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