How Continents Form

Accretions of terranes. The development of a series of mountain belts along a continent's margins increases the size of the continent by adding new continental crust (accretion). In most cases, a continent consists of an older core (craton) surrounded by progressively younger rocks. Mountain ranges are sometimes called tectonostratigraphic terranes, or just terranes, which represent regions of geologic continuity distinct from neighboring mountain ranges. Terranes can range up to thousands of square kilometers in area. Accreted terranes are those that appear to have formed in place along a continent's margin through accumulation and orogeny. A suspect terrane is one that does not fit the regional pattern or has conflicting age dates; an exotic terrane is one that did not form naturally through accretion and has likely collided with the continental margin. Exotic terranes have distinctive rock types, metamorphic and structural histories, and ages of formation. Paleomagnetic data can sometimes be used to reconstruct an exotic terrane's path of migration. Such terranes can be island arcs, microcontinents such as New Zealand, or rifted fragments of distant continents. North America is composed of over fifty distinct geologic terranes; twelve of these have been accreted to western North America during the past 200 million years.