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1. Plate tectonics again!
The theory of plate tectonics can be used to explain both earthquakes and volcanoes. According to this theory the outer, solid, part of the earth (the Lithosphere) is broken into large plates that move over a liquid interior. Both dense oceanic crust and lighter continental crust is attached to these plates and when they move, so do the continents. This is explained in greater detail in the previous article on earthquakes.The North American continent and the Pacific Ocean are moving past one another along a long boundary or fault. Most of California sits on the Pacific plate. When it moves, California goes with it. Movement at the plates often occurs suddenly and results in waves of energy passing through the crust resulting in earthquakes.
Another consequence of plate tectonics is volcanic activity - a broad term that refers to melted (molten) rock or magma is forced from the earth's interior to its surface. Just as a map of earthquake activity outlines the major plates in the lithosphere, the volcanic activity is concentrated at the margins of plates (with a few exceptions).
