How do magnetic reversals provide evidence of sea floor spreading?
The direction of the Earth’s field reverses every few hundred thousand years or so. Sometimes, the new direction lasts for up to a million years, sometimes just a few thousand years. This magnetic reversal provides the main evidence for continental drift. On cooling down from the molten state, hot rocks containing a magnetisable material such as iron or nickle are magnetised in the direction of the Earth’s magnetic field at that time. Geomagnetic surveys across the North Atlantic in the 1950’s produced the mirror-like patterns of magnetism either side of the Mid Atlantic Ridge. In 1963, this pattern was interpreted as a record of the widening of the Atlantic due to continental drift. Liquid magma emerges continuously from the Mid Atlantic Ridge. As it cools its iron and nickel are magnetised in the direction of the Earth’s field, so that as the ocean floor widens, a giant ‘magnetic tape recorder’ is made. The further the rock is from the Ridge, the earlier it was formed from the magma.
