2.8 The Appalachian Mountains: Continents Collide: Plate Tectonics

2.8 The Appalachian Mountains: Continents Collide: Plate Tectonics

The Appalachian Mountains provide great scenic beauty near the eastern margin of North America from Alabama to Newfoundland. In addition, mountains that formed contemporaneously with the Appalachians are found in the British Isles, Scandinavia, Northwestern Africa, and Greenland. The orogeny that generated this extensive mountain system lasted a few hundred million years and was one of the stages in assembling the supercontinent of Pangaea. Detailed studies in the central and southern Appalachians indicate that the formation of this mountain belt was more complex than once thought. Rather than forming during a single continental collision, the Appalachians resulted from three distinct episodes of mountain building.

This over simplified scenario begins roughly 750 million years ago with the breakup of a pre-Pangea supercontinent Rodinia, which rifted North America from Europe and Africa. This episode of continental rifting and seafloor spreading generated the ancestral North Atlantic. Located within this developing ocean Basin was a fragment of continental crust that had been rifted from North America.

Then, about 600 million years ago, plate motion dramatically changed and the ancestral North Atlantic began to close. Two subduction zones probably formed. One of these was located seaward of the coast of Africa and gave rise to a volcanic arc similar to those that presently rim the Western Pacific. The other developed on the continental fragment that lay off the coast of North America.

Between 450 and 500 million years ago, the marginal sea located between this crustal fragment and North America began to close. The ensuing collision deformed the continental shelf and sutured the crustal fragment to the North American Plate. The metamorphosed remnants of the continental fragment are recognized today as the crystalline rocks of the Blue Ridge and Western Piedmont regions of the Appalachians. In addition to the pervasive regional metamorphism, igneous activity placed numerous plutonic bodies along the entire continental margin, particularly in New England.

A second episode of mountain building occurred about 400 million years ago. In the southern Appalachians, the continued closing of the ancestral North Atlantic resulted in the collision of the developing volcanic arc with North America. Evidence for this event is visible in the Carolina slate belt of the Eastern Piedmont, which contains metamorphosed sedimentary and volcanic rocks characteristic of an island arc.

The final orogeny occurred somewhere between 250 and 300 million years ago, when Africa collided with North America. At some locations the total landward displacement of the Blue Ridge and Piedmont provinces may have exceeded 250 kilometers, 155 miles. This event displaced and further deformed the shelf sediments and sedimentary rocks that had once flanked the Eastern margin of North America. Today these folded and thrust faulted sandstones, limestones, and shales make up the largely un-metamorphosed rocks of the Valley and Ridge Province. Outcrops of the folded, thrust faulted structures that characterize collision mountains are found as far inland as central Pennsylvania and Western Virginia.

Geologically speaking, shortly after the formation of the Appalachian Mountains, the newly-formed supercontinent of Pangaea began to break into smaller fragments. Because this new zone of rifting occurred east of the suture that formed between Africa and North America, a remnant of Africa remains welded to the North American Plate.

Other mountain ranges that exhibit evidence of continental collisions include the Alps and the Urals. The Alps are thought to have formed as a result of a collision between Africa and Europe during the closing of the Tethys Sea. The Urals, on the other hand, formed during the assembly of Pangea when Baltica, Northern Europe, and Siberia, Northern Asia, collided.