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Plate Tectonics

What is plate tectonics?

The earth’s crust and outer layers (down to about 100 km depth) is divided into large, nearly rigid pieces called plates. These plates move slowly about the earth’s surface, at speeds of a few centimeters per year. Thus the earth’s surface is like a big jigsaw puzzle with moving pieces. This system of moving plates is called plate tectonics.

Faults between plates

In some places two plates are pulling apart, and new material wells up from depth to add new material to each plate. This process forms the mid-ocean ridges. In other places two plates are converging, and this requires one of the plates to sink into the mantle. The sites of this sinking, or subduction, are called subduction zones. Their surface expression is the deep ocean trenches. In yet other places two plates are moving past each other, neither diverging nor converging. These boundaries are called transform faults, famous examples being the San Andreas fault in California and the Anatolia fault in Turkey.

The reason the plates are moving is evidently because they are part of a system of mantle convection.


Notes: Last modified: Wed, 19 Dec 2001