The key to the world of geology is change; nothing remains the same. Life has evolved from simple combinations of molecules in the sea to complex combinations in man. The land, too, is changing, and earthquakes are daily reminders of this. Earthquakes result when movements in the earth twist rocks until they break. Sometimes this is accompanied by visible shifts of the ground surface; often the shifts cannot be seen, but they are there; and everywhere can be found scars of earlier breaks once deeply buried. Today's earthquakes are most numerous in belts where the earth's restlessness is presently concentrated, but scars of the past show that there is no part of the earth that has not had them.

The effects of earthquakes on civilization have been widely publicized, even overemphasized. The role of an earthquake in starting the destruction of whole cities is tremendously frightening, but fire may actually be the principal agent in a particular disaster. Superstition has often blended with fact to color reports.

We have learned from earthquakes much of what we now know about the earth's interior, for they send waves through the earth which emerge with information about the materials through which they have traveled. These waves have shown that 1800 miles below the surface a liquid core begins, and that it, in turn, has a solid inner core.

Earthquakes originate as far as 400 miles below the surface, but they do not occur at greater depths. Two unsolved mysteries are based on these facts. (1) As far down as 400 miles below the surface the material should be hot enough to be plastic and adjust itself to twisting forces by sluggish flow rather than by breaking, as rigid surface rocks do. (2) If earthquakes do occur at such depths, why not deeper?