The Great Smoky Mountains are one of the oldest mountain ranges in the world, and the rocks there reveal a fascinating history.
The oldest rocks that can be found in the Great Smokies, and throughout the Southern Appalachians, are over a billion years old. This bedrock traces its origins to the formation of an ancient supercontinent called Rodinia (There is actually evidence of several supercontinents that have formed and broken over the course of Earth’s history!). Much of the core of North America is composed of remnants of the Rodinia supercontinent.
Around 750 million years ago, the supercontinent began to pull and break apart. As low areas were created, new oceans formed over time. One of the oceans was the Ocoee Basin, which formed near present-day western Carolinas, eastern Tennessee, and northern Georgia, where the Great Smokies sit today. Rivers and streams carried massive amounts of sediment into this basin over millions of years. These layers of sediment were eventually cemented into layers of rocks over nine miles thick.
Younger sedimentary rocks in the Great Smokies were formed 450 to 540 million years ago. These rocks were formed when what is now the Appalachian region was a shallow marine continental margin. Sediments deposited here formed limestone rocks and fossils can be found among them. Fossils here include worm burrows and shells of tiny crustaceans.
Deformation and Mountain Building
About 470 million years ago, the continents that had pulled apart started moving towards each other again. The movement of continental plates is slow, occurring at a rate of a few inches per year over many millions of years. But eventually, what we now know as Africa collided with the eastern edge of ancestral North America around 270 million years ago, part of the formation of the supercontinent Pangea. As ancestral North America and Africa collided, the entire Appalachian region was uplifted, creating mountains whose elevations were likely higher than the Rockies today. Continental collisions generate huge amounts of pressure and heat, which has a number of interesting effects.
One of those effects is the creation of plutons. As the continental blocks rode over each other, the tremendous pressure and heat melts some rocks. If the molten rock makes it to the surface it forms volcanoes or lava flows. If it stays underground, however, it cools and hardens forming igneous blocks of rock called plutons. The heat and pressure also metamorphized the sedimentary rocks that had previously existed in the area. The process of metamorphosis turned sedimentary rocks like sandstone and shale into quartzite and slate, respectively.
Yet another process associated with the continental collision was folding and faulting. Faults are fractures that occur between blocks of rock where movement occurs (the link provides information from USGS on different types of faults and examples throughout the world including animations). Earthquakes are the result of sudden movement of the blocks of rock at a fault. During this time, earthquakes would have been commonplace in the Southern Appalachians. In fact, we can see evidence of faulting in the Great Smoky Mountains National Park today. Typically, younger rocks lie on top of older rocks, but in Cades Cove in the Great Smoky Mountains the limestone of the valley is younger than the surrounding mountains. This is a result of the older rocks being pushed over top of the younger rocks when faulting occurred. Over time, the older rocks were weathered away giving as “window” to see the younger rocks underneath.
Around 240 million years ago, the continents began pulling apart again in the breakup of Pangea. This continental breakup is what formed the Atlantic Ocean as we know it. In fact, the Atlantic Ocean is still getting wider today. The previously uplifted Appalachian Mountains were subject to intense erosion. The Appalachian Mountains that we see today are just the remnant cores of the mountains that stood some 100 million years ago. The sediment was carried away towards the Gulf of Mexico and Atlantic Ocean. As sandstone and quartzite from the mountains is weathered, eroded, and transported away from the mountains it is broken into smaller and smaller pieces ultimately forming sand. Some of that sand eventually finds its way to the beaches of the Gulf and Atlantic.
Rates of erosion were not constant across the Appalachian Mountains. Different types of rocks erode at different speeds. Rocks made from hard pebbles and sand are more resistant to weathering and erosion and form the peaks of the mountains we see today. Softer rocks made from fine-grained sediments, like silt and clay, are more easily weathered and are found in lower areas. The difference in weatherability between rock layers is what allows for the waterfalls that can be found in the Smokies today. Water has amazing erosional power, particularly in times of flood and high-water flow.
Starting about three million years ago, Earth experienced a series of ice ages, during which huge sheets of ice extended up or down from the poles. During the last period of glaciation, around 1.1 million to 11 thousand years ago, glaciers dominated the landscape just some 200 miles north of Great Smoky Mountain National Park. The effects of glaciation can be seen in both the geologic record and in the wildlife and plants present in the park today. One piece of evidence of the glaciers and freezing temperature is the presence of boulders in Great Smoky Mountains National Park. Water permeates through the rock layers and then expands when frozen, pushing the rocks apart. As this happened over and over again boulders were formed as rock was broken apart into giant chunks. Cold temperatures also pushed animals and plants native to more northern climates southward and into the Southern Appalachians. Many of those animals can still be found in the park today.