Geology Reference
In-Depth Information
(a)
(b)
Figure 3.1.
Spalling of granite due to bush fires (a) near the Portugal-Galicia frontier line of Portela d'Home;
(b) at the Devil's Marbles, Northern Territory. Most fire-induced heating produces thin flakes and
slivers, only 2-3 mm thick, but here the plates spalled off are several centimetres in thickness.
But field observations suggest that, at the very least, moisture-related weathering acts far more
rapidly and widely than do insolational effects, and many of the features attributed to heating and
cooling must be due to other processes. Flaking and spalling demonstrably extend to depths of
many metres, far beyond the effect of diurnal or even secular temperature changes (
Fig. 3.2).
Again, in the Nile valley and in the adjacent deserts the surfaces of granite blocks exposed to the Sun's
rays (Barton, 1916) for a few thousands of years show no signs of alteration or disintegration,
