The Appalachian Mountains and Plate Tectonics

 

 

            The cataclysms that are manifest in earthquakes, eruptions and tsunamis are testimony to the power of geologic forces.  As we seek to understand the vastness of the cosmos and the minutiae of genetic processes, we are humbled by the inexorable passage of geologic time and the occasion of geological events. Understanding the phenomenology of these events has been the quest of mankind for millennia. But it wasn't until the late 18th century that Scotsman James Hutton made the seemingly innocuous observation that what was then occurring had always occurred. This first principle of geology, known as uniformitarianism, provides the basis for such observable features as rock strata that are similar in structure and composition to sediments collecting in streambeds and, by association, the notion of sedimentary rocks.  But uniformitarianism does not adequately account for earthquakes.

            That the continents may have a relationship to each other was likely first manifest when accurate maps became available in the 18th century and the striking similarity between the east coast of South America and the west coast of Africa was first noted.  Alfred Wegener, a German meteorologist, first introduced the theory of continental drift in an article published in 1912. By reassembling the continents that contained similar glacial striations and similar assemblages of fossil plants (Glossopteris flora), he posited that South America, Africa, India, Australia and Antarctica were at one time a single landmass he named Gondwanaland (from a region in central India which gave its name to Gondwana sedimentary rocks that were found to exist on other continents, thus supporting the theory).

            By the middle of the 20th century, the theory of independent floating continental landmasses was supplanted by the theory of plate tectonics; the continents being higher elevation portions of their respective plates.  This was precipitated by several antecedent observations.  Oceanographers had determined that the ocean ridges are connected in a worldwide system and that the ridges were actually areas where the crust was being pulled apart and new crust was forming. At about the same time, it was noted that earthquakes occurred in areas where the ocean basin trenches dip beneath the edge of a continent like South America or an island arc, like Japan.  These zones were named subduction zones to account for the resorption of the continental crust into the mantle, thus maintaining a geologic balance for the crust created in the expanding ocean ridges. In 1968, three American geophysicists named Isacks, Oliver and Sykes originated the theory of plate tectonics, with the earth divided into about twelve major plates and several minor ones separated by oceanic ridges and subduction zones.

             The story of the Appalachian Mountains begins where the Eurasian Plate and the North American Plate come together.  It is a complicated story with a global scale, a continental scale and a local (Blue Ridge) scale.  It is now theorized that the continents have come together through plate movements as many as six times in the last 3 billion years, about once every 500 million years.  However, it is only the last three that are of direct concern; the geologic history of the Appalachians, as it is currently understood, being therein explained.  About 1.2 million years ago, the Grenville orogeny (from the Greek oros meaning mountain -- the formation of mountains through structural disturbance of the earth's crust) occurred as the plates came together to form a single large landmass named Rodinia. 

            The crust at the interface buckled and formed the Grenville Mountains, believed to have been as high as the Rocky Mountains are today, as they were formed by a similar process. The temperatures and pressures that were generated within the mountains by the grinding plates were such that the crustal rocks melted to form magma. This magma slowly cooled, forming the backbone of the current Appalachian Mountains, most visible as Old Rag Granite at Old Rag Mountain and as granodiorite of the Pedlar Formation at Mary's Rock. As the Grenville Mountains eroded during this early period, the coarse grained, conglomerate Swift Run Formation was formed.

            According to the plate tectonic model, the super continent Rodinia broke up about 750 million years ago, resulting in the formation of the Iapetus Ocean, the proto-Atlantic.  (Iapetus was one of the twelve titans of Greek mythology and was the father of the god Atlas, for whom the Atlantic is named). It is theorized that these breakups occur because the large continental mass acts like an insulator over the molten magma underneath. The magma eventually heats up to the point that it rises to the surface and drives the continents apart. In the case of the Blue Ridge, the magma flowed out onto the surface and covered the Old Rag Granite, the Pedlar and the Swift Run formations with thick layers of extrusive, volcanic igneous rock which we know as the Catoctin Formation.

