ABSTRACT
In 2003, Mark Eberhardt, professor of chemistry and geochemistry at the Colorado School of Mines in the USA, published a book with the title “Why things break” (Eberhardt, 2003). In this book and with reference to the history of tool making, the author notes the antiquity of using shaped rock pieces as tools and that only certain types of rock, i.e. flint, obsidian and petrified wood, fractured to produce a sharp cutting edge. He also explains the distinction between bending (when planes of atoms slide across one another) and breaking (when the planes of atoms are pulled apart). He explains that the tendency to break or bend in a polycrystalline material is dependent on a dislocation’s ability to move across grain boundaries: when this movement is suppressed, the material fractures. As with metals, rock fracture begins at the tip of a pre-existing crack although, as we shall see, the initiation of such crack growth does not necessarily lead to total structural collapse of the rock volume in question.
