How does magnetic reversals found in the sea floor rocks cause continental drift?

Answer 1

Magnetic reversals found in the sea floor do not cause continental drift. It does however provide evidence that sea floor spreading occurs (and hence is evidence for the theory of plate tectonics).

This is because the iron rich minerals in basaltic rocks (such as magnetite) are at high temperatures when they are first intruded into the crust and at these high temperatures they record the orientation of Earth's magnetic field. As they cool, this record of the orientation of the magnetic field is "locked" in place.

The Earth's magnetic field is known to "reverse polarity" - the north and south magnetic poles switch over time and so the orientation of the magnetic field in newly cooled rocks will change to match this.

Geophysicists had recognised that there were alternating bands of rocks with these reversed orientations of the magnetic field. These bands ran parallel to the Mid-Ocean-Ridges.

In the 1960s after significant magnetic survey work was undertaken it was realised that the alternating bands on one side of the oceanic ridge were a mirror image to the bands on the other side.

So why is this evidence for sea floor spreading / plate tectonics?

Well it was realised that if the newly intruded rock was to move away from the Mid-Ocean-Ridge (M-O-R) and be replaced with new younger rock, this rock would continue to record the orientation of the Earth's magnetic field as and when reversals occurred. As such this would lead to the stripes of alternating magnetic orientation that are symmetrical about the M-O-R. As such movement of the plates away from a M-O-R and the reversals of Earth's magnetic field best explained the presence of these bands and so confirmed the idea that sea floor spreading was occurring.

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