            One of the dilemmas facing geophysicists is whether the formation of supercontinents always occurs in the same direction; that the Atlantic keeps opening and closing (the so-called accordion or introversion theory).  The other alternative is that the continents break up and continue to move apart until they form a new supercontinent at the antipodes of the first (the exterior ocean or extroversion theory).  Recent research to resolve this dilemma uses the amounts of the "rare earth" elements Samarium (Sm) and Neodymium (Nd). Samarium is smaller than Neodymium and will stay in the solid mantle whereas the larger Neodymium will migrate to the liquid melted magma. The crux of the argument is that the Sm/Nd ratio (which is 0.32 for the earth as a whole) is 0.2 in crustal rocks (more Nd, less Sm) and 0.5 in mantle rocks (more Sm, less Nd).

            Using this ratio, geophysicists can determine when a crystallized rock left the mantle, and thus whether a specimen from oceanic lithosphere (earth's crust) is younger or older than one from the continental landmass. In the exterior ocean model, the ocean will be older than the continent.  Based on this theory, the continent Pannotia formed after the breakup of Rodinia due to extroversion, as the continents now known as Australia, Antarctica, and the amalgamation of South America and Africa collided about 550 million years ago. This had no appreciable effect on the Appalachians, however, and will hence no longer concern us here.

            What does concern us is what happened to the Grenville Mountains.  They eroded, probably much more rapidly than uniformitarianism might indicate, as there was no vegetation to check the flow.  This occurred for hundreds of millions of years, depositing layers of sediments up to 60,000 feet in depth on top of the Swift Run and Catoctin Formations.  Sand particles were deposited and gradually compressed into the quartzite of the Weverton Formation. As the Iapetus Ocean widened and became deeper, finer grained sediments formed the Harpers (Hampton) Formation and the succeeding Antietam (or Erwin) Formation. the Weverton, Harpers and Antietam Formations are collectively known as the Chilhowee Group.   The Skolithos Ichnofossils, trace fossils of the burrowing of marine worms, are evidence of the tidal basin paleoenvironment of these formations.     

            In keeping with the theory that supercontinents must eventually break up due to the magma heating effect, Pannotia broke up and set in motion the movement of the tectonic plates that ultimately resulted in the closure of the Iapetus Ocean and the creation of Pangea (from the Greek pan meaning all and ge, earth).  This process started in the Ordovician Period, about 450 million years ago. As the Eurasian plate descended under the North American plate, the hardened sediments that had been formed by the erosion of the earlier Grenville Mountains were uplifted in what is known as the Taconic orogeny, affecting primarily the northern Appalachians. As the subduction of the Eurasian plate continued, the continental landmasses moved closer and closer until they collided in the Devonian Period, about 400 million years ago. This second event, known as the Acadian orogeny, thrust up the northern Appalachians a second time.

            The central and southern Appalachians were formed by the collision of the African plate with the combined Eurasian and North American plates in the Alleghany orogeny in the Permian period, about 250 million years ago.  This was the final uplift of the Blue Ridge Province.  The supercontinent Pangea did not stay together long, by geologic standards.  Northern and southern continental landmass groupings first opened to create the Sea of Tethys about 200 million years ago (Tethys was the sister and consort of Oceanus, the Greek god of the ocean; in keeping with the Iapetus tradition, both were titans).  Europe and North America forming the northern Laurasia, and South America, Africa, India, Australia and Antarctica forming the southern Gondwanaland (remember Wegener). And then the erosion began yet again, ultimately revealing what we now see, the 1.2 billion year old plutonic core surrounded by the magmas and sediments that are the inferred evidence of a complex history.

            It is beyond our capacity as ephemeral transients to truly comprehend the time frames that encompass the geologic events that have created our small corner of the earth.  It is only when an irresistible geologic force meets an immovable geologic object and cataclysm results that we take note.  But one can stand atop Old Rag Mountain and imagine what has happened, and seek to understand where we may fit in. Or maybe we have it all wrong and there is something obvious that we have missed.  Only geologic time will tell.  And we will therefore never really know.