. However, the
magnitude of the contrast, the time over which the contrast occurs, and the lag between
the time of maximum surface temperature and that of the maximum at depth within the
rock, must also be considered. Second, the rock must have pores of a capillary size such
that entrapped moisture cannot escape when water expands during the heating cycle.
Here, micro-crack porosity appears crucial.
Several laboratory studies suggest that the threshold value for thermal shock approxi-
mates to a rate of temperature change of 2 °C/min (Richter and Simmons, 1974; Yatsu,
1988).
It is signifi cant that most weathering phenomena attributed to thermal shock have been
observed in arid environments, irrespective of whether these are hot or cold. For example,
V. N. Konishchev and V. V. Rogov (1993) observe that an engineering study in Tashkent,
Tadjikstan, reported numerous micro-cracks in granite of heterogeneous mineralogical
composition that were the result of sharp temperature changes and large temperature
amplitudes. Furthermore, it appears that there are many similarities in the rock-thermal
regimes of hot and cold regions where aridity is a central characteristic. For example, in
a quartz-monzonite boulder in the Mohave Desert, California, the maximum rock surface
temperature over an 8-hour period ranged between 48 °C and 23 °C (Roth, 1965). In
Antarctica, Russian scientists have measured daily temperature ranges of 40-42 °C in
Victoria Land and monitored rates of heating and cooling of 0.8 °C/min and 15-20 °C/h
on the ice-free nunataks of the Queen Maud Mountains (see Bardin et al., 1965, and
