Posted by kayla on Tuesday, October 2, 2012 at 3:01pm.
The first key piece of evidence for plate tectonics was the paper by Fred Vine and Drum Matthews showing that the ocean floor had magnetic stripes that are symmetric about the ocean ridge. Basaltic magma is erupted at the ridge and the magnetite in it lines up with the Earth's magnetic field. The plates move apart and more material is erupted and intruded and adopts the magnetic field. Every now and then (the last time was 0.78 million years ago) the Earth's magnetic field reverses. When this happens the next batch of magma adopts an orientation the reverse of the previous one. This occurs many times over millions of years. Thus, going away from the ridge, one passes over "normal" and "reversed" polarity sections of the crust and they are mirror images on each side of the ridge. Once that was published, the majority of geological opinion changed to a positive view of plate tectonics
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