Why is oceanic crust predominantly basalt while continental crust is mainly granitic in composition?

Answer 1

Basalt is very dense while granite is lighter and can "float" on the basaltic oceanic crust. The Continental plates shift with the oceanic plates as the heavier basalt sub-ducts under the lighter granite.

Answer 2

The lithosphere isn't just composed of basalt, there are also granites and sedimentary rocks. Basalt is a common rock because this is actually the lithospheric rock with the closest composition to the mantle and is therefore common whenever magma erupts to the surface from the mantle, such as at seafloor spreading zones (basalt is the rock found on the ocean floor). Igneous rocks such as granite are formed from recycling other rocks due to plate tectonics and subduction.

Answer 3

Firstly, the lithosphere is composed of the crust and uppermost rocky mantle. Secondly, the crust is divided into oceanic and continental varieties. The oceanic crust is primarily the igneous rock basalt, and the continental crust is primarily the igneous rock granite (or the chemical equivalent). The majority of the crust is therefore basaltic in origin.

Answer 4

Over many hundreds of millions of years, our current continental crust was formed from the differentiation of minerals from the once molten Earth. As the magma cooled and crystallized, less dense mineral material rose toward the surface as the heavier minerals sank. Because of its relative density, continental crust, which is predominately granitic, tends to float on top of the asthenosphere, and resists subduction at colliding plate margins. Some granitic continental crustal rock is nearly 4 billion years old.

Oceanic crust has undergone the same differentiation, but its source material is magma from the mantle, which is composed mostly of dark, heavy minerals. Basalt is the product of this solidified magma, and the ocean floor is subjected to subduction back into the mantle at colliding plate margins. In fact, no oceanic crustal material is older than 200 million years in age.