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Waves

Plate Tectonics - Plates separating.

Plates under land masses are called continental plates.
Plates under the ocean are called oceanic plates.
Oceanic plates grow outwards from an underwater plate boundary,
(a mid-ocean ridge like the Mid-Atlantic ridge).
This is called sea floor spreading.

Hot magma rises up through the boundary,
and cools rapidly in the cold sea water, forming basalt mountains
(see the GCSE Chemistry site).
As the oceanic plates move apart, more magma rises to fill the gap.
The basalt mountains are therefore the same on each side of the ridge,
since they were formed at the same time from the same rising magma.
See the evidence for this on the next page.

Hot magma may burst violently out of a mid-ocean ridge
from underwater volcanoes.
Volcanoes and earthquakes under the ocean can cause enormous waves
called "tsunami", pronounced "tsoo-nar-mee"
(also incorrectly called "tidal waves").
These waves can do great damage when they reach land.

New oceanic crust is continually being formed.
Where the oceanic plate meets a continental plate, the plates collide.
The oceanic plate is forced down into the mantle and melts,
it is recycled forming new magma.
The oceanic plates are not more than 200 million years old.
That's about how long it takes for the largest plate to be recycled.

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