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Waves

Plate Tectonics - 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 handy 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 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 magnetic field direction,
it was found that this was also symmetrical on each side of the ridge.

This is the clearest evidence for sea floor spreading and continental drift,
as it shows that rocks 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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