Geology of Great Smoky Mountains National Park
Geology of the Smokies
Sedimentary Rock is Formed
The landscape of the Great Smokies has undergone profound changes throughout the ages. The rocks in this area are mostly sedimentary. That is, they were formed by deposits of silt, sand, and gravel carried down from higher regions by erosion and deposited into a sea. As more and more sediments were deposited and buried, they became layers of hard rock tens of thousands of feet thick. This process took millions of years and, for most of the rocks in the park, occurred before the Cambrian period at the beginning of the Paleozoic Era more than 545 million years ago.
Sunrise in Great Smoky Mountains by William Britten
Life on earth at the time of the formation of this sedimentary rock is thought to have been very primitive: bacteria, some algae, and small invertebrate marine animals. These primitive life forms left very few readily seen fossils in this sedimentary rock. As time progressed into the Paleozoic Era, life forms became more complex. Recognizable fossils have only been documented in the unmetamorphosed limestone in Cades Cove and White Oak Sink, and in shale and sandstone along the Foothills Parkway.
Between about 310 to 245 million years ago, the eastern edge of the North American tectonic plate collided with the African tectonic plate becoming part of a "supercontinent" known as Pangaea. Continental collisions take place at a rate of a few inches per year over many millions of years and are the result of continuing global-scale plate tectonics. Evidence of earlier plate tectonic geologic events are found in rocks of the Great Smoky Mountains attesting to an incredibly long and active geologic history in this area. During one of these earlier continental collisions, tremendous pressures and heat were generated, which changed or “metamorphosed” the sedimentary rocks found in the Smokies. For example, sandstone became recrystallized to metasandstone or quartzite, and shale became slate, as the rocks were heated, squeezed, and welded together.
The last great episode of mountain building uplifted the entire Appalachian mountain chain from Newfoundland, Canada to Alabama. These mountains probably were much higher than they are today, with elevations similar to today's Rocky Mountains. As the African tectonic plate gradually pushed against the edge of the North American tectonic plate, the original horizontal layers of the rocks were bent or folded and broken by faults. Huge masses of older, deeply buried rocks were pushed northwestward, up and over younger rocks along a large, nearly flat-lying thrust fault, known as the Great Smoky fault. In Cades Cove, younger limestone was overridden by much older metasandstone along the Great Smoky fault during this time of mountain building. Today, erosion has removed some of these older rocks exposing the younger limestone in the floor of the cove. The older rocks lying above the limestone were pushed tens to hundreds of miles from the southeast to their present location.
Although small earthquakes occur in eastern Tennessee, they probably are not associated with the numerous ancient geologic faults in Great Smoky Mountains National Park. These ancient faults, such as the Great Smoky fault, may have formed when earthquakes were more common. Today, no evidence of recent movement along these faults has been found in the park.
Wind and Water
Following this final episode of Appalachian mountain building, the supercontinent of Pangaea broke apart, and the North American and African tectonic plates gradually moved to their present position with the newly formed Atlantic Ocean between them. The new rugged highlands, the ancient ancestors of the Smokies, were subject to intense erosion from water and wind, as well as breakdown from mechanical agents such as freezing and thawing, and chemical dissolution of rocks. As mountain valleys were carved, tremendous quantities of eroded sediment were transported toward the Atlantic Ocean and Gulf of Mexico by rivers and streams.
It is fascinating to realize that much of the beautiful sand that makes up our Atlantic coastal beaches was derived from recycling of eroded metasandstone that once formed the highest mountain peaks in the ancestral Appalachians. Today, however, all streams that drain the Great Smoky Mountains National Park flow eventually into the Mississippi River and into the Gulf of Mexico.
As the mountains were worn down, the layers of rock most resistant to the forces of erosion were left to form the highest peaks in the park today, such as the hard metasandstone on top of Clingmans Dome. Most of the beautiful waterfalls in the park were formed where downcutting streams encountered ledges of very resistant metasandstone that erode more slowly than the adjacent slate or metasiltstone.
Today, streams and rivers are the dominant forces that erode the mountains, although landsliding, chemical dissolution, and rocks cracking from roots and freezing and thawing all contribute to the wearing away of the Smokies. Geologists estimate that the mountains are being eroded about 2 inches every thousand years. Although this seems incredibly slow by human time scales, the 200 million years of geologic time that have passed since the Smokies formed is ample time to erode and lower them to their present height.
Glaciation and Abundance
From almost two million to ten thousand years ago, the movement of ice sheets during the Pleistocene Epoch further modified the environment of the Smokies. Although the glaciers did not reach this far south, their influence on the climate of the Smokies region produced alpine conditions on the higher crests. The numerous boulders found on the slopes and in the streams today originated during this time as they were broken off cliffs and ledges by freeze and thaw activity. Various species of Canadian-zone flora and fauna retreated south to suitable habitats in the Smokies.
The glacial influence on the Smokies climate, coupled with the range of elevation and the southwest/northeast layout of these mountains, accounts for much of the striking variety of living things found in the park.
Most other mountainous regions were formed by uplifts similar to those that formed the Great Smoky Mountains. The highlands of the Smoky Mountains have been a long term product of tectonic uplift and subsequent erosion over a period of hundreds of millions of years, as opposed to younger (tens of millions of years) mountains, like the Rockies. The latter have higher relief and are more rugged since they have not been around long enough to be eroded down more.