Plates in Motion

Today we know Earth's outer shell, the lithosphere, is divided into about 15 major tectonic plates plus several smaller ones. These rigid slabs include crust plus the uppermost mantle, and they move over the weaker, semi-fluid asthenosphere below, driven by heat from the planet's interior.

Where Plates Meet

Where plates meet, three things can happen. At divergent boundaries, plates pull apart and new crust forms, as in Iceland. At convergent boundaries, one plate dives beneath another in a process called subduction. At transform boundaries, plates grind sideways past each other, building strain that releases as earthquakes.