Although the Appalachians are older than the Sierra Nevada and the Rocky Mountains, they are not the oldest mountain range in North America. All mountain ranges older than the Appalachians were eroded long ago, and their remains have been largely flattened by glaciers and buried under sediments. Great mountain ranges like the Himalayas have formed in North America a dozen times, but the only evidence they ever existed is rocks deformed by high pressure and heat a few miles below their peaks, which formed billions of years ago.
Mountains form when rocks are compressed between convergent crustal plates, but the most spectacular mountain ranges form when two continents collide and an ocean closes in. These crustal collisions are called orogenies, the technical term for mountain formation, and each orogeny adds rock to the edge of the continent, causing the continent to grow over millions of years.
Throughout Earth’s history, nearly all of the continents have converged, or come together, into supercontinents at various times. When Earth was occupied by one continent and one ocean, there were many supercontinents. Mountain building that occurred during the formation of the two most recent supercontinents is responsible for the formation of the Appalachian Mountains on the east coast of North America.
Precambrian: Supercontinent Rodinia and the Grenville Orogeny
The supercontinent Rodinia formed during the late Precambrian supercontinent about 1.25 to 980 million years ago, long before the first animals existed on Earth, when North America was only a fraction of its present size and is called Laurentia. During the Precambrian, there was no life on land and the continent was bare rock surrounded by oceans that were home to single-celled organisms and some of the earliest multicellular organisms. Laurentia was on the equator, and the Grenville orogeny had formed a huge mountain range along the edge of the continent that would later become the east coast of North America.
Evidence of this Grenville orogeny can be seen primarily in the western Appalachians, the area shaded dark brown on the map at right, as debris from what would later become South America, Africa, and Europe collided with Laurentia. Many of the rocks from these ancient mountain ranges are still buried beneath the modern Appalachian peaks. Millions of years from now, these deeper, older rocks will be exposed by rivers and landslides, and will continue to shape the landscape through the continuing erosion of modern mountains.
Precambrian: Breakup of the Iapetus Ocean and Rodinia Island
Eventually, all supercontinents break up. The Iapetus Ocean formed east of the Grenville Mountains as Rodinia broke apart about 800-650 million years ago. The temperature of the Earth dropped sharply, and a Snowball Earth event trapped the world in ice. Ice is a master of erosion, scraping away mountains and washing their remains into the ocean, washing nutrients into the water. Eventually the world warmed, the ice melted, and continents emerged that were separated again by plate tectonics. Between Laurentia and the Amazon (part of South America) and Baltica (part of Europe), the Iapetus Ocean widened, but it only existed for a few hundred million years before it was closed by mountain building. Its short existence coincided with a rise in oxygen levels in the Earth’s atmosphere and oceans that supported more complex life in the now oxygen-rich and nutrient-rich oceans. The Precambrian ended with the Cambrian explosion, a biodiversification event in these marine areas between the remnants of Rodinia, which marked the beginning of the history of the plants and animals that currently dominate on land.
Paleozoic Era: Supercontinent Pangaea and the Appalachian Orogeny
Less than 300 million years later, the Iapetus Ocean closed and another supercontinent formed. The east coast of North America collided with Europe and Africa, forming the supercontinent Pangaea. Earthquakes and volcanic eruptions caused the Appalachian Mountains to rise, again breaking the edge of the continent. In the map above, all the dark brown areas are mountain building events related to the formation of Pangaea. Parts of the Appalachian Mountains are now found in Africa, Spain, and Scotland, because these lands were once connected by ancient mountain building events that formed Pangaea before the dinosaurs existed. The ancestors of the dinosaurs, giant reptiles, amphibians, and insects, lived on Pangaea, the only supercontinent in the history of Earth to support life. It was also the site of the first forests, whose remains were buried and transformed into the coal deposits that humans rely on.
As Africa and Europe approached North America, the islands between them and the rocks of the Iapetus Ocean floor were compressed, metamorphosed, and faulted into mountains the height of the Himalayas. Buried rocks of the Grenville Orogeny were caught in the shock zone. As rocks were added to the edge of North America, the continent expanded eastward, forming the parallel region of the modern Appalachian Mountains. The orogeny ended, and the first life-bearing supercontinent stretched from pole to pole across the equator. The great ocean surrounding Pangaea is called Panthalassa, Greek for “all seas.” The mountains that formed during the formation of Pangaea were eroded as rivers cut into the rocks that formed in the center of the giant mountain range.
Mesozoic Era: Atlantic Ocean and Breakup of Pangaea
Pangaea was a land of high mountains and vast deserts. Reptiles were especially adapted to this arid world. When the first dinosaurs began to walk the Earth, the vast landmass of Pangaea began to split into the eight continents we know today. The Atlantic Ocean stretched between North America, South America, Africa and Europe.
As Pangaea broke apart, thick lava flows covered the land. Remnants of the eruptions that marked the disappearance of continents and the birth of oceans can be seen in the Eastern Appalachian Mountains and along the Hudson River in the Palisades of New Jersey. Tectonic activity along the east coast of North America gradually subsided, but erosion continues to shape the mountains we know today.
The Appalachians as we know them are remnants of a once-great mountain range that recorded the formation and disappearance of two supercontinents and an ocean, and the formation of the still-growing Atlantic Ocean. Today, we walk over rocks that were once buried miles below their peaks and see the cores of ancient mountains exposed by eons of wind, rain, snow, ice, and landslides. We peer deep into past supercontinents and use rocks preserved in the mountains to piece together the geological history of North America.
