Plates Move Apart
Hidden beneath the seas, along a global system of mountain ridges, Earth’s plates are growing and spreading apart. Each year these oceanic spreading ridges erupt more than three times as much molten rock as do all the volcanoes on land. Magma rises from Earth’s mantle at spreading ridges and cools on and beneath the ocean floor, adding to the plates on either side. The growing plates inch away from the ridges—widening ocean basins and rafting apart entire continents.

Plates Come Together
Where plates come together, or converge, we see some dramatic manifestations of plate tectonics. At convergent margins, continents grow as plates are consumed.

Plates Pass By
Why do earthquakes shake California? The state straddles two plates that are moving past each other like trains on opposite tracks. The plate boundary is marked by a zone of active faults—breaks in the rock and ground surface caused by plate movements. The most famous of these is the 1200-km (750-mi) long San Andreas Fault. The San Andreas fault is a transform fault, a kind common on the sea floor but rarely found on land.