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Earth & Space Science: Session 3

A Closer Look: Mapping Earth's Interior

How do we know the nature of Earth's interior structure?

Much of what we know about Earth's interior comes from seismic waves. Seismic waves are waves of energy that can be caused by earthquakes. The two main types of seismic waves are body waves and surface waves. Body waves travel through the Earth’s interior in all directions. Surface waves travel only along the surface of the Earth, like ripples on water. It is the behavior of body waves that gives us clues about the nature of Earth's interior. There are two types of body waves: primary waves (P waves) and secondary waves (S waves).

What are P and S waves?

p and s waves
P and S waves.

P waves stand for “primary waves.” They’re considered to be primary because they travel faster than S waves and, after any given earthquake, will reach a seismic recording station first. In the video, two children simulate P waves by holding opposite ends of a Slinky on the floor. One child pushes the end of the Slinky towards the other child. As a wave moves down the Slinky, the coils can be seen to push forward and compress, then pull back and open up again. This simulates the action of P waves. P waves are compressional waves that exert a force in the direction that the wave travels. These waves push through rock in the same way that sound waves push though air.

S waves stand for “secondary waves.” In the video, two children hold opposite ends of a Slinky on the floor and one child moves the Slinky from side to side. In this case, as the wave moves down the Slinky, the coils can be seen to shake side to side, elastically springing back. S waves are shear waves that exert a force perpendicular to the direction that the wave travels.

What are the differences between P and S waves?

Scientists have learned about Earth’s internal structure by studying how these waves travel through the Earth. The technique is straightforward — it involves measuring the time it takes for both types of waves to reach seismic stations from the epicenter of an earthquake. Since P waves travel faster than S waves, they’re always detected first. The farther away from the epicenter, the larger the time interval between the arrival of P and S waves — and if the Earth were built of a uniform substance, that would be the only variation measured.

Scientists, however, noticed variations that could not be accounted for based simply on the distance traveled from the epicenter. For instance, they noticed places in the Earth through which S waves didn’t travel. Geologists inferred that these sections of the Earth were liquid, through which S waves (which, remember, are shear waves) cannot travel. You may not know it, but you are probably already familiar with this phenomenon. In a bathtub, if you submerge your arm underwater and push your hand straight out from your body, you can see a wave arrive as it hits the edge of the tub. Consider this an example of a P wave. If you then move your hand side to side in the water, you should notice that the wave does not hit the edge of the tub in front of you. Consider this to represent an S wave. What happened to it?

Solids and liquids both transmit P waves because their particles transfer energy in the direction of the wave as they compress and elastically spring back along its length. As with the child at the end of the Slinky, the seismograph at the end of a P wave detects a “push” — the energy of this action. Although S waves don’t compress, they still travel through solids because the particles in solids elastically spring back even when moved only from side to side. This is not a property of liquids. In liquids, the energy of an S wave simply dissipates. So technically, the second Slinky described above represented an S wave travelling through a solid.

What were scientists able to learn from P and S waves?

earth cut out
Simulation of an Earth cross section.

The absence of S waves in certain places along with an understanding of S wave behavior in solids and liquids led scientists to conclude that the outer core is liquid and effectively absorbs S waves. As the number of seismic readings increased along with their precision, a worldwide community of scientists uncovered patterns that indicated a much more complicated picture of the Earth’s interior than was previously believed. Scientists have been able to distinguish the layers of the Earth that are made of different materials that transmit waves at different speeds. Based on these seismic observations, the Earth’s interior has been divided into the following layers:

  • Crust: A very thin, solid outer layer. The oceanic crust is about 5 km (3 miles) thick. The continental crust is from 30–40 km (18–24 miles) thick.
  • Moho: The boundary between the crust and the mantle.
  • Mantle: The layer beneath the crust. The mantle is about 2885 km (1790 miles) thick.
  • Upper mantle: Includes a solid layer fused to the crust. This layer combined with the crust is called the lithosphere. Beneath this is the asthenosphere, which is a partly molten layer. The asthenosphere is thought to be the layer upon which tectonic plates ride. The upper mantle is about 700 km (420 miles) thick.
  • Lower mantle: Is composed of solid rock under conditions of extremely high temperature and pressure. This layer is about 2,185 km (1,370 miles) thick.
  • Outer Core: A layer about 2,270 km (1,400 miles) thick, having the properties of a metallic liquid.
  • Inner Core: A solid, metallic, spherical layer about 1,216 km (755 miles) thick.
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