Why is gneiss in earths surface?

Gneiss is a metamorphic rock that is formed when heat and pressure is applied to an igneous or sedimentary rock. The old rock does not melt completely but reorganizes it's minerals to form banks of foliation giving it the light/dark bands. Gneiss is formed when you have a parent rock that is rich in silica. Since most of the continental crust is silica rich, gneiss it a common rock. It is the highest grade of metamorphic rock in the metamorphic sequence. The sequence goes like this: slate - schist - phyllite - rock.