Two kinds of crust

Not all crust is the same. Oceanic crust is thin (~7 km), dark, dense basalt. Continental crust is thick (~30–70 km), lighter-coloured granite-rich rock — and crucially, it is less dense.

That density difference decides almost everything at plate boundaries. When two plates meet, the denser one tends to sink. Cold oceanic lithosphere (crust plus uppermost mantle) can become denser on average than the warmer mantle below and sink. Basaltic crust alone is not denser than mantle rock. Continental crust is more buoyant, though parts can be carried down during collisions.