A Brief Geological History of the Appalachian Trail

It is satisfying to be able to look at a simulated view of the earth from space and clearly see the trace of the entire Appalachian Trail (AT)(Figure 2). The approximate 2200 mile long footpath extends from Mt. Katahdin, Maine to Springer Mountain, Georgia mostly following ridgelines made of tough, resistant rocks, like sandstones and granites. The entire collection of mountain chains, each having its own unique geological make up, origin and history, are collectively known as the Appalachians. These peaks and highlands are merely the roots of a much greater, once more massive mountain chain created during at least four orogenies, or mountain building episodes, beginning over a billion years ago. These discreet orogenies, from oldest to youngest, are known as: Grenvillian, Taconian, Acadian, and Alleghanian (Figure 3). To understand how and why these each occurred requires a brief discussion of plate tectonics.

In 1912, Alfred Waggener, a German geophysicist and meteorologist was the first scientist to propose the continental drift hypothesis, the idea that the earth’s crust is comprised of individual slabs, or plates, that slowly move around the surface of our planet, driven by some powerful unknown forces. Originally met with much skepticism, criticism and debate, this theory was eventually embraced by the scientific community following many decades of research that yielded overwhelming evidence. The theory evolved into plate tectonics, and by the 1960s and 1970s was being taught to geology students all over the world. Even as a student in the 1970s, I can still recall being taught the “theory” of plate tectonics.

Our 4.6 billion year old planet reveals an incredible story about its origin and geological history. Nearly all of the physical and geological processes involved in the creation of the earth are cyclical, on multiple and seemingly unending time scales. There are the very short duration cycles such as tides, sunrises and sets, and longer duration multi-millennial astronomical cycles, known as Milankovitch cycles, which control glacial periods. Finally, there are hundreds of million year periods, known as Wilson cycles. These very long-term cycles were named after Dr. John Tuzo Wilson, a famous Canadian geologist/geophysicist. Wilson cycles begin with the rifting, or breaking up of a supercontinent, which in turn forms a new ocean by sea floor spreading. The final phase is the coming back together of the fragmented continents to reform a new supercontinent, thus completing the cycle. Over the past 3.6 billion years (or Giga-annum, abbreviated as Ga) of geologic time there may have been as many as 7 supercontinents, (Vaalbara, Ur, Kenorland, Nuna, Rodinia, Pannotia and Pangaea) and thus 7 Wilson cycles, each fragmenting, drifting and returning together again by the process of plate tectonics. The Appalachians were formed during two of the last three Wilson cycles, involving the formation and break up of the two ancient supercontinents, Rodinia and Pangaea.

During the formation of Rodinia, much of the North American continent was already assembled but was still missing parts of its present day eastern margin. About 1.3 Ga, a long-lasting 250 million year, (or Mega-annum, abbreviated as Ma) period of mountain building events, collectively known as the Grenvillian orogeny occurred in multiple phases, accreting large swaths of continental crust known as Grenville terrane, onto the now eastern part of North America. The mountains formed during this orogeny were Himalayan in scale, and eventually eroded over time, delivering sediment into nearby shallow seas, such as the Ocoee Basin. This important sedimentary basin contains incomprehensible volumes of eroded sand, silt, and clay-sized particles, which would eventually become rock and comprise the majority of the bedrock in the Great Smokies.

The crustal plates assembled to create this supercontinent remained in place for about 250 million years, as the great mountains continued to erode. Around 750 million years ago Rodinia’s plates began to diverge from one another, as deep convection currents in the highly viscous but fluid lower mantle began to drive these individual jigsaw pieces of the earth’s crust in new directions. East (present coordinates) of the newly attached Grenville terrane, a rift developed, eventually resulting in the formation of a new ocean, known as the Iapetus. A new Wilson Cycle was beginning. As these plates wandered around the early planet, the deep powerful forces that pulled them apart began to once again reunite them in fits and starts, where multiple collisions and retreats finally resulted in a reunification of earth’s last supercontinent, known as Pangaea.

During the assembly of Pangaea, three more collisions, or orogenies, occurred along the present day eastern North American continental margin. Each one of these collisions caused a crumpling (faulting and folding) of the crust and the formation of long extensive mountain chains. In addition, emplacement of igneous intrusions (plutonic rocks) and extrusions (volcanic rocks) occurred throughout these time periods. The three orogenies all took place during the Paleozoic era, and are named Taconian, Acadian and Alleghanian (Figure 4).

The Taconian occurred about 460 Ma when a volcanic island arc collided with Laurentia, resulting in the formation of a new mountain range along the present day Piedmont province, extending from present day Alabama to Newfoundland. As these mountains eroded, copious amounts of sediment (sand, silt and mud) entered surrounding low areas and were deposited in rivers, shorelines and deltas. It was at this time period that the massive pulses of Silurian sandstones were deposited, resulting in some of the most resistant bedrock and ridge forming units in the Appalachians.

After about 100 million years, another collision occurred between a microcontinent, Avalonia, and Laurentia during the Acadian orogeny roughly 380 Ma. More mountain chains formed, known as the Acadian Mountains and slowly eroded sending sediment into growing sedimentary basins to the west, such as the Appalachian, Warrior, Illinois and Michigan Basins. These growing basins slowly depressed the existing continental crust, which had been worn down over the millennia to near flattened, eroded terrain, known as peneplains. The most significant deposit during this time was the Devonian age Catskill delta, spreading sediment for hundreds of miles westward into several actively subsiding basins.

Finally, the ancient continents of Baltica and Gondwana collided with Laurentia, closing the Iapetus Ocean and forming another mountain chain about 300 Ma. This was the final of the three Paleozoic orogenies, and caused folding and thrust faulting of pre-existing rock layers, resulting in an immensely wide and long new mountain range, stretching from present-day eastern Greenland to south Texas. Each one of these orogenies caused varying degrees of either further burial, or partial exhumation of the rocks that were already deeply buried in the piedmont and Blue Ridge areas. The tremendous heat and pressure associated with these orogenies resulted in the formation of metamorphic rocks from previous sedimentary or igneous rocks. In addition, some volcanism occurred during these collisions, resulting in extrusive igneous, or volcanic rock deposits. During relaxation or extensional phases (interorogenic periods) the crust rifted or separated in some places, allowing for deep mafic (lower mantle) molten igneous rocks to rise and flow onto the surface as flood basalts. These rocks can be seen along the AT in places such as the Shenandoahs.

After about 50 million years this supercontinent began to rift, or pull apart once again, first opening up long rift valleys in the early part of the Triassic that formed extensive lakes similar to the east African rift valleys of today. Continued rifting along the continental margin between North America and Africa soon began to open up a new ocean basin we know today as the Atlantic. This began a new Wilson cycle, still going on today. As the Atlantic continued to open, there were places along our continent’s margin, which continued to rift or crack, episodically allowing more magma to rise up from deep within the earth and eventually cool to the rocks we presently see at the surface.

The rocks exposed along the Appalachian Trail are the remnants of once much greater mountain chains, which over time have continued to erode to their mere roots. Over the past centuries, many thousands of well-trained geologists from all over the world have worked diligently to reconstruct the geologic history of this fascinating portion of earth’s surface we know today as the Appalachian Mountains (figure 5).