A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at the surface. Shields themselves can be very complex: they consist of vast areas of granitic or granodioritic gneisses, usually of tonalitic composition, and they also contain belts of sedimentary rocks, often surrounded by low-grade volcano-sedimentary sequences, or greenstone belts. These rocks are frequently metamorphosed greenschist, amphibolite, and granulite facies.[citation needed] It is estimated that over 50% of Earth's shields surface is made up of gneiss.[4]