WORKING GROUP 2
GREENWOOD
95
depth about 2.25 °C; and at 30 inches the variation measured on one rock
was 1 °C. (Figs. 1 and 2).
I hese results confirm that steep temperature gradients are restricted to near
surface levels and that thermal disruption of flakes by differential volume
change of rock layers must be limited to a matter of a few inches.
Fig. 2. Diurnal temperature varation in grey quartz-biotite-feldspar metasediment
Wadi Gharish, Aden Protectorate. January
Temperature of rock surface
Temperature at 3 1 / 2 inches penetration
It is suggested that the more profound effects of thermal changes arise from
the disorganisation of the rock fabric by repeated differential volume change
of component rock minerals, a process in which more penetrative periodic and
seasonal temperature changes play a part. The stress developed in an assem
blage of minerals having differing thermal properties is cumulative, due to
fatigue and because the minerals do not regain original dimension after heating.
Rocks are affected to depths mainly controlled by their thermal conductivity.
Rock porosity is increased and chemical weathering is facilitated throughout
the zone of derangement. Weathering solutions become concentrated above the
plane limiting effective disorganisation and at this level volume changes of
chemical origin - mainly hydration - operate to optimum effect against the
resistance of fresh confining rock. Layers of considerable thickness may be
disrupted by combined mechanical and chemical weathering, whilst piecemeal
disintegration can also occur. Surface flaking or spalling is less important in
rock reduction than more penetrative fabric disorganisation.
The following factors influence the response of different rocks to thermal
disintegration: thermal conductivity and expansion coefficient of rocks and
component minerals; rock texture and degree of heterogeneity of component
minerals; porosity; rock structure; surface colour and heat capacity.
Brief comment on certain of these factors must include the fact that low con
