94 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1966 
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It is perhaps less evident that chemical weathering should be important but 
in fact determination of the net weathering response of various rock types 
depends upon assessment of the balance existing between chemical and ther 
mal effects. 
To comprehend the effect of thermal disintegration on rocks of differing 
composition consideration must be given to the process in general. The ther 
mal disruption of rocks is generally attributed to differential volume change of 
outer layers relative to a more stable inner layer. To this process the separation 
of sheets or layers many feet thick has been ascribed. This requires the devel 
opment of steep temperature gradients at comparable depths within the rock. 
In recent work in the Aden Protectorate a Remote Reading Differential 
Thermometer was used to measure simultaneously rock surface and interior 
temperatures, by means of pencil-thin thermistor probes. Experiments showed 
that at 3 1 /2 inches depth within the rock the maximum diurnal temperature 
was reached two hours after the rock surface attained its maximum tempera 
ture. Measurements on a number of rocks indicate a similar time lag, in the 
order of 4 hours at 6 inches penetration, and of about 10 hours at 18 inches 
penetration. Heat flow is influenced by several factors such as lithological 
characteristics and local climate conditions, but these figures illustrate the 
general rate of heat transference. 
Experiments made also show that the magnitude of diurnal temperature 
variation decreases sharply with increasing depth of penetration. Thus, at 3 1 ¡2 
inches penetration, temperature variation approximated to within 2-3 °C of 
surface changes; where surface variations ranged between 20 to 30 °G, tem 
perature variation at 6 inches penetration averaged about 5 °C; at 18 inches 
Fig. 1. Diurnal temperature variation in grey bedded limestone 
Wadi Hajar, Aden Protectorate. January 
Temperature of rock surface 
Temperature at 13 inches penetration