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Determining Planet Layers from Seismic Waves

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Earth's Composition

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Lesson Focus

Determining Planet Layers from Seismic Waves

Earth & Space Science

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Students use P and S waves to find the number and type of layers for several planets, ending with Earth.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that primary (P) waves from earthquakes travel through both solids and liquids, and that they travel faster through solids than liquids.
  • Explain that secondary (S) waves from earthquakes travel only through solid material.
  • Use primary (P) waves and secondary (S) waves to determine the number of layers in a planet, and whether they are solid or liquid.

Everything You'll Have Covered

Seismic waves are waves that move through the Earth, usually as the result of an earthquake. Seismic waves are measured and recorded by seismometers (or seismographs) and are studied by seismologists, geologists, and other scientists. There are two types of seismic waves: body waves and surface waves. This Activity Object focuses on how body waves called primary (P) waves and secondary (S) waves help us to understand the interior of the Earth.

P waves alternately compress and dilate the material through which they travel in the direction of propagation, and can travel through both solids and liquids. In solids, P waves travel almost twice as fast as S waves. In air, P waves take the form of sound waves, so they travel at the speed of sound. The speed of sound is approximately 330 m/s in air, 1450 m/s in water, and 5000 m/s in granite.

S waves displace the material through which they travel perpendicular to the direction of propagation, and travel only through solids. Their speed through a given material is about 40 percent less than that of P waves, so S waves arrive after P waves at a seismic station. S waves are larger in amplitude than P waves and are more destructive during an earthquake.

People have never journeyed to the center of the Earth, except in science fiction, but we know what the interior of the Earth is like. Similar to the way ultrasound waves help us to visualize a baby in utero, primary (P) and secondary (S) seismic waves help us to visualize the interior of the Earth as they travel through the layers. We live and walk on the crust of the Earth. This solid layer is thinner beneath the oceans (7 km) than beneath the continents (30-50 km).

The next layer is called the mantle, and is 2885 km thick. The boundary between the crust and the mantle is called the mohorovicic discontinuity, or the moho. It lies almost entirely within the lithosphere. However, beneath mid-ocean ridges, the moho defines the lithosphere-asthenosphere boundary. The mantle has two layers, the upper mantle, which is 670 km thick, and the lower mantle, which is 2215 km thick. This layer is mostly solid, except for the asthenosphere, which is 70-200 km thick. It contains rocks that are almost at their melting point and flow slowly under pressure. The uppermost part of the mantle, along with the crust, comprises the lithosphere, which is where earthquakes occur.

Below the mantle is the layer called the core, which is comprised of metal. The outer core is a liquid layer, while the inner core is a solid layer. The pressure, density, and temperature increase going from the outermost layer of the Earth to the innermost layer. This means the temperature is lowest at the crust and highest at the inner core. Temperatures within each layer increase with depth. Temperatures in the crust average about 200C-400C. Temperatures in the mantle average about 1000C-3700C. Temperatures in the core average about 3700C-4300C.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with the concepts of earthquakes, reflection, refraction, and seismic waves.
Course Earth & Space Science
Type of Tutorial Concept Development
Key Vocabulary asthenosphere, core, crust