Geology of the Appalachians

Geological history

Overview

The Appalachian Mountains formed through a series of mountain-building events over the last 1.2 billion years:

• The Grenville orogeny began 1250 million years ago (Ma) and lasted for 270 million years.
• The Taconic orogeny began 450 Ma and lasted for 10 million years.
• The Acadian orogeny began 375 Ma and lasted 50 million years.
• The Alleghanian orogeny began 325 Ma and lasted 65 million years.

Proterozoic era

Grenville orogeny

The first mountain-building tectonic plate collision that initiated the construction of what are today the Appalachian Mountains occurred during the Mesoproterozoic era at least one billion years ago when the pre-North-American craton called Laurentia collided with other continental segments, notably Amazonia. All the other cratons of the Earth also collided at about this time to form the supercontinent Rodinia, which was surrounded by one single ocean. The Grenville orogeny occurred between the colliding cratons. The present Appalachian Mountains have at least two areas which are made from rock that was formed during this orogeny: the Blue Ridge Mountains and the Adirondacks.

Breakup of Rodinia

After the Grenville orogeny, Rodinia began to break up. The mountains formed during the Grenvillian era underwent erosion from weathering, glaciation, and other natural processes, resulting in the leveling of the landscape. The eroded sediments from these mountains contributed to the formation of sedimentary basins and valleys.

For example, in what is now the southern Applachian Mountain, the Ocoee basin was formed. Seawater filled the basin. Rivers from the surrounding countryside carried clay, silt, sand, and gravel to the basin, much as rivers today carry sediment from the midcontinent region to the Gulf of Mexico. The sediment spread out in layers on the basin floor. The basin continued to subside, and over a long period of time, probably millions of years, a great thickness of sediment accumulated.

Eventually, the tectonic forces pulling the two continents apart became so strong that the Iapetus Ocean formed off the eastern coast of the Laurentian margin. The rocks of the Valley and Ridge province formed in this ocean over millions of years. Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became limestone, similar to modern oceans. The weathering of limestone exposed at the land surface produces the lime-rich soils that are so prevalent in the fertile farmland of the Valley and Ridge province.

During the break-up of Rodinia, around 600 million to 560 million years ago, volcanic activity was present along the tectonic margins. Mount Rogers, Whitetop Mountain, and Pine Mountain in the Blue Ridge Mountains are all the result of volcanic activity that occurred around this time. Evidence of subsurface activity (dikes and sills intruding into the overlying rock) is present in the Blue Ridge as well. For instance, mafic rocks have been found along the Fries Fault in the central Blue Ridge area of Montgomery County, Virginia.

Paleozoic era

During the earliest part of the Paleozoic, 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 shallow seas. Thick layers of sediment and carbonate rock were deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.

During the middle Ordovician (about 458-470 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 crust, the Iapetus, collided with and began sinking beneath the North American craton. With the creation of this new subduction zone, the early Appalachians were born.

Volcanoes grew along the continental margin, 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.

Mountain building continued periodically throughout the next 250 million years, comprising the Caledonian, Acadian, Ouachita, Hercynian, and Alleghanian orogenies. Continent after continent was thrust and sutured onto the North American craton as Pangea 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 in the Pennsylvanian period, Africa/Gondwana was approaching the North American craton. The collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountains area of Texas. Continental collisions raised the Appalachian-Ouachita chain to a lofty mountain range on the scale of the present-day Himalayas. The massive bulk of Pangea was completed during the Permian period when Gondwana plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the middle of the super-continent.

Mesozoic era and later

Pangea began to break up about 220 million years ago, in the 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.

By the end of the Mesozoic, 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. The ridges of the Appalachian Mountain core represent erosion-resistant rock that remained after the rock above and beside it was eroded away.