Shield (geology)

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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. 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 metamorphose 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...
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