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Plate Tectonics - What is the Evidence for Sea Floor Spreading?

The Earth acts as a giant magnet. The magnetic North Pole
is close to the geographic North Pole (true North). This is useful
if you are trying to get around using a compass. At
various times in the past, the Earth's magnetic field has reversed.
The North Pole has become the South Pole. This happens
(on average) about every half million years (no one knows why).
Using the same compass could have been confusing!

Iron is a magnetic metal. When molten iron cools and turns solid
it becomes (weakly) magnetised in the direction of the
Earth's magnetic field. Therefore, rocks which contain iron
also become magnetised as the molten magma cools
and sets as new basalt. The rocks make a record of the
Earth's magnetic field direction at the time they were formed.

When the symmetrical basalt mountains on each side
of the Mid-Atlantic ridge were checked for magnetism
 it was found that the magnetic field direction
was also symmetrical on each side of the ridge.

Magnetic Reversal at a Mid-Ocean Ridge

This is the clearest evidence for sea floor spreading
and continental drift, as it shows that rocks that are the
same distance from the ridge on each side
must have formed at the same time from
the same magma and then moved apart later.

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