A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. These rocks are older than 570 million years and sometimes date back to around 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 . Shields occur on all continents.
7-470: The Western Plateau is Australia's largest drainage division and is composed predominantly of the remains of the ancient rock shield of Gondwana . It covers two thirds of the continent; 2,700,000 square kilometres (1,000,000 sq mi) of arid land, including large parts of Western Australia , South Australia , and the Northern Territory . For comparison, it is roughly the same size as
14-410: Is highly unpredictable. Shield (geology) 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 sedimentary platform while shield only describes the basement. The term shield , used to describe this type of geographic region, appears in
21-689: Is made up of gneiss . 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 closer to
28-475: 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." A shield is that part of the continental crust in which these usually Precambrian basement rocks crop out extensively at
35-513: 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 has amounted to tens of meters, though this
42-443: The surface. Shields 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 . It is estimated that over 50% of Earth's shields surface
49-463: The whole of continental Europe from Poland west to Portugal . Rain rarely falls in this region and aside from a handful of permanent waterholes, surface water is absent at all times except after heavy rain. Most of the territory is flat sandy or stony desert with a sparse covering of shrubs or tussock grasses. Average rainfall varies from one area to another and is quoted at 189 millimetres (7.4 in) to 398 millimetres (15.7 in) per year, but
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