Geology of 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 mountainsreveals 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 buildingplate collisions that culminated in the construction of the supercontinent Pangaeawith the Appalachians and neighboring Anti-Atlas(now in Morocco) near the center. These mountain ranges were once higher than today's Himalaya mountain range, which was also formed by continental collision.
During the earliest
PaleozoicEra, the continent that would later become North Americastraddled the equator. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal PlainProvince. During this interval, the region was periodically submerged beneath shallow seas. Thick layers of sediment and carbonate rockwas deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.cite web|url=http://wrgis.wr.usgs.gov/docs/parks/province/appalach.html|title=Geologic Provinces of the United States: Appalachian Highlands Province|work=USGS Geology in the Parks|lastaccess=2007-09-02 (public domain source)]
During the middle
OrdovicianPeriod (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. The once quiet, Appalachian passive margin changed to a very active plate boundarywhen 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.
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, erosionbegan 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 (
PennsylvanianPeriod) Africawas approaching North American craton. The collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountainsarea 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 ( PermianPeriod ) when Africa ( Gondwana) plowed into the continental agglomeration, with the Appalachian-Ouachita mountainsnear the core.
Mesozoic Era and later
Pangea began to break up about 220 million years ago, in the Early
MesozoicEra (Late TriassicPeriod). 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
MesozoicEra, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the CenozoicEra 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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