Precambrian and Paleozoic Period
The understanding of Appalachian geology has advanced greatly in the last few decades, owing to the development of the plate tectonics theory. Many geologic concepts, timeframes, and theories, once held as fact, have changed radically with the development of the new plate tectonic theory. Scientific theories will continue to change with advances in research.
According to this theory, the Earths upper layer, or lithosphere (composed of the crust plus the uppermost mantle), is divided into a small number of rigid plates that rest on weaker underlying rock (the aesthenosphere) and move relative to one another at rates of a few centimeters per year. Two adjacent plates may move apart along a zone of divergence, with new crust forming along their margins as magma rises into the gap between them. Alternatively, they may slide past each other along a zone of lateral movement. Finally, they may collide along a zone of convergence. Upon collision, one plate commonly slides beneath the other at a "subduction zone." Where two continental plates collide, however, they are too light to be subducted, and the result is a massive collision, producing zones of intense compression characterized by melting, metamorphism, and more peripherally, folding and thrust faulting.
In Precambrian times, North America was joined to Europe and Africa.
During the late Precambrian, the North American plate and the African plates began to spread apart near what is now the Atlantic Coast of the Eastern United States. This spreading produced an ancestor of the modern Atlantic Ocean, as well as two seaways separating the North American Continent from two strips of land. During the Paleozoic Era, however, the plates reversed direction and began to converge. Slowly but inexorably the two strips of offshore land were crushed against the east coast. As convergence continued, the African oceanic plate began diving beneath the American plate. Finally, near the end of the Paleozoic the African Continent itself collided with the North American Continent. This collision produced tremendous compressive forces that thrust huge slabs of crust many kilometers to the west. It also folded formerly flat-lying rock layers. This compression, thrusting, and folding created many of the geologic structures seen in the Appalachians today. At the same time, vertical forces pushed up high mountain ranges, perhaps similar to the Swiss Alps today. During these times, erosion of the high mountains poured huge volumes of sediments into the seaways and onto the downwarped crust to the west of the mountains. These sediments now make up the rocks of the Ridge and Valley and the Appalachian Plateau Provinces.
Encyclopedia ID: p1541
