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Earth Story: The Appalachians

The Appalachians are older than the Sierra Nevadas and the Rockies, but they are not the oldest mountain chain in North America. All the mountain chains older than the Appalachians have long since eroded away, their remnants mostly leveled by glaciers and buried by sediments. Massive mountains chains like the Himalayas have formed in North America more than a dozen times, but the only evidence they ever existed are the rocks that were morphed by the high pressures and temperatures miles beneath the peaks that formed billions of years ago.


Types of orogenies caused by tectonic convergence
Types of convergent tectonic boundaries differentiated by what kind of crust is converging, from https://csmgeo.csm.jmu.edu/geollab/fichter/wilson/orogytyp.html

Mountain chains form as rocks are compressed between converging tectonic plates, and the grandest of them form when an ocean is closed as 2 continents crash into each other. These tectonic collisions are called orogenies, a fancy word for mountain formation, and every orogeny adds more rock to the edge of a continent allowing it to grow over millions of years.


A map of the mountains chains that formed during the last 2 supercontinent formation events
Geologic map of North America east coast mountains that formed during 2 orogenies from https://www.sciencedirect.com/topics/earth-and-planetary-sciences/grenvillian-orogeny

Multiple times throughout Earth history, nearly all the continents converged, or assembled, into supercontinents. Many supercontinents have existed in the past, at times when the Earth is dominated by one continent and one ocean. The orogenies that occurred during the formation of the two most "recent" supercontinents are responsible for the creation of the Appalachians Mountains of the east coast of North America.


Precambrian: Supercontinent Rodinia & the Grenville Orogeny

A map of Rodinia Supercontinent and the mountain chains that formed as the supercontinent assembled
The beige cratons are the continents that converged at the green bands, which designate orogenies that occurred as Rodinia formed ~1.1Ga (=1,100Ma = million years ago). https://antarcticsun.usap.gov/science/2497/

As Supercontinent Rodinia formed during the late Precambrian Super-Eon ~1250-980 million years ago, long before the first animals existed on Earth, the North American continent was a fraction of its current size and is referred to as Laurentia. During the Precambrian there was no life on land, continents were bare rock surrounded by oceans that harbored single-celled organisms and a few of the earliest multi-cellular organisms. Laurentia was on the equator as the Grenville Orogeny was forming a massive mountain range along the edge of the continent, the same edge that would later be North America's east coast.


A map of the rocks that formed in the mountain chains as Rodinia assembled
Map with locations of Grenville Orogeny Rocks https://earthathome.org/hoe/se/geologic-history/

Pieces of what would later be South America, Africa & Europe crashed into Laurentia, and the evidence of this Grenville Orogeny is visible mostly in the western Appalachians, in areas colored darker brown on the map to the right. Much of these ancient mountain chain rocks are still buried beneath the peaks of the modern Appalachians. In million of years from now these deeper older rocks will be exposed by rivers and landslides that continue to shape the landscape by the ongoing erosion of the modern mountains.


Precambrian: The Iapetus Ocean & the Break-Up of Rodinia

A map of Earth continents after Rodinia
Pieces of Rodinia drifting apart forming the Iapetus Ocean ~565Ma, from https://earthathome.org/hoe/ne/geologic-history/

Eventually every supercontinent breaks up. As Rodinia broke apart ~800-650 million years ago, the Iapetus Ocean formed to the east of the Grenville mountains. Global temperatures plummeted and a Snowball Earth event locked the world in ice. Ice is a master of erosion, and it abraded the mountains and washed their remains into the oceans, flushing nutrients into the water. Eventually the world warmed and the ice melted, revealing continents once again separated by plate tectonics. Between Laurentia and Amazonia (a piece of South America) and Baltica (a piece of Europe), the Iapetus Ocean widened but would only exist for a few hundred million years before it too was closed by an orogeny. It's short existence coincided with rising global atmospheric and oceanic oxygen levels, which supported more complex life forms in the now oxygenated and nutrient-rich oceans. The Precambrian ended with the Cambrian Explosion, a biological diversification event in these oceans between the remnants of Rodinia, an event that initiated the history of plants and animals that now dominate the land.


