Conversations with Professor Prestwich #6
WHEN ROCKS BREAK

Dear Geoneophytes,

Most rocks have been broken during some stage of their histories: like eggshells they can be, perhaps surprisingly, brittle. Joints are the most common evidences of this brittle behavior; they record gentle flexing and pulling on the rocks by relatively modest forces. Faults record much more substantial stresses; I have appended two examples of normal faults from a mining district in northern England. Normal faults (recording extension of the earth’s crust) are one of three principal types of fault, the others are reverse (recording compression) and strike-slip (recording shear stress).

The movement of faults is the chief cause of earthquakes. Fortunately few faults are on the move in Great Britain, although I understand that it is different in the USA.

Cordially, perhaps apprehensively,

Joseph Prestwich