During the earliest Paleozoic Era, 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 Period (about 480-440 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event (Taconic orogeny) in North America.
Taconic Orogeny
The Taconic orogeny was a great mountain building period that perhaps had the greatest overall effect on the geologic structure of basement rocks within the New York Bight region. The effects of this orogeny are most apparent throughout New England, but the sediments derived from mountainous areas formed in the northeast can be traced throughout the Appalachians and midcontinental North America.
Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to grow progressively narrower. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to fold gradually downward. In this manner, shallow-water carbonate deposition that had persisted on the continental shelf margin through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. Sometime during this period a convergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along a subduction zone with an eastward-dipping orientation. Partial melting of the down-going plate produced magma that returned to the surface to form the offshore Taconic island arc. By the Late Ordovician, this island arc had collided with the North American continent. The sedimentary and igneous rock between the land masses were intensely folded and faulted, and were subjected to varying degrees of intense metamorphism. This was the final episode of the long-lasting mountain-building period referred to as the Taconic Orogeny.
When the Taconic Orogeny subsided in the New York Bight region during Late Ordovician time (about 440 million years ago), subduction ended, culminating in the accretion of the Iapetus Terrane onto the eastern margin of the continent. This resulted in the formation of a great mountain range throughout New England and eastern Canada, and perhaps to a lesser degree, southward along the region that is now the Piedmont of eastern North America. The newly expanded continental margin gradually stabilized. Erosion continued to strip away sediments from upland areas. Inland seas covering the Midcontinent gradually expanded eastward into the New York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically-quiet period persisted until the Late Devonian time (about 360 million years ago) when the next period of mountain-building began, the Acadian orogeny.
Acadian Orogeny
The Acadian orogeny is a middle Paleozoic mountain building event (orogeny), especially in the northern Appalachians, between New York and Newfoundland. The Acadian orogeny most greatly affected the Northern Appalachian region (New England northeastward into the Gaspé region of Canada). The Acadian orogeny should not be regarded as a single tectonic event, but rather as an orogenic era. It spanned a period of about 50 million years, from 375 to 325 million years ago. In Gaspé and adjacent areas, its climax is dated as early in the Late Devonian, but deformational, plutonic, and metamorphic events extended into Early Mississippian time. During the course of the orogeny, older rocks were deformed and metamorphosed, and new faults formed and older faults were reactivated.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of the Great Basin.
The cause of this great period of deformation is a result of the
plate-docking of a small continental landmass called
Avalonia (named after
the
Avalon Peninsula
of Newfoundland). The docking of Avalonia onto the composite margin of
Ganderia and Laurentia resulted in the closing of a portion of the
Rheic Ocean.
Avalonia was gradually torn apart as plate tectonic forces accreted the
landmass onto the edge of the larger North American continent. Today,
portions of the ancient Avalonia landmass occur in scattered outcrop belts
along the eastern margin of North America. One belt occurs in Newfoundland,
another forms the bedrock of much of the coastal region of New England from
eastern Connecticut to northern Maine
A period of lithospheric thinning that followed the Acadian orogeny created volcanoes, such as the large Mount Pleasant Caldera in southwestern New Brunswick, Canada.
Alleghenian (Appalachian) Orogeny
The Alleghenian orogeny or Appalachian orogeny is one of the geological mountain-forming events (orogeny) that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling "Alleghany Orogeny" (sic) originally proposed by H.P. Woodward (1957, 1958) is preferred usage. Approximately 350 million to 300 million years ago, in the Carboniferous period, when Gondwana (specifically what became Africa) and what became North America collided, forming Pangaea. This collision exerted massive stress on what is today the Eastern Seaboard of North America, resulting in a large-scale uplift of the entire region. Closer to the boundary between the colliding plates, tectonic stresses contributed to the metamorphosizing of the rock (i.e. the transformation of igneous and sedimentary rock into metamorphic rock). These stresses concurrently caused faults (mostly thrust faults and some strike-slip faults) as well as folding. The immense region involved in the continental collision, the vast temporal length of the orogeny and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians could have risen as high or perhaps even higher than the present-day Himalaya.
The Appalachian Orogeny is responsible for the creation of the mountains themselves and is not responsible for the topography that now typifies the Piedmont and coastal plain regions east of the mountain chain. The heavily-eroded hills of Piedmont are remnants of the sizeable mountain chain, while the coastal plain is made up of the material that was washed away in that process. Thus, the coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years ago to the present.
Evidence for the Appalachian orogeny stretches for many hundreds of miles on the surface from Alabama to New Jersey and can be traced further subsurface to the southwest. In the north it enters a region of confused topography associated with earlier orogenies, but clearly the Applachian deformation extends northeast to Newfoundland.
The mountains were once rugged and high, but in our time are now eroded into only a small remnant. Sediments that were carried eastward form part of the continental shelf. Sediments that were carried westward form the Allegheny and Cumberland Plateau, which in some areas are popularly called mountains, but are actually simply uplifted and eroded plateaus. Carbonates and fine sediments from these mountains were carried farther to form limey rocks in a shallow sea that was later uplifted and forms the bulk of Tennessee, Kentucky, Ohio, and Indiana.
A portion of the Alleghenian mountain system departed with Africa when Pangaea broke up and the Atlantic Ocean began to form. Today, this forms the Anti-Atlas mountains of Morocco. The Anti-Atlas have been geologically uplifted in relatively recent times, and are today much more rugged than their Alleghenian relatives.
Erosion
Pangea began to break up about 220 million years ago, in the Early Mesozoic Era (Late Triassic Period). As Pangea rifted apart a new passive tectonic margin was born and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and the mountains began to wear away.
By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present formed. 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.
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Source: Geology of the Appalachians