Model for Structure of Earth

It is easy to observe that the Earth has a magnetic field

This is ``easy'' to explain under the hypothesis that the Earth has a fluid, electrically conducting core that it heated from inside.

The heat from the inside is from radioactive decay. (Some evidence: the earth is warmer as you go down in deep mines.)

How can this heat get out from the middle to the surface. We need to investigate thermal energy transfer.

Our conclusion:

• A strongly heated fluid will undergo convective motion.

The final link: both physics theory (which is unfortunately rather complicated) and observation show that a moving, electrically conducting fluid can make a magnetic field.

Thus our hypothesis of a fluid iron core for the earth accords nicely with the observation that the earth has a magnetic field.

Simplicio: Well, if the magnetic field of the earth comes from fluid motions inside the earth, it should not be stable. The magnetic field should change!

Sagredo: Usually, your observations are not very bright, but that is a very good objection.

Salviati: You are both right. Actually there is good geological evidence that the magnetic field of the earth does change. In fact, it completely reverses direction from time to time (on a very long time scale compared to a human lifetime though). Thus the model is confirmed by observation.

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