How do plate tectonics cause volcanoes?

1the pattern

About three quarters of the world's active volcanoes lie along the Ring of Fire, the belt of plate boundaries around the edge of the Pacific Ocean. Volcanoes cluster along plate edges because that is where plate motion gives rock a reason to melt.

Volcanoes cluster along plate boundaries, most of all the Pacific's Ring of Fire.

2solid mantle

Earth's mantle is mostly solid rock, hot enough to flow only very slowly. It does not sit under the crust as a sea of magma. Rock melts when its temperature rises, when the pressure on it drops, or when water mixes in and lowers its melting point. Plate motion triggers the last two.

The mantle is solid; plate motion melts it by dropping pressure or adding water, not mainly by adding heat.

3subduction

Oceanic plates are cooler and denser than continental plates, so where they collide the ocean plate bends and sinks into the hot mantle. That is subduction. It makes deep ocean trenches and the volcanic arcs on the overriding plate.

At a subduction zone the dense ocean plate dives beneath a continent.

4water released

The sinking ocean plate carries water locked in its minerals and seafloor sediments. Under rising pressure and temperature, about 100 kilometres down, those minerals change and release the water, which rises into the mantle wedge between the slab and the overriding plate.

Roughly 100 km down, the sinking plate squeezes out water that rises into the mantle above.

5melt and rise

Water lowers the temperature at which mantle rock melts, so the wedge above the sinking plate partly melts. This is called flux melting. The new magma is lighter than the solid rock around it, so it rises through the continent plate and erupts at the surface, building a volcano. The sinking slab itself mostly does not melt.

Water lowers the mantle's melting point; the magma it makes rises and erupts as a volcano.

6pull apart

Where two plates move apart, as along the mid-ocean ridge, hot solid mantle rises to fill the gap. Rising rock is under less and less pressure, and lower pressure lets it melt even though it has not got hotter. This decompression melting makes the magma that builds new ocean floor, and the volcanoes of places like Iceland.

Where plates pull apart, rising mantle melts because the pressure on it drops.

7hotspot

A hotspot is a long-lived plume of hot rock rising from deep in the mantle, not tied to a plate edge. As a plate drifts across it, a volcano forms over the plume, then is carried off and dies while a new one grows. The result is a chain of islands that get older away from the hotspot, as in Hawaii, where the Pacific Plate moves about 7 to 10 centimetres a year.

A fixed hotspot melts through a moving plate, leaving a chain of ever-older volcanoes.

8the whole idea

The whole picture: the mantle is solid, and volcanoes appear where something makes it melt. At subduction zones, water from the sinking plate lowers its melting point. At spreading ridges, rising rock loses pressure and melts. Hotspots, which do not sit on plate edges, melt through a drifting plate and leave island chains.

Volcanoes form where plate motion melts solid mantle: water at collisions, low pressure at ridges, plus hotspots.