Shield (geology)
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Oceanic crust ██ 0-20 Ma ██ 20-65 Ma ██ >65 Ma Geologic province ██ Shield ██ Platform ██ Orogen ██ Basin ██ Large igneous province ██ Extended crust
A shield is that part of a craton in which the (usually) Precambrian basement rocks crop out extensively at the surface. By contrast, in a platform the shield, or more accurately then referred to as the crystalline "basement", is overlain by horizontal or subhorizontal sediments. A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that are tectonically stable areas with no active orogenic belts. In all cases, the age of these rocks is greater than 570 million years and sometimes date back 2 to 3.5 billion years. They have been little affected by any tectonic events in the Cambrian, 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. 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 and are surrounded by a sediment covered platform. Together, the shield, platform and basement are the parts that comprise the craton. But the 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. In the marginal areas of stable shields, complex sequences of many orogeny events have been documented covering the past few hundred million years. The margins surrounding a shield generally constitute relatively mobile zones of intensity and tectonic or plate-like dynamic mechanisms.
Shields are normally the nucleus of continents and most are bordered by belts of folded Cambrian rocks. In fact, the growth of continents has occurred as a result of the accretion of younger rocks that underwent deformations during successions of mountain building (orogeny) 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 constitute. Shield margins have been subjected to geotectonic forces that have both destroyed and rebuilt the margins of the shields and of the cratons that they partially comprise.
Continental shields occur on all continents. For example, the Canadian Shield forms the nucleus of North America - and the North American craton - and extends from Lake Superior on the south to the Arctic Islands on the north, and from western Canada eastward across to include most of Greenland. Other shields include the Amazonian Shield (sometimes referred to as the Brazilian Shield) on the eastern bulge portion of South America (the Guiana Shield lies to the north and the Platian Shield to the south); the Baltic or Fennoscandian Shield located in eastern Norway, Finland and Sweden; the African Shield (sometimes called the Ethiopian Shield); the Australian Shield that occupies most of the western half of Australia, the Arabian-Nubian Shield on the western edge of Arabia, and the Antarctic Shield. In Asia, an area in China and North Korea is sometimes referred to as the China-Korean Shield. And 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 and south. The Ural Mountains to the west of the Angaran Shield, and the Himalayas to the south, are the mobile zones that separate it from the Baltic Shield to the west and the Indian Shield to the south.
The term "shield" was originally translated from German schild by H. B. C. Sollas in Suess's Face of Earth in 1901.
