Is the oceanic crust homogenous or heterogeneous?
Oceanic crust has, on top, sediments that lie on top of pillow basalts. These are formed from lavas erupted under the ocean and quenched to large (very hard) pillows. They are fine-grained to glassy and much has reacted with water. Beneath them are intrusions that fed the basalts. These are often said to be dike swarms but I think they are rather more varied than that. In any case they are medium grained basaltic rocks (diabases). Beneath and around them are more coarse-grained basaltic rocks — gabbros. Beneath those are cumulates — piles of crystals that formed from liquids and accumulated — that are mostly olivines and pyroxenes. Beneath those are olivine-only cumulates but that is, at least seismically out of the crust and into the mantle. Pretty heterogeneous.
