Most of what we know about the inner structure of the Earth comes from ... and geologists now think they are composed of relatively dense oceanic crust which broke off the rest of its plate ...

The diagram below shows the structure of the earth ... Continental and oceanic plates all fit together to form the outer crust of the planet. Eight major plates are named on the diagram below.

The Pacific structure appears to have 50 percent more fresh oceanic crust mixed through it than the African province, the researchers found. That makes for a greater difference in composition ...