Module 2

Earth’s Interior

A hole in rocky earth showing glowing lava

Figure 1. Lava Flow, Hawaii Volcano Observatory


Earth is a dynamic planet, but not everything happens on the surface! A lot of the exciting activity is also happening in Earth’s interior. Before we move into a discussion of Plate Tectonics (Module 3), we first need to learn something about the inside of the Earth. Unfortunately, geologists don’t have any way to see what’s going on in the interior, so they have to use other tools that can help visualize its inner workings.

Studying the Earth’s interior poses a significant challenge due to the lack of direct access. Many processes observed at the Earth’s surface are driven by the heat generated within the Earth, however, making an understanding of the interior essential. Volcanism, earthquakes, and many of the Earth’s surface features are a result of processes happening within the Earth.

Much of what we know regarding the Earth’s interior is through indirect means, such as using seismic data to determine Earth’s internal structure. Scientists discovered in the early 1900s that seismic waves generated by earthquakes could be used to help distinguish the properties of the Earth’s internal layers. The velocity of these waves (called primary and secondary waves, or P and S waves) changes based on the density of the materials they travel through. As a result, seismic waves do not travel through the Earth in straight lines, but rather get reflected and refracted, which indicates that the Earth is not homogeneous.

The Earth’s interior consists of an inner and outer core, the mantle, and the crust. Located in the center of the Earth is the inner core, which is very dense and under incredible pressure, and is thought to be composed of an iron and nickel alloy. It is solid, and surrounded by a region of liquid iron and nickel called the outer core. The outer core is thought to be responsible for the generation of the Earth’s magnetic field. A very large portion of the Earth’s volume is in the mantle, which surrounds the core. This layer is less dense than the core, and consists of a solid that can behave in a plastic (deformable) manner. The thin outer layer of the Earth is the crust. The two types, continental and oceanic crust, vary from each other in thickness, composition, and density.

Module Objectives

At the completion of this module you will be able to:

  1. Explain the variations in the composition and characteristics of Earth’s different layers
  2. Compare the characteristics, behavior, and velocities of the two types of seismic body waves, and how they can be used to interpret Earth’s interior.
  3. Describe temperature variations within the interior and how this relates to mantle convection.
  4. Explain the origins of Earth’s magnetic field.
  5. Describe the isostatic relationship between the crust and the mantle, and the implications of that relationship for geological processes on Earth.

Activities Overview

See the Schedule of Work for dates of availability and due dates.

Be sure to read through the directions for all of this module’s activities before getting started so that you can plan your time accordingly. You are expected to work on this course throughout the week.


Physical Geology by Steven Earle

  • Chapter 9 (Earth’s Interior)

Module 2 Assignment: Exploring Earth’s Layers and Seismic-wave Travel Times

10 points

After you complete the reading, you can start working on Module 2 Assignment – Exploring the Earth’s Layers and Seismic-wave Travel Times.

Module 2 Quiz

10 points

Module 2 Quiz has 10 multiple-choice questions and is based on the content of the Module 2 readings and Assignment 2.

The quiz is worth a total of 10 points (1 points per question). You will have only 10 minutes to complete the quiz, and you may take this quiz only once. Note: that is not enough time to look up the answers!

Make sure that you fully understand all of the concepts presented and study for this quiz as though it were going to be proctored in a classroom, or you will likely find yourself running out of time.

Keep track of the time, and be sure to look over your full quiz results after you have submitted it for a grade.

Your Questions and Concerns…

Please contact me if you have any questions or concerns.

General course questions: If your question is of a general nature such that other students would benefit from the answer, then go to the discussions area and post it as a question thread in the “General course questions” discussion area.

Personal questions: If your question is personal, (e.g. regarding my comments to you specifically), then send me an email from within this course.