Regional deformation of rocks

Deformation of rocks occurs at a variety of scales, from the atomic to the scale of continents and entire tectonic plates. Deformation at the continental-to-plate scale produces distinctive regional structures. Cratons, large stable blocks of ancient rocks that have been stable for a long time (since 2.5 billion years ago), form the cores of many continents and represent continental crust that was formed in the Archean Era. Most cratons are characterized by thick continental roots made of cold mantle rocks, by a lack of earthquakes, and by low heat flow.

Orogens, or orogenic belts, are elongate regions that represent eroded mountain ranges, and they typically form belts around older cratons. Characterized by abundant folds and faults, they typically show shortening and repetition of the rock units by 20-80 percent. Young orogens are mountainous—for instance, the Rocky Mountains have many high peaks, and the slightly older Appalachians have lower peaks.

Continental shields are places where ancient cra-tons and mountain belts are exposed at the surface, whereas continental platforms are places where younger, generally flat-lying sedimentary rocks overlie the older shield. Many orogens contain large portions of crust that have been added to the edges of the continental shield through mountain-building processes related to plate tectonics. Mountain belts may be subdivided into three basic types: fold and thrust belts, volcanic mountain chains, and fault block ranges.

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