Plate tectonics and Earth's interior

Earth is not a uniform ball of rock. Density sorted it into layers early in its history: a solid iron-nickel inner core under enormous pressure, a liquid outer core whose churning motion generates Earth's magnetic field, a thick solid mantle, and a thin, brittle outer crust. The mantle is solid rock — it does not flow like a liquid on human timescales — but under sustained pressure over millions of years it deforms and creeps the way glacier ice or hot glass does, in slow convective currents driven by heat escaping from the core and by radioactive decay within the mantle itself.

The crust is broken into a dozen or so large tectonic plates that ride on top of this slowly convecting mantle. Where plates spread apart at mid-ocean ridges, magma rises and cools to create new crust; where a denser oceanic plate meets a lighter continental one, it plunges (subducts) back into the mantle, pulling the rest of its plate along behind it — a process called slab pull that turns out to be a bigger driver of plate motion than convection dragging plates from below. Where plates collide, converge, or slide past one another, the accumulated strain is eventually released as earthquakes, and rising magma at boundaries and hotspots produces volcanism.

None of this happens quickly on a human timescale — plates move a few centimetres a year, about the rate fingernails grow — but over hundreds of millions of years it has assembled and broken apart entire supercontinents, opened and closed ocean basins, and built every mountain range on Earth.