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For centuries, earth scientists have known where the major earthquakes occurred. They also knew they occurred in the same locales as the most violent volcanoes, a fact that led to centuries of arguments as to which was the cause of the other. Now we know that they are not cause and effect; they are both associated with the convergent plate margins. During the mid-1900s, another major zone of both volcanic and earthquake activity was discovered, namely the divergent margins, the most important site being the oceanic ridges. Since then, we have also come to understand the occurrence of volcanic activity within the plates as being located over hot spots beneath the plates. All of this new knowledge has been the result of the theory of plate tectonic. In our discussions of volcanism, we learned that the observed difference in volcanic activity between convergent and divergent plates is due to the type of magma involved. We must now explain why the earthquakes associated with convergent plate margins are of much higher magnitude than those associated with divergent plate margins. For this we must review our discussion of stress and strain.