They are created at mid-oceanic ridges and destroyed at subduction zones. The upper layer of each lithospheric plate is composed of crust. The crust is composed of either continental crust or oceanic crust. Continental crust is typically made up of relatively less dense rock like granite. The oceanic crust is typically made up of more dense rock like basalt.
Oceanic crust carries sediments that have been deposited on the oceanic floor.
Some lithospheric plates have just oceanic crust. Others have some oceanic crust and some continental crust. Oceanic crust is generally thin, usually between 5 and10 kilometres thick. Continental crust is generally between 25 and 50 kilometres thick. Under large mountain ranges the crust can be over 80 kilometres thick.