Earth, Composition of the crust

Most of the Earth's surface is covered by oceanic crust. The composition of this crust is documented on the basis of rocks sampled by deep-sea dredging and by studies of ophiolites, which are slivers of oceanic crust and uppermost mantle that have been tectonically emplaced on land. The oceanic crust is found to be relatively uniform in composition (Table 3). It consists