Chapter 3. Earth’s Interior

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Explain the variations in the composition and characteristics of Earth’s different layers
  • Compare the characteristics and behaviour of the two types of seismic body waves
  • Summarize the variations in seismic-wave velocity as a function of rock type and temperature and pressure conditions
  • Explain some of the ways that seismic data can be used to understand planetary interiors
  • Describe the temperature variations within Earth and their implications for internal processes such as mantle convection
  • Explain the origins of Earth’s magnetic field and the timing of magnetic field reversals
  • Describe the isostatic relationship between the crust and the mantle, and the implications of that relationship for geological processes on Earth

In order to understand how Earth works, and especially the mechanisms of plate tectonics, we need to know something about the inside of our planet — what it’s made of, and what goes on in there. We have a variety of ways of knowing, and these will be discussed in this chapter, but the one thing we can’t do is go down and look! Fortunately there are a few places where mantle rock is exposed on Earth’s surface, and we have some samples of material from the insides of other planetary bodies, in the form of meteorites that have landed on Earth (Figure 3.1). We also have a great deal of seismic information that can help us understand the nature of Earth’s interior.

Left: a fragment of the Tagish Lake meteorite, discovered in 2000 on the ice of Tagish Lake, B.C. It is a stony meteorite that is dominated by mafic silicate minerals, and is similar in composition to Earth’s mantle. Right: part of the Elbogen meteorite discovered in Germany around 1400 CE. It is an iron meteorite, similar in composition to Earth’s core. Both samples are a few centimetres across. [left from: http://www.nasa.gov/centers/goddard/images/content/557996main_tagish-lake-meteorite.jpg] right from: http://upload.wikimedia.org/wikipedia/commons/d/dc/Elbogen_meteorite%2C_8.9g.jpg]

Figure 3.1 Left: a fragment of the Tagish Lake meteorite, discovered in 2000 on the ice of Tagish Lake, B.C. It is a stony meteorite that is dominated by mafic silicate minerals, and is similar in composition to Earth’s mantle. Right: part of the Elbogen meteorite discovered in Germany around 1400 CE. It is an iron meteorite, similar in composition to Earth’s core. Both samples are a few centimetres across. [left from: http://www.nasa.gov/centers/goddard/images/content/557996main_tagish-lake-meteorite.jpg] right from: http://upload.wikimedia.org/wikipedia/commons/d/dc/Elbogen_meteorite%2C_8.9g.jpg]

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