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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 28:
Inside the Earth

Key Ideas:

Interior Structure of the Earth:

Earth's Magnetic Field

Crust is broken into tectonic plates:

Surface of the Earth

The Earth's surface is

Surface rocks are primarily silicates.

Surface layers have been subjected to

Interior of the Earth

The Earth's interior is hot and dense: It got this way through the process of Differentiation: An important process seen in rocky bodies throughout the Solar System.

Journey to the Center of the Earth

Solid Inner Core: (5100-6370 km deep)

Molten Outer Core: (2900-5100 km deep)

Mantle: (100 to 2900 km deep)

The thin solid Crust floats on the Mantle. Only ~100 km thick.

Earth's Magnetic Field

Convection currents get setup in the molten outer core because of the bottom-to-top difference in temperature: This setups the Geo-Dynamo: The Earth's Magnetic Field extends beyond the surface into a Magnetosphere that interacts with the solar wind.


How do we know this is what the Earth looks like inside?

Different kinds of seismic waves are caused by Earthquakes in the crust:

In addition, there are surface waves that roll along the surface.

Seismic Waves
[Adapted from figure 2 in Don L. Anderson's article Planet Earth in The New Solar System, 4th ed., Beatty, Petersen, & Chaikin, eds (1999, Cambridge University Press). This is a "still" of the animated PowerPoint graphic I used in lecture. Click on image for full-size version (11Kb GIF)]

Seismologists use P- and S-waves from earthquakes to map the interior of the Earth like a doctor uses MRI or ultrasound to map the inside of a person.

The Crust of the Earth

The Earth's crust is broken into 16 rigid plates

These plates float on the Mantle above a complex transition zone:

This allows the plates to slide around.

Plate Tectonics

The crustal plates slide around on top of the Mantle:

Plate Motions:

Continental Drift:

Transform Boundary

Where two plates are sliding past each other

These boundaries form Transverse Faults:

Convergent Boundary

Where two plates are colliding together.

The collision results in two processes:


[Note: the powerful earthquake in Indonesia that triggered the Indian Ocean tsunami in December 2004 was in one of these deep subduction zones]

Crust Buckling:

Divergent Boundary

Where two plates are moving apart.

Mid-Atlantic Ridge:

Sea-floor spreading was the first hard evidence of tectonic motions.

Hot Spots

Locations in the middle of plates: As the plate slides over the fixed hot-spot, get long chains of shield volcanoes.

The Dynamic Earth

The Earth is a dynamic, actively evolving planet. It is active today because the Earth's interior is still hot and molten.

We will use the Earth as the basis of comparison when we look at other rocky bodies in the Solar System.


My geologist colleagues will likely be most upset with me for having compressed their life's work into one brief lecture. The subject is rich and fascinating, but what I want to emphasize here are the basic processes at work in the interior of our planet. When we consider the other terrestrial planets in the Solar System, we will return to these ideas, and see how they are similar and different from the Earth.

If you would like to learn more about this fascinating subject, you can start with this excellent hypertext summary of plate tectonics, This Dynamic Earth by W. Jacquelyne Kious and Robert Tilling of the US Geologic Survey.

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Updated: 2007 October 27
Copyright Richard W. Pogge, All Rights Reserved.