The setting where two plates collide and subduction occurs is manifested by which feature on earth?

Sometimes an entire ocean closes as tectonic plates converge, causing blocks of thick continental crust to collide. A collisional mountain range forms as the crust is compressed, crumpled, and thickened even more. The effect is like a swimmer putting a beach ball under his or her belly—the swimmer will rise up considerably out of the water. The highest mountains on Earth today, the Himalayas, are so high because the full thickness of the Indian subcontinent is shoving beneath Asia.

The Appalachian Mountains formed during a collision of continents 500 to 300 million years ago. In their prime they probably had peaks as high as those in the modern zone of continental collision stretching from the Himalayas in Asia to the Alps in Europe. But over the past 300 million years, the Appalachians have eroded to more modest heights.

Collisional Mountain Range Development

Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

The Appalachian Mountains extend from the Maritime Provinces of Canada all the way to northern Georgia and Alabama. The continental collision zone extends even farther southwestward, but young sediments of the Gulf coastal plain cover most of it. It does surface as the Ouachita Mountains of western Arkansas and southeastern Oklahoma, and the Marathon Mountains of west Texas. The Brooks Range is another, younger zone of continental collision, stretching across northern Alaska.

Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Appalachain Mountains [12 parks]

• ACAD—Acadia National Park, Maine—[Geodiversity Atlas] [Park Home]

• BISO—Big South Fork National River and Recreation Area, North Carolina & Tennessee—[Geodiversity Atlas] [Park Home]

• BLRI—Blue Ridge Parkway, Virginia & North Carolina—[Geodiversity Atlas] [Park Home]

• BLUE—Bluestone National Scenic River, West Virginia—[Geodiversity Atlas] [Park Home]
• CHAT—Chattahoochee River National Recreation Area, Georgia—[Geodiversity Atlas] [Park Home]
• DEWA—Delaware Water Gap National Recreation Area, Pennsylvania—[Geodiversity Atlas] [Park Home]

• GARI—Gauley River National Recreation Area, West Virginia—[Geodiversity Atlas] [Park Home]

• GRSM—Great Smoky Mountains National Park, North Carolina & Tennessee—[Geodiversity Atlas] [Park Home]
• NERI—New River Gorge National River, West Virginia—[Geodiversity Atlas] [Park Home]

• OBED—Obed Wild and Scenic River, Tennessee—[Geodiversity Atlas] [Park Home]

• RUCA—Russell Cave National Monument, Alabama—[Geodiversity Atlas] [Park Home]

• SHEN—Shenandoah National Park, Virginia—[Geodiversity Atlas] [Park Home]

NPS photo.

NPS photo.

NPS photo.

Several National Park Service sites are in the mountains that lie inland from the Atlantic and Gulf coasts of the United States. A visit to one of those sites reveals not only beautiful mountain scenery, but also rocks and topography that tell a story of ancient episodes of drifting plates and crashing continents.

NPS Sites in Appalachian-Ouachita-Marathon Collisional Mountain Range

The Appalachian Mountains are part of a collisional mountain range that includes the Ouachita Mountains of Arkansas and Oklahoma, and the Marathon Mountains in west Texas. After 300 million years the mountains have eroded deeply and are covered in places by young sediments of the Atlantic and Gulf coastal plains. Four-letter codes refer to some of the National Park Service sites listed near the top of this page. The Blue Ridge Parkway follows the crest of the mountains in Virginia and North Carolina, connecting Shenandoah and Great Smoky Mountains national parks. Hot Springs National Park is in the Ouachita Mountains, while the Marathon Mountains extend into Big Bend National Park. Acadia National Park is in the northern Appalachians in Maine. D-D’ is the line of the cross section showing in the tectonic provinces of the Appalachian Mountains.

Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Tectonic Development of the Appalachian—Ouachita—Marathon Mountain Range

The Appalachian Mountains, along with the Caledonide Mountains in Greenland, the British Isles and Scandinavia, as well as the Atlas Mountains in northeastern Africa, are parts of a continental collision zone that formed 500 to 300 million years ago. Later rifting opened the Atlantic Ocean, isolating the mountains as separate ranges on different continents.

Iapetus Ocean Opens. Land that will later become Florida is part of Africa. The Yucatan Peninsula and Cuba may have been on the north side of South America.

Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Iapetus Ocean Closes. Pangea forms as the continents collide. The Appalachians are part of a larger zone of continental collision that includes the Marathon and Ouachita mountains in the southern United States, the Atlas Mountains in Africa, and the Caledonide Mountains in Greenland, the British Isles, and Scandinavia.

Atlantic Ocean and Gulf of Mexico Open. The modern oceans originated about 200 million years ago when Europe, Africa and South America ripped away from North America. Fragments of the collision zone mountains were left on three continents: the Appalachians in North America, the Atlas in Africa, and the Caledonides in Europe. The Atlantic continues to widen along the Mid-Atlantic Ridge. The Gulf of Mexico ceased opening about 100 million years ago, when tectonic activity shifted eastward to the Caribbean region. Continental collision is occurring today where Africa and India ram into Europe and Asia, forming the Alpine-Himalayan chain.

The continents of Africa, South America, Australia and Antarctica were originally part of a large supercontinent called Gondwanaland. Prior to 500 million years ago, the ancient North American continent was separated from Gondwanaland and Eurasia by a large body of water known as the Iapetus Ocean. As the Iapetus Ocean closed along subduction zones, various volcanic islands and continental fragments (terranes) collided with the continental edges and became permanently attached (accreted). Eventually, the entire Iapetus Ocean closed and the continents collided to form the supercontinent of Pangea.

Paleogeographic map from Ron Blakey, Northern Arizona University, http://cpgeosystems.com/paleomaps.html

The tectonic history of the Appalachian Mountains involves opening an ancient ocean along a divergent plate boundary, closing the ocean during plate convergence, and then more divergence that opened the Atlantic Ocean. Tracing this large-scale development helps us understand the place of origin of the various geologic provinces of the Southern Appalachian Mountains, as well as the rocks and structures observed along the Blue Ridge Parkway, the C&O Canal National Historical Park, and other NPS sites in the region.

Tectonic Evolution of the Southern Appalachian Mountains

750 Million Years Ago

Iapetus Ocean Opens. Continental blocks destined to become North America and Gondwanaland drift apart. The eastern edge of ancient North America developed into a passive continental margin, similar to the modern East Coast. As the margin cooled and subsided, thick sedimentary strata covered the earlier mountains and rift valleys.

400 Million Years Ago

Iapetus Ocean Narrows during Subduction. Oceanic sediments and volcanic islands were at times added to the edge of North America.

300 Million Years Ago

Iapetus Ocean Completely Closes. The Southern Appalachians develop as the African portion of Gondwanaland crashes in, forming the supercontinent of Pangea.

Atlantic Ocean Opens. Ancient ocean rocks are left behind as the Piedmont Province, along with a sliver of Africa that now lies beneath the Coastal Plain of Florida and offshore regions of Georgia and the Carolinas (purple). The Blue Ridge Province is part of North America’s ancient continental margin, while the Valley and Ridge Province contains sedimentary layers of North America that were folded and faulted during the collision.
illustrations above modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172. About 750 million years ago a very ancient supercontinent began to rip apart. The resulting continental rift zone was much like today’s Basin and Range Province, with long mountain ranges separated by down-dropped valleys. Sedimentary and volcanic layers that were deposited in the rift valleys are exposed in Shenandoah National Park and along the Blue