American geography

East Coast - The Appalachians

The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor.Strong evidence that these rocks were deformed during plate collision.

The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangea with the Appalachians near the center.

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 shallows seas. Thick layers of sediment and carbonate rock was 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 440-480 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 plate, the Iapetus, collided with and began sinking beneath the North American craton. With the birth of this new subduction zone, the early Appalachians were born.

Along the continental margin, volcanoes grew, coincident with the initiation of subduction. Thrust faulting uplifted and warped older sedimentary rock laid down on the passive margin. As mountains rose, erosion began to wear them down. Streams carried rock debris downslope to be deposited in nearby lowlands.

This was just the first of a series of mountain building plate collisions that contributed to the formation of the Appalachians. Mountain building continued periodically throughout the next 250 million years (Caledonian, Acadian, Ouachita, Hercynian, and Allegheny orogenies).

Continent after continent was thrust and sutured onto the North American craton as the Pangean supercontinent began to take shape. Microplates, smaller bits of crust, too small to be called continents, were swept in, one by one, to be welded to the growing mass.

By about 300 million years ago (Pennsylvanian Period) Africa was approaching North American craton. The collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountains area of Texas. Continent vs. continent collision raised the Appalachian-Ouachita chain to lofty, Himalayan-scale ranges. The massive bulk of Pangea was completed near the end of the Paleozoic Era (Permian Period ) when Africa (Gondwana) plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the core.

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.

1. Acadian orogeny

The Acadian orogeny is a middle Paleozoic deformation, especially in the northern Appalachians, between Alabama and Newfoundland. 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. The Acadian should be regarded, not as a single orogenic episode, but rather as an orogenic era.

It appears to have been 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.

2. Alleghenian orogeny

The Alleghenian orogeny or Appalachian orogeny is the geological mountain-forming event (orogeny) that formed the Appalachian Mountains and Allegheny Mountains.

Approximately 350 million to 300 million years ago, in the Carboniferous period, the combined continents of Europe and Africa (Gondwana) collided with North America to form the supercontinent of Pangaea.

This collision exerted massive pressure on what is today the Eastern Seaboard of North America, resulting in an enormous uplift of the entire region, called the Appalachian orogeny. Pressure and heat over millions of years "cooked" the rock, folding, twisting and faulting it. Farther west the collision was gentler and resulted in less faulting and more wrinkling, creating ridges and valleys which became the Appalachian mountains.

Closer to the point of impact, the immense pressure turned igneous and sedimentary rock into metamorphic rock and broke it in numerous places, creating faults which were very susceptible to being worn away by wind and water. Over time these erosive forces wore the landscape down to nearly sea level. This worn-down landscape is now the Piedmont.

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. 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.

3. 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-buliding 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.

4. Aftermath of the Taconic Orogeny

As the Taconic Orogeny subsided in early Silurian time, uplifts and folds in the Hudson Valley region were beveled by erosion. Upon this surface sediments began to accumulate, derived from remaining uplifts in the New England region.

The evidence for this is the Silurian Shawangunk Conglomerate, a massive, ridge-forming quartz sandstone and conglomerate formation, which rests unconformably on a surface of older gently- to steeply-dipping pre-Silurian age strata throughout the region. This ridge of Shawangunk Conglomerate extends southward from the Hudson Valley along the eastern front of the Catskills. It forms the impressive caprock ridge of the Shawangunk Mountains west of New Paltz (town), New York. To the south and west it becomes the prominent ridge-forming unit that crops out along the crest of Kittatinny Mountain in New Jersey.

Through Silurian time, the deposition of coarse alluvial sediments gave way to shallow marine fine-grained muds, and eventually to clear-water carbonate sediment accumulation with reefs formed from the accumulation of calcareous algae and the skeletal remains of coral, stromatoporoids, brachiopods, and other ancient marine fauna.

The episodic eustatic rise and fall of sea level caused depositional environments to change or to shift laterally. As a result, the preserved faunal remains, and the character and composition of the sedimentary layers deposited in any particular location varied through time. The textural or compositional variations of the strata, as well as the changing fossil fauna preserved, are used to define the numerous sedimentary formations of Silurian through Devonian age preserved throughout the region.