Shadows from the Core

Š1995 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?

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?

1) studying rocks and other material on the surface
2) exploring natural holes, canyons or caverns
3) drilling our own holes
4) studying lava or other material that comes up from underground
5) taking X-ray pictures of the earth
6) studying meteorites, planets or other space objects
All of the methods listed above may help us a little. But if you think of the earth as an apple, then the deepest holes we have drilled or explored are only tiny scratches on the apple peel. We also know that lava, water, oil, and other material underground are not deep enough compared to the whole apple. And X-rays cannot travel through the solid rock of the earth. So which method is the most helpful?
The best method for studying the interior of the earth is the X-ray method -- if we replace the X-rays with a different type of ray or wave that can penetrate solid rock. Can you name some waves that are good at traveling through rock? You guessed it -- waves from an earthquake!

What can seismic waves tell you about the interior of the earth? If you can put the clues together, you are about to find out. The clues come from the activity Can You Read a Quake?, which showed you seismograms from eight cities around the world. Since all eight Seismograms came from only one earthquake, you can use them as clues to the interior of the earth. Here is how:

Notice that only 4 cities in the table received all three types of waves. In the other four cities, some of the waves disappeared. Why? What happened deep in the earth? Here are some clues:
Clue1. Remember that P waves are pressure waves (push-pull) and S waves are shear waves (side-to-side). Both types of waves travel well in solid rock. In fact, they travel faster when rock is more rigid, like vibrations on a stiff spring. But when they hit liquid/melted material, P waves slow down significantly, and S waves die out completely.
Clue 2. This is how P and S waves would travel if all 8 cities received them:
Clue 3. The diagram above did not actually occur. Some waves disappeared. Here is what really happened:
Do the diagrams above tell you anything about the interior? Do you want more information? There is another major clue: Wave reflections or echoes.
If a P wave tries to dive straight through the center of the earth, some of the wave energy is able to penetrate the center and arrive on the opposite side of the globe. However, not all of it makes it through. Some of the wave energy bounces backwards before it gets to the center, like a sound wave that echoes off of a wall. By studying these P-wave echoes, geologists think that the P waves are reflecting off of several different layers inside the earth that act like walls. What would make the waves bounce back? This is the next major clue:
Clue 4. This diagram shows that the four largest echoes seem to happen at depths of about 30 km, 150 km, 400 km, 2900 km, 4600 km.

Your Turn to Draw the Earth...
Now it is time to put the clues together If you sliced the earth open like an apple, what would it look like? Draw the following diagram and answer the questions (ask your instructor if you will do this as homework):
A. Draw a large copy of the simple diagram shown below. On this diagram draw the layers inside the earth at the correct depth levels. Should you draw straight, flat layers like stacked pancakes? or large wedges like the inside of an orange? or round (spherical) layers like an onion? Use clue #3 to decide how deep and how thick to draw each layer. You should have at least 5 different layers, some thick and some thin.
B. Label the top thin layer on the outside the CRUST.
C. Label the layer at the very center the INNER CORE.
D. Show that at least one of the layers near the center is liquid (melted). Label this melted layer the OUTER CORE.
E. You should have two layers between the CRUST and the OUTER CORE. These two layers are named UPPER MANTLE (next to the CRUST) and LOWER MANTLE (next to the OUTER CORE).
F. Which layer is the thickest? Which layer is the thinnest?

Return to the main lesson ("Earthquake!").

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Shadows from the Core

Š1995 The Regents of the University of California

Your Turn to Draw the Earth...