The rigid lithosphere is broken into plates whose edges meet at plate boundaries. These narrow zones are the sites of most earthquakes, volcanoes, and mountain building. Three primary types exist: divergent, convergent, and transform.

At divergent boundaries plates move apart and new crust forms. At convergent boundaries plates collide and one may subduct or the crust may thicken into mountains. At transform boundaries plates slide horizontally past each other. Boundaries may also be diffuse zones rather than sharp lines.

The global pattern of plate boundaries outlines the present plate tectonic regime. Historical geology records the opening and closing of oceans as boundaries are created and destroyed. Understanding plate boundaries is fundamental to assessing geologic hazards and reconstructing past continental configurations.

• Edge where two tectonic plates meet
• Site of most earthquakes and volcanoes
• Divergent, convergent, or transform types
• Defines the current plate tectonic framework