One of the main sources of information about the Earths composition comes from understanding the relationship between peridotite and basalt melting. Peridotite makes up most of Earths mantle. Basalt, which is highly concentrated in the Earths oceanic crust, is formed when magma reaches the Earths surface and cools down at a very fast rate. When magma cools, different minerals crystallize at different times depending on the cooling temperature of that respective mineral. This ultimately changes the chemical composition of the melt as different minerals begin to crystallize. Fractional crystallization of elements in basaltic liquids has also been studied to observe the composition of lava in the upper mantle. This concept can be applied by scientists to give insight on the evolution of Earth's mantle and how concentrations of lithophile trace elements have varied over the last 3.5 billion years.