They don't as such. Evidence for magnetic reversals comes from the fact that some rocks are reversely magnetized and some are normally magnetized. Divergent boundaries, or to be more accurate, mid ocean ridges, just provide a setting where there is continual production of igneous rock that is capable of systematically recording the magnetic field reversals. That is to say, the divergent boundary produces the means of recording the evidence, not the evidence itself.
A divergent boundary lays down new rock. When molten any magnetic crystals in the lava orient with the global magnetic field. When it cools and freezes it can no longer change orientation.
Measure the orientation of rock moving away from the boundary and you find the rock shows reversals in the field present at the time it froze.
Answers & Comments
They don't as such. Evidence for magnetic reversals comes from the fact that some rocks are reversely magnetized and some are normally magnetized. Divergent boundaries, or to be more accurate, mid ocean ridges, just provide a setting where there is continual production of igneous rock that is capable of systematically recording the magnetic field reversals. That is to say, the divergent boundary produces the means of recording the evidence, not the evidence itself.
A divergent boundary lays down new rock. When molten any magnetic crystals in the lava orient with the global magnetic field. When it cools and freezes it can no longer change orientation.
Measure the orientation of rock moving away from the boundary and you find the rock shows reversals in the field present at the time it froze.