ABSTRACT

CONTINENTS AND OCEANS Oceanic crust is mainly basalt and dolerite, 5-10 km thick, forming all the ocean floors; it is created and destroyed at plate boundaries. Continental crust is mainly granite and gneiss, 20-80 km thick; of lower density than oceanic crust, it floats higher on the mantle and forms all the continents, submerged continental shelves and adjacent islands. It is too light to be subducted, so is almost indestructible; it may be eroded or added to by accretion of sediment and rock scraped off subducting oceanic plates. Individual plates may be either or both crustal types. Continent coasts may or may not be plate boundaries.