Seafloor Magnetism

Did you know:
The center of Earth is too hot to support an electric field. But, movement of molten rock creates the magnetic field in the outer core. Heavy elements like iron make the journey from the mantle to the crust via volcanic activity.

In most elements, electrons are paired and each electron of the pair spins in an opposite direction. Metals are different. All of their electron pairs spin in the same direction. This creates the magnetic poles of metals like iron.

NOVA: When Our Magnetic Field Flips - Good interactive that shows a simulated reversal of our magnetic field. Scientists do not fully understand how the reversal occurs. This is their best hypothesis.

Magnetic ‘zebra stripes’:
Seafloor lavas have built-in magnetic “clocks” that reveal their age. When seafloor lava solidifies at the seafloor, its magnetic crystals are quenched in alignment with Earth’s magnetic field, and the rocks’ magnetic “polarity” is preserved. But Earth’s magnetic field has reversed many times over the planet’s history—with the magnetic north sometimes facing south, or vice versa, as it is today.
New seafloor is created at mid-ocean ridges (with the prevailing magnetic signature) and spreads out in both directions, creating a symmetrical “zebra-stripe” pattern of alternating rocks with either “reversed” or “normal” polarity.  (Woods Hole Oceanographic Institute)
This is one way that scientists can calculate the relative age of Earth's crust at divergent boundaries.