Shadows from the Core

1995 The Regents of the University of California
Can you solve the mystery of the disappearing waves?

To do this activity...
You will need some basic vocabulary, including an understanding of P and S waves. The activity Can You Read a Quake? is recommended. You will also find the activities Race of the Waves andWhere Did It Hit? helpful but not necessary.



Many years ago geologists noticed something strange about seismic waves that travel across the earth by going deep through the interior. Whenever an earthquake occurs, it sends out seismic waves that spread out in all directions through the earth until they arrive somewhere else on the surface. Some P and S waves do not dive too deeply; these waves travel no more than a few thousand kilometers before arriving safely back at the surface somewhere else. But other waves dive deeper towards the center of the earth and seem to disappear before they reach the other side. Why do they disappear?

The answer to this puzzle is found somewhere in the middle of the earth. What does the middle of the earth look like? If you could slice it open like an apple, what would you see? Rock? fire? lava? oil? water? Here are some diagrams of the earth that people have drawn throughout history. Which one do you think is most like the real earth?

Image of Earth Model Models of the Interior of the Earth How can you draw a correct picture of the earth if you have never seen the interior? Perhaps we can gather clues about the interior by the methods listed below. Which method do you think is most helpful?