A shield is a large area of exposed Precambriancrystallineigneous and high-grade metamorphic rocks that form tectonically stable areas. These rocks are older than 570 million years and sometimes date back 2 to 3.5 billion years. They have been little affected by tectonic events following the end of the Precambrian, and are relatively flat regions where mountain building, faulting, and other tectonic processes are minor, compared with the activity at their margins and between tectonic plates. The term shield, used to describe this type of geographic region, appears in the 1901 English translation of Eduard Suess's Face of Earth by H. B. C. Sollas, and comes from the shape "not unlike a flat shield" of the Canadian Shield which has an outline that "suggests the shape of the shields carried by soldiers in the days of hand-to-hand combat." Shields occur on all continents.
Lithology
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 granulitefacies. It is estimated that over 50% of Earth's shields surface is made up of gneiss.
Erosion and landforms
Being relatively stable regions the relief of shields is rather old with elements such as peneplains being shaped in Precambrian times. The oldest peneplain identifiable in a shield is called a "primary peneplain", in the case of the Fennoscandian Shield this is the Sub-Cambrian peneplain. The landforms and shallow deposits of northern shields that have been subject to Quaternary glaciation and periglaciation are distinct from those found in closer to the equator. Shield relief, including peneplains, can be protected from erosion by various means. Shield surfaces exposed to sub-tropical and tropical climate for long enough time can end up being silicified, becoming hard and extremely difficult to erode. Erosion of peneplains by glaciers in shield regions is limited. In the Fennoscandian Shield average glacier erosion during the Quaternary amounts to tens of meters, albeit this was not evenly distributed. For glacier erosion to be effective in shields a long "preparation period" of weathering under non-glacial conditions may be a requirement. In weathered and eroded shields inselbergs are common sights.
