Appalachian Mountains - Geology

Geology

A look at rocks exposed in today's Appalachian mountains reveals elongated belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor, which provides strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges, some 480 Ma, marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians near the center. Because North America and Africa were connected, the Appalachians formed part of the same mountain chain as the Little Atlas in Morocco. This mountain range, known as the Central Pangean Mountains, extended into Scotland, from the North America/Europe collision - see Caledonian orogeny.

During the middle Ordovician Period (about 496-440 Ma), 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 down slope to be deposited in nearby lowlands. The Taconic Orogeny was just the first of a series of mountain building plate collisions that contributed to the formation of the Appalachians, culminating in the collision of North America and Africa (see Appalachian orogeny).

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.