Prior to 1965 geologists assumed that the two giant rock plates meeting at the San Andreas Fault generate heat through friction as they grind past each other, but in 1965 Henyey found that temperatures in drill holes near the fault were not as elevated as had been expected. Some geologists wondered whether the absence of friction-generated heat could be explained by the kinds of rock composing the fault. Geologists' pre-1965 assumptions concerning heat generated in the fault were based on calculations about common varieties of rocks, such as limestone and granite; but "weaker" materials, such as clays, had already been identified in samples retrieved from the fault zone. Under normal conditions, rocks composed of clay produce far less friction than do other rock types.

In 1992 Byerlee tested whether these materials would produce friction 10 to 15 kilometers below the Earth's surface. Byerlee found that when clay samples were subjected to the thousands of atmospheres of pressure they would encounter deep inside the Earth, they produced as much friction as was produced by other rock types. The harder rocks push against each other, the hotter they become; in other words, pressure itself, not only the rocks' properties, affects frictional heating. Geologists therefore wondered whether the friction between the plates was being reduced by pockets of pressurized water within the fault that push the plates away from each other.

The passage mostly agree that Heney's findings about temperature in the San Andreas Fault made the greatest contribution in that they

revealed an error in previous measurements of temperature in the San Andreas Fault zone

indicated the types of clay present in the rocks that form the San Andreas Fault

established the superiority of a particular technique for evaluating data concerning friction in the San Andreas Fault

suggested that geologists had inaccurately assumed that giant rock plates that meet at the San Andreas Fault generate heat through friction

confirmed geologists' assumptions about the amount of friction generated by common varieties of rocks, such as limestone and granite


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