A compass works because Earth has a magnetic field, as if a giant bar magnet lay along its axis. Seismic waves showed the outer core is liquid iron. Here we see how that field is made, how rocks record it, and why the record turned out to be some of the strongest evidence for moving plates.
Why it works
Iron loses its permanent magnetism when it is hot: above about $580^\circ\text{C}$ (the Curie temperature) magnetite can't hold a direction. The core is thousands of degrees hotter, so it cannot be a permanent magnet. Instead, liquid iron churns in the outer core. Moving electric conductors in a magnetic field make electric currents, and those currents make the field: a self-sustaining geodynamo. A dynamo can drift and even flip, and the record shows it does.
- Recording. As lava cools through about $580^\circ\text{C}$, tiny magnetite grains line up with the field and freeze in place: a compass needle in stone.
- Reversals. Lava flows of different ages point two opposite ways. The whole field has flipped many times at irregular intervals; the last flip was about 0.77 million years ago.
- Inclination. Field lines are flat at the equator and point steeply down near the magnetic poles. For a dipole field, $\tan I = 2\tan\lambda$, so the dip $I$ frozen in a rock tells the latitude $\lambda$ where it formed (tangent again).
Key idea
Earth's field is made by moving liquid iron in the outer core, not a solid magnet. Cooling lava records the field's direction and dip, so rocks keep a timeline of reversals and a record of the latitude where they formed.
Worked examples
Worked example: Latitude from a rock
Old lava records an inclination of $49^\circ$. At what latitude did it cool?
- $\tan\lambda = \dfrac{\tan 49^\circ}{2} = \dfrac{1.150}{2} = 0.575$.
- $\lambda = \tan^{-1}(0.575) \approx 30^\circ$.
- If the same rock now sits at $55^\circ$ N, its plate has carried it about $25^\circ$ north, roughly $25 \times 111 \approx 2,800\,\text{km}$.
Answer: About $30^\circ$ from the equator.
Worked example: How deep can rock stay magnetic?
With a surface temperature of $15^\circ\text{C}$ and a gradient of $25^\circ\text{C/km}$, how deep does the crust reach $580^\circ\text{C}$?
- Warming needed: $580 - 15 = 565^\circ\text{C}$.
- Depth: $565 \div 25 \approx 23\,\text{km}$.
- Below a few tens of kilometers, no rock can hold permanent magnetism, which is why the field must come from flowing metal.
Answer: About $23\,\text{km}$.
Common mistake
Picturing a giant bar magnet of solid iron in the core is a natural guess, and the field's shape does look like one. But a hot magnet loses its magnetism, and a permanent magnet could not flip back and forth. Check: the core is above about $4,000^\circ\text{C}$, far past iron's Curie temperature of about $770^\circ\text{C}$.