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.[back]
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