Shield (geology)

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Geologic provinces of the world (USGS)

A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas.[1] In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years.[2] 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 greatly diminished compared with the activity that occurs at the margins of the shields and the boundaries 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"[3] 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."[4]

Shields occur on all continents.

Lithology[edit]

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.[5]

Erosion and landforms[edit]

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",[6] in the case of the Fennoscandian Shield this is the Sub-Cambrian peneplain.[7]

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.[6] Shield relief, including peneplains, can be protected from erosion by various means.[6][8] 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.[8] Erosion of peneplains by glaciers in shield regions is limited.[8][9] In the Fennoscandian Shield average glacier erosion during the Quaternary amounts to tens of meters, albeit this was not evenly distributed.[9] For glacier erosion to be effective in shields a long "preparation period" of weathering under non-glacial conditions may be a requirement.[8]

List of shields[edit]

See also[edit]

Notes[edit]

  1. ^ Kearey, Philip (2001). The New Penguin Dictionary of Geology. p. 243.
  2. ^ Bastedo, Jamie. "Canadian Shield". thecanadianencyclopedia.ca.
  3. ^ Suess, Eduard; Sollas, William Johnson; Sollas, Hertha B. C. (3 June 2018). "The face of the earth (Das antlitz der erde)". Oxford, Clarendon press – via Internet Archive.
  4. ^ Miall, Andrew D. "Geological Regions". thecanadianencyclopedia.ca.
  5. ^ Austrheim, Håkon; Corfu, Fernando; Bryhni, Inge; Andersen, Torgeir B. (2003). "The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway" (PDF). Precambrian Research. 120: 149–175.
  6. ^ a b c Fairbridge, Rhodes W.; Finkl Jr., Charles W. (1980). "Cratonic erosion unconformities and peneplains". The Journal of Geology. 88 (1): 69–86.
  7. ^ Lidmar-Bergström (1988). "Denudation surfaces of a shield area in southern Sweden". Geografiska Annaler. 70 A (4): 337–350.
  8. ^ a b c d Fairbridge, Rhodes W. (1988). "Cyclical patterns of exposure, weathering and burial of cratonic surfaces, with some examples from North America and Australia". Geografiska Annaler. 70 A (4): 277–283.
  9. ^ a b Lidmar-Bergström, Karna (1997). "A long-term perspective on glacial erosion". Earth Surface Processes and Landforms. 22: 297–306.