Paleozoic Era: Supercontinent Pangaea & the Appalachian Orogeny

A map of Pangaea with mountain chains highlighted in dark brown
Pangaea Supercontinent ~255 million years ago from https://ucmp.berkeley.edu/history/wegener.html

After less than 300 million years the Iapetus Ocean closed as another supercontinent formed. The east coast of North America crashed against Europe and Africa forming Supercontinent Pangaea. Once again the edge of the continent crumpled as earthquakes and volcanic eruptions uplifted the Appalachian Mountains. In the map above, all the dark brown regions are orogenies associated with the formation of Pangaea. Pieces of the Appalachians are also found in Africa, Spain, & Scotland today, as these lands were once joined by ancient orogenies that assembled Pangaea before the dinosaurs existed. The predecessors of the dinosaurs, the giant reptiles, amphibians, and insects inhabited the lands of Pangaea, the only supercontinent in Earth history to harbor life. This was also the land of the first the first forests, the remnants of which would be buried and morphed into the coal deposits that our species is so reliant upon.


A map of the Appalachians, Ouachitas, and Marathon Mountains, remnants of the assembly of Pangaea
Modern Appalachian Mountains geologic map with National Park locations, from https://www.nps.gov/subjects/geology/plate-tectonics-collisional-mountain-ranges.htm

As Africa and Europe approached North America, the islands in between and the rocks of the Iapetus Ocean floor were squeezed, metamorphosed, and faulted into Himalaya-height mountains. The buried rocks of the Grenville Orogeny were caught within the crumple zone. As rocks were added on to the edge of North America the continent grew eastward, creating parallel provinces of the modern Appalachians. The orogeny ended and the first Supercontinent to harbor life spanned from pole to pole as it straddled the equator. The great ocean that surrounded the Pangaea is called Panthalassa, Greek for "all sea". The mountains that formed during Pangaea's assembly eroded as rivers carved down towards the rocks that formed in the heart of the massive mountain chain.


Mesozoic Era: The Atlantic Ocean & the Break-up of Pangaea

Pangaea Supercontinent 240Ma
Pangaea Supercontinent 240Ma at the beginning of the Mesozoic Era, from https://paleonerdish.wordpress.com/2015/01/28/the-triassic-paleoclimate/

Pangaea was a land of tall mountains and vast deserts. The reptiles were especially suited to this dry world. As the first dinosaurs were walking upon the Earth the vast lands of Pangaea began to be pulled apart into the 8 continents we have today. Between North America, South America, Africa and Europe the Atlantic Ocean opened.

A side view of the rocks and faults within the Appalachians
Geologic cross-section (side view) of the Appalachians from https://gotbooks.miracosta.edu/geology/regions/blue_ridge.html

As Pangaea broke up, the land was inundated by thick lava flows. The remnants of the eruptions that heralded the demise of a continent and the birth of an ocean are found in the eastern Appalachians, and along the Hudson River in the Palisades of New Jersey. The tectonic activity along the east coast of North America slowly subsided, but erosion continues to shape the land into the mountains we know today.


A maps of the modern day Atlantic ocean and the mountains that formed prior to the ocean's existence
The mountains that formed during the Appalachian Orogeny ~250Ma are in yellow and the Mid-Atlantic Ridge forms the Atlantic Oceanfrom https://iat-sia.org/about/

The Appalachians that we know are all that remains of a once massive mountains chain that recorded the formation and demise of two supercontinents and an ocean, and the formation of the Atlantic Ocean which continues to widen today. Today we walk upon the rocks that were once buried many miles beneath the peaks, we view the heart of the ancient mountains have been uncovered by years of wind, rain, snow, ice, and landslides. We see into the depths of supercontinents of the past and use the rocks preserved within the mountains to piece together the geologic history of North America.

Appalachian Mountains view from a peak
View within the Appalachians, from https://peakvisor.com/range/appalachian-mountains.html
Satellite view of the Appalachians
Appalachians from above, from https://earthobservatory.nasa.gov/images/80088/appalachian-mountains

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