Scientists use telescopes and space probes to gather information about the planets and the stars. They use microscopes to examine unseen worlds of life on Earth. They use computers and other instruments to gather information about atoms, the building blocks of all matter. Most of the information scientists have gathered about the Earth's interior has come not only from complex instruments but also from earthquakes.

Earthquakes and Seismic Waves

Earthquakes are produced when a part of the Earth's uppermost layer moves suddenly. During an earthquake, the ground shakes and trembles. Sometimes the movement is so violent that buildings crash to the ground and roads and highways are destroyed. Earthquakes produce shock waves that travel through the Earth. These shock waves, which are actually waves of energy, are called seismic (SIGHz-mihk) waves. You can make a simple model to show how shock waves move. Fill a sink or basin half full with water and then drop a small pebble onto the center of the water's surface. You will observe waves that move outward from the pebble's point of impact in circles of ever-increasing size.

All earthquakes produce at least two types of seismic waves at the same time: P waves and S waves. These waves are detected and recorded by a special instrument called a seismograph (SIGHZ-Muh-graf). You will learn more about how a seismograph works and about seismic waves in Chapter 11. What you need to know now is that seismic waves penetrate the depths of the Earth and return to the surface. During this passage, the speed and direction of the waves change. The changes that occur in the movement of seismic waves are caused by differences in the structure and makeup of the Earth's interior.

Exactly how have P waves and S waves helped scientists develop a model of the Earth's inner structure? At a depth of 2900 kilometers below Earth's surface, P waves passing through the Earth slow down rapidly. S waves disappear. Scientists know that P waves do not move well through liquids and that S waves are stopped completely. So the changes in the movement of the two seismic waves at a depth of 2900 kilometers indicate something significant. Do you know what it is? You are right if you say that 2900 kilometers is the beginning of a liquid layer of the Earth. At a depth of 5150 kilometers, P waves increase their speed. This increase indicates that P waves are no longer traveling through a liquid layer. Instead, P waves are passing through a solid layer of the Earth.

After observing the speeds of P waves and S waves, scientists have concluded that the Earth's center, or core, is actually made up of two layers with different characteristics.

The Earth's Core

Both layers of the Earth's core are made of the elements iron and nickel. The solid, innermost layer is called the inner core. Here iron and nickel are under a great deal of pressure. The temperature of the inner core reaches 5000'C. Iron and nickel usually melt at this temperature. The enormous pressure at this depth, however, pushes the particles of iron and nickel so tightly together that the elements remain solid.

The radius, or distance from the center to the edge, of the inner core is about 1300 kilometers. The inner core begins at a depth of about 5150 kilometers below the Earth's surface. The presence of solid iron in the inner core may explain the existence of the magnetic fields around the Earth. Scientists think the iron produces an effect similar to the effect around a magnet-that is, a magnetic field. Have you ever experimented with iron filings and a bar magnet? If so, were you able to observe the pattern of the filings around the magnet? This pattern identifies the magnetic field. Perhaps your teacher can help you do this activity so that you can see a magnetic field for yourself.

Surrounding the inner core is the second layer of the Earth, called the outer core. The outer core begins about 2900 kilometers below the Earth's surface and is about 2250 kilometers thick. The outer core is also made of iron and nickel. In this layer, the temperature ranges from about 2200'C in the upper part to almost 5000'C near the inner core. The heat makes the iron and nickel in the outer core molten, or changed into a hot liquid.

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1. What-evidence has caused scientists to conclude that the Earth's core is made of two different layers?

2. Name two types of seismic waves. How are these waves the same? How are they different?

3. How are the inner and the outer cores of the Earth alike? How do they differ?

4. Predict what would happen to P waves and S waves if the Earth's outer core were solid and its inner core were liquid.