Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how do plate tectonics cause volcanoes
Map the world's volcanoes and you trace the edges of Earth's plates. About three quarters of them sit on the Ring of Fire around the Pacific.
Here is the surprise: the mantle is not a sea of liquid. It is hot but solid rock. Plates make volcanoes by helping that rock melt.
Take a collision. A dense ocean plate meets a continent and dives beneath it. This is subduction, and it is where most volcanoes begin.
As the plate sinks, about a hundred kilometres down, the squeeze forces water out of its rocks. That water rises into the hot mantle above.
Wet rock melts at a lower temperature, so the mantle above starts to melt. The slab itself doesn't. The light magma rises and erupts as a volcano.
Now pull plates apart instead. Hot mantle rises to fill the gap, and as it rises the pressure drops, so it melts, with no extra heat at all.
Hotspots break the pattern. A fixed plume of hot rock melts through a plate drifting overhead, leaving a chain of islands, like Hawaii.
So plates make volcanoes by melting solid rock: water where plates collide, lower pressure where they part. Hotspots are the rare exception.
Plates never melt the Earth, they just nudge solid rock into the few places where it gives way, and that is where volcanoes rise.