Plates are now pulling apart primarily along the system of great submarine ridges in the world's oceans.

Hot material from the deeper mantle wells up into the gap, and some of it melts and is erupted on the surface as lava or is injected near the surface to crystallize as other igneous rocks.

The ridge stands high because its material is hot, and hence low in density. As the plates move apart, the ridge material gradually cools and contracts, and its surface sinks.

Ridges generally form step-like alternations of spreading centers perpendicular to the direction of motion and of strike-slip faults parallel to that direction.

Where plates converge, one tips down and slides beneath the other.

Generally, an oceanic plate slides ("subducts") beneath a continental plate (for example, along the west coast of South America) or another oceanic plate (for example, the east side of the Philippine Sea plate).

A trench is formed where the under-sliding plate tips down, and the ocean-floor sediment it carries is scraped off against the front of the overriding plate.

Part 1: Broken Crust
Part 3: The Changing Face of the Earth
Part 4: Volcanoes

From: Hamilton, 1976, Plate Tectonics and Man: Reprint from: USGS Annual Report, Fiscal Year 1976