Geology is the science of the study of the structures of the earth and the processes that affect those structures. Scientists who study the earth are known as ¡§geologists.¡¨ One way geologists learn about the inner part of the earth is by studying the way earthquake waves travel through the various layers of the earth.

The outer layer of the earth is the crust ¡V a layer of solid rock that covers the entire surface of the earth. The thickness of the crust ranges from about 5 miles beneath the oceans to roughly 25 miles beneath the continents. Compared with the other layers, the crust is very rigid, thin, and brittle. We know more about the crust than the other layers because we can see it.

In 1909, a Yugoslavian seismologist (i.e., a scientist who studies earthquakes) named Andrija Mohorovicic discovered that earthquake waves change speed after they pass through...