Shield (geology)

Shield (geology)
Geologic provinces of the world (USGS)
  Shield
  Orogen
  Basin
  Extended crust
Oceanic crust:
  0–20 Ma
  20–65 Ma
  >65 Ma

A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years[citation needed]. They have been little affected by tectonic events following the end of the Precambrian Era, 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 was originally translated from German Schild by H. B. C. Sollas in Eduard Suess's Face of Earth in 1901.

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.

Shields are normally the nucleus of continents and most are bordered by belts of folded Cambrian rocks. Because of their stability, erosion has flattened the topography of most of the continental shields; however, they commonly do have a very gently convex surface. They are also surrounded by a sediment covered platform. By contrast, in a platform the shield, more accurately referred to then as the crystalline "basement", is overlain by horizontal or subhorizontal sediment. Together, the shield, platform and basement are the parts that comprise the stable interior portion of the continental crust known as the "craton."

The margins surrounding a shield generally constitute relatively mobile zones of intense tectonic or plate-like dynamic mechanisms. In these areas, complex sequences of mountain building (orogeny) events have been documented over the past few hundred million years.

For example, the Ural Mountains to the west of the Angaran Shield, are atop the mobile zone that separates the shield from the Baltic Shield. Similarly, the Himalayas are on the mobile boundary between the Angaran and Indian shields. Shield margins have been subject to geotectonic forces that have both destroyed and rebuilt the margins and the cratons that they partially comprise. In fact, the growth of continents has occurred as a result of the accretion of younger rocks that underwent deformations during series of mountain building processes. In a sense, these belts of folded rocks have been welded onto the borders of the preexisting shields, thus increasing the size of the proto-continents that they make up.

Continental shields occur on all continents, for example:

See also

Notes



Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Shield (disambiguation) — Shield may refer to:Protection* Shield, a hand held protective device meant to intercept attacks * Electromagnetic shielding * Shielded cable * Energy shield * Deflector shield, a protective energy field * Heat shield, which protects a spacecraft …   Wikipedia

  • Geology of Colombia — refers to the geological composition of the Republic of Colombia that determines its geography. The territory of Colombia covers vast areas within the South American plate, where most of the emerged land is, the Caribbean plate and the Nazca… …   Wikipedia

  • Geology of Mars — Mars   Mars as seen by the Hubble Space Telescope Designations …   Wikipedia

  • Geology of Venus — Radar image of the surface of Venus, centered at 180 degrees east longitude Venus is a planet with striking surface characteristics. Most of what is known about its surface stems from radar observations, mainly images sent by the Magellan probe… …   Wikipedia

  • Geology of North America — The geology of North America, like most topics of scientific study, is undergoing progressive investigation by numerous public and private sector earth scientists, academicians, and students. In that regard, the detailed picture is subject to… …   Wikipedia

  • Geology of the Lassen volcanic area — The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U.S.. The park is located in the southernmost part of the Cascade… …   Wikipedia

  • Geology of the Pacific Northwest — The geology of the Pacific Northwest refers to the study of the composition (including rock, minerals, and soils), structure, physical properties and the processes that shape the Pacific Northwest region of the United States and Canada. The… …   Wikipedia

  • Geology of the North Sea — Contents 1 Tectonic structure 2 Precambrian 3 Finnmarkian, Athollian, Caledonian Orogenies …   Wikipedia

  • Geology of Minnesota — The geology of Minnesota is the study of the rock, minerals, and soils of the U.S. state of Minnesota, including their formation, development, distribution, and condition. The state s geologic history can be divided into three periods. The first… …   Wikipedia

  • Geology of the Australian Capital Territory — Australia grew around three fused pieces of very ancient continental crust (cratons). The geology of the Australian Capital Territory includes rocks dating from the Ordovician around 480 million years ago, whilst most rocks are from the Silurian …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”