Another thing to keep in mind: these tectonic plates are heavy. Like unimaginably heavy. They can be thousands of miles wide and tens of miles thick and are as dense as rock. They exert huge pressure on that soft sludgey core, which is incidentally being heated by a low-level nuclear furnace in the center of the Earth. So the plates are more like a lid on a boiling pot that is foaming over. They're all getting jostled by insanely powerful pressure from underneath. Every once in a while a pimple appears and goes kaboom.