Geology of the Appalachians

Overview The Appalachian Mountains formed through a series of mountain-building events over the last 1.2 billion years: • The Grenville orogeny began 1250 million years ago (Ma) and lasted for 270 million years. • The Taconic orogeny began 450 Ma and lasted for 10 million years. • The Acadian orogeny began 375 Ma and lasted 50 million years. • The Alleghanian orogeny began 325 Ma and lasted 65 million years. Proterozoic era Grenville orogeny The first mountain-building tectonic plate collision that initiated the construction of what are today the Appalachian Mountains occurred during the Mesoproterozoic era at least one billion years ago when the pre-North-American craton called Laurentia collided with other continental segments, notably Amazonia. All the other cratons of the Earth also collided at about this time to form the supercontinent Rodinia, which was surrounded by one single ocean. The Grenville orogeny occurred between the colliding cratons. The present Appalachian Mountains have at least two areas which are made from rock that was formed during this orogeny: the Blue Ridge Mountains and the Adirondacks. Breakup of Rodinia After the Grenville orogeny, Rodinia began to break up. The mountains formed during the Grenvillian era underwent erosion from weathering, glaciation, and other natural processes, resulting in the leveling of the landscape. The eroded sediments from these mountains contributed to the formation of sedimentary basins and valleys. For example, in what is now the southern Applachian Mountain, the Ocoee basin was formed. Seawater filled the basin. Rivers from the surrounding countryside carried clay, silt, sand, and gravel to the basin, much as rivers today carry sediment from the midcontinent region to the Gulf of Mexico. The sediment spread out in layers on the basin floor. The basin continued to subside, and over a long period of time, probably millions of years, a great thickness of sediment accumulated. Eventually, the tectonic forces pulling the two continents apart became so strong that the Iapetus Ocean formed off the eastern coast of the Laurentian margin. The rocks of the Valley and Ridge province formed in this ocean over millions of years. Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became limestone, similar to modern oceans. The weathering of limestone exposed at the land surface produces the lime-rich soils that are so prevalent in the fertile farmland of the Valley and Ridge province. Evidence of subsurface activity (dikes and sills intruding into the overlying rock) is present in the Blue Ridge as well. For instance, mafic rocks have been found along the Fries Fault in the central Blue Ridge area of Montgomery County, Virginia. Paleozoic era reconstruction showing the Appalachian Basin area during the Middle Devonian period. volcanic rocks that became a barrier and forced the mountains to push up around it. 2012 image from NASA's Aqua satellite. image.During the earliest part of the Paleozoic, the continent that would later become North America straddled the equator. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal Plain province. During this interval, the region was periodically submerged beneath shallow seas. Thick layers of sediment and carbonate rock were deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated. During the middle Ordovician (about 458-470 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event (Taconic orogeny) in North America. The once quiet Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic crust, the Iapetus, collided with and began sinking beneath the North American craton. With the creation of this new subduction zone, the early Appalachians were born. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures. The ridges of the Appalachian Mountain core represent erosion-resistant rock that remained after the rock above and beside it was eroded away. == Physiographic provinces==

The geographic boundaries of the Appalachian Mountains follow a definition that accounts for all the land mass in the United States and Canada used by the US Geological Survey and the Geologic Survey of Canada using physiography. The US uses the term Appalachian Highlands, and Canada uses the term Appalachian Uplands, to define contiguous regions that have similar geology, topography, history, and native plant and animal communities. The Appalachian Mountains are not synonymous with the Appalachian Plateau, which is one of the provinces of the Appalachian Highlands. Appalachian Basin The Appalachian Basin is a foreland basin containing Paleozoic sedimentary rocks of early Cambrian through early Permian age. From north to south, the Appalachian Basin province crosses New York, Pennsylvania, eastern Ohio, West Virginia, western Maryland, eastern Kentucky, western Virginia, eastern Tennessee, northwestern Georgia, and northeastern Alabama. The northern end of the Appalachian Basin extends offshore into Lakes Erie and Ontario as far as the United States–Canada border. The province covers an area of about and is long from northeast to southwest and between wide from northwest to southeast. The northwestern flank of the basin is a broad homocline that dips gently southeastward off the Cincinnati Arch. A complexly thrust faulted and folded terrane (Appalachian Fold and Thrust Belt or Eastern Overthrust Belt), formed at the end of the Paleozoic by the Alleghanian orogeny, characterizes the eastern flank of the basin. Metamorphic and igneous rocks of the Blue Ridge Thrust Belt that bounds the eastern part of the Appalachian Basin Province were thrust westward more than over lower Paleozoic sedimentary rocks. Discovery of oil in 1859 in the Drake Well, Venango County, Pennsylvania, marked the beginning of the oil and gas industry in the Appalachian Basin. The discovery well opened a prolific trend of oil and gas fields, producing from upper Devonian, Mississippian, and Pennsylvanian sandstone reservoirs that extend from southern New York, across western Pennsylvania, central West Virginia, and eastern Ohio, to eastern Kentucky. The Blue Ridge Thrust Belt Province underlies parts of eight states from central Alabama to southern Pennsylvania. Along its western margin, the Blue Ridge is thrust over the folded and faulted margin of the Appalachian basin, so that a broad segment of Paleozoic strata extends eastward for tens of miles, buried beneath these subhorizontal crystalline thrust sheets. At the surface, the Blue Ridge consists of a mountainous to hilly region, the main component of which are the Blue Ridge Mountains that extend from Georgia to Pennsylvania. Surface rocks consist mainly of a core of moderate-to high-rank crystalline metamorphic or igneous rocks which, because of their superior resistance to weathering and erosion, commonly rise above the adjacent areas of low-grade metamorphic and sedimentary rock. The province is bounded on the north and west by the Paleozoic strata of the Appalachian Basin and on the south by Cretaceous and younger sedimentary rocks of the Gulf Coastal Plain. It is bounded on the east by metamorphic and sedimentary rocks of the Piedmont Province. The Adirondack and New England Provinces include sedimentary, meta-sedimentary, and plutonic igneous rocks, mainly of Cambrian and Ordovician age, similar lithologically to rocks in the Blue Ridge and Piedmont Provinces to the south. The uplifted, nearly-circular Adirondack Mountains consist of a core of ancient Precambrian rocks that are surrounded by upturned Cambrian and Ordovician sedimentary rocks. == Notes and references ==