Why are there different rock types found in the continental and oceanic crust?
It has to do with the way the rocks are formed.
Since the oceanic plates are heavier and more dense sediments undergo more pressure as they are being formed.
The lava that forms the oceanic crust comes directly from the mantle. It is more dense and is formed of basaltic rock.
The lave that forms the continental crust comes from melted oceanic crust that has been subducted under the lighter continental crust. The melted basalt is combined with melted ocean sediments that have accumulated in layers on top of the basalt as well as parts of the lighter continental crust. This lava that does not reach the surface is granite. Granite is much lighter than basalt.
Oceanic crust comes directly from the mantle is much more dense.
Continental crust comes from melted crustal material and is much lighter.