Ashield is a large area of exposed Precambriancrystallineigneous and high-grade metamorphic rocks that form tectonically stable areas.[1] These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years.[citation needed] 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.
Shields occur on all continents.
The term shield cannot be used interchangeably with the term craton. However, shield can be used interchangeably with the term basement. The difference is that a craton describes a basement overlayed by a sedimentaryplatform while shield only describes the basement.
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"[2] 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."[3]
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";[5] in the case of the Fennoscandian Shield, this is the Sub-Cambrian peneplain.[6]
The landforms and shallow deposits of northern shields that have been subject to Quaternary glaciation and periglaciation are distinct from those found closer to the equator.[5] Shield relief, including peneplains, can be protected from erosion by various means.[5][7] 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.[7] Erosion of peneplains by glaciers in shield regions is limited.[7][8] In the Fennoscandian Shield, average glacier erosion during the Quaternary has amounted to tens of meters, though this was not evenly distributed.[8] For glacier erosion to be effective in shields, a long "preparation period" of weathering under non-glacial conditions may be a requirement.[7]
In weathered and eroded shields, inselbergs are common sights.[9]
In Asia, an area in China and North Korea is sometimes referred to as the China-Korean Shield.
The Angaran Shield, as it is sometimes called, is bounded by the Yenisey River on the west, the Lena River on the east, the Arctic Ocean on the north, and Lake Baikal on the south.
The Indian Shield occupies two-thirds of the southern Indian peninsula.
^Merriam, D. F. (2005). Encyclopedia of Geology. Selley, Richard C., 1939-, Cocks, L. R. M. (Leonard Robert Morrison), 1938-, Plimer, I. R. Amsterdam: Elsevier Academic. p. 21. ISBN9781601193674. OCLC183883048.
