How Tectonic Plates Move

You know Earth has a rocky mantle. It is mostly solid, but rock can deform and flow over millions of years. Warm, less dense rock rises; cooler, denser rock sinks. This circulation is mantle convection. Plates are part of this moving system. Gravity helps drive it: sinking dense slabs pull plates, and plates slide away from high ocean ridges. Plate motion is not caused only by currents dragging plates from below.

Divergent plates pull apart. At ridges, rising mantle partly melts as pressure falls; magma cools into new crust. Convergent plates push together. Colliding continents thicken and lift crust into mountains. If one plate sinks beneath another, it releases water that helps mantle above it melt, feeding volcanoes. Transform plates slide sideways past each other, as at the San Andreas Fault. This sideways motion does not itself create or destroy crust. All three boundary types can have earthquakes.

Volcanoes can form where plates pull apart and above sinking plates.

The Ring of Fire follows many plate boundaries around the Pacific, including edges of the Nazca and Philippine Sea plates. Many earthquakes and volcanoes cluster along these boundaries. Along South America, the Nazca Plate sinks beneath the South American Plate. Water released from it helps mantle above it melt. This feeds Andean volcanoes. Some volcanoes and earthquakes occur within plates, so a hazard map is not a complete boundary map.

You learned that most continental land once formed Pangaea. Continental land can split, and an ocean basin can open as plates spread. Later, subduction can consume ocean floor and narrow the basin. When continents collide, that ocean can close. This opening and closing of an ocean basin is a Wilson cycle. It can take hundreds of millions of years. It is one part of supercontinent history.

The cycle describes an ocean basin, not one fixed set of continents rejoining forever.