Plate tectonics

The theory that unified the subject.

Moving plates

The outer shell is broken into plates that move a few centimetres a year, carried by slow circulation in the mantle. Continents are passengers on them.

Plates pull apart at ridges, collide at mountains and slide past at faults.
Matching coastlines, fossils and rock sequences across oceans were the first clues.

Consequences

Earthquakes, volcanoes, mountain ranges and ocean basins all occur where plates meet. A map of earthquakes is effectively a map of plate boundaries.