The Earth Contents

2. Plate Tectonics

Oceans expand and contract with continual plate movement

As more ocean crust forms, the existing crust gets transported away from the mid-ocean ridge, and is cooled and sinks. This process can continue for a long time: oceanic crust welling up in the centre of the ocean to form new crust which is carried away from the mid-ocean ridge as if on a conveyor belt. By way of an example, the Atlantic Ocean has been growing through separation of the two plates for 135 million years, and growth is still continuing to the present. However, continued growth is only possible with contraction elsewhere, simply because the surface of the Earth is always fully covered by the existing plates. Therefore contraction occurs where the dense oceanic crust collides with, and is forced beneath (or subducted), the adjacent, lighter continental crust. The best example of a contracting ocean today is the Pacific Ocean.

Give an example of an expanding ocean and a contracting ocean.