Tectonic Plates

You know Earth's crust is its thin, rocky outside layer. Beneath it is mantle rock. The crust and the stiff uppermost mantle together form Earth's rigid outer shell, called the lithosphere. This shell is split into huge pieces called tectonic plates. A plate can carry land, ocean floor, or both. The Ring of Fire's earthquake and volcano clusters lie near plate edges.

Plates rest on hotter, softer mantle rock underneath. This supporting rock is mostly solid. It can slowly change shape and flow over long periods, so the solid plates above it can move. A mountain moves with its plate: being made of solid rock does not mean it is fixed in place. The supporting mantle is not an ocean of melted rock.

Both the plate and the supporting mantle are mainly solid, but they behave differently over long periods.

Plates usually move a few centimetres each year, roughly like fingernails growing. This steady creep is too slow to feel. Sudden slipping at a plate edge can still cause an earthquake. Small yearly moves add up: plates moving apart carry places farther apart. On a movement number line, count from the hop's start to its end. If the same distance is added each year, multiply distance per year by the number of years. Keep the same distance unit.