The Appalachians are old. 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, some 480 million years ago, marks the
first of several mountain building plate collisions that culminated
in the construction of the supercontinent Pangea
with the Appalachians near the center.
List of National Parks exhibiting Appalachian Geology
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
rock was deposited on the shallow sea bottom when the region
was submerged. When seas receded, terrestrial sedimentary
deposits and erosion dominated.
The making of a supercontinent
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 Mountain 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.
Contorted ridges of erosion-resistant rocks of the Appalachian Mountain core show up as red on this image. Notice the stream running down the center of the image. This is an old stream that was well established before the Cenozoic Appalachian uplift began. As the Appalachians rose some streams downcut so rapidly that they sliced through resistant rocks rock layers and geologic structures, rather than establishing new courses that flowed around them. Image from NASA.
The breaking of a supercontinent
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.
Old mountains, young topography
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.