in the theory of tectonic plates, at a convergent boundary betweena continental plate and an oceanic plate, the denser plate usuallysubducts underneath the less dense plate. It is well known thatoceanic plates subduct under continental plates, and thereforeoceanic plates are more dense than continental plates.My question is why are the oceanic plates always denser than thecontinental plates. I'm aware that the difference in density can beattributed to the plates differing compositions, but what I'minterested in is why these plates have different composition in thefirst place giving rise to their relative difference indensities.