Why Do Tectonic Plates Move?

Earth's hard outer shell is not one unbroken skin. It is divided into plates that carry oceans and continents and move very slowly. The movement is too gradual to feel directly, but satellite measurements, the age and magnetic record of the seafloor, and the distribution of earthquakes and volcanoes show that it is real. Earthquakes, volcanoes, and mountain building often concentrate near plate boundaries.

There is hot mantle below the plates, but plates do not simply float on a sea of magma. Mantle rock can flow extremely slowly over long timescales, and Earth's internal heat relates to convection and density differences within it. That flow, together with gravitational forces acting at plate boundaries, contributes to plate motion. The real planet is much more complex than a single tidy conveyor belt.

When an old, cold, dense oceanic plate sinks toward the mantle at a subduction zone, it can pull the rest of the plate behind it. This is called slab pull and is considered an important driver of plate motion. At a mid-ocean ridge, hot material rises, new oceanic crust forms, and plates separate. The tendency for a plate to slide away from an elevated ridge is called ridge push.

Plates do not only move apart. At some boundaries they meet and one descends beneath another; at others they slide horizontally past each other. Even while a boundary is temporarily locked by friction, other parts of a plate may continue moving. When accumulated strain is released, an earthquake occurs. Plate tectonics explains the broad setting for many earthquakes, but it does not predict the exact moment of a particular quake.

Over very long periods, plate motion builds and reshapes mountain ranges, trenches, island arcs, and ocean ridges. Typical speeds may resemble fingernail growth, yet over millions of years they can open oceans and rearrange continents. Researchers continue to study the relative importance of the forces involved, but gravity, density differences, and internal heat are connected in Earth's long-term circulation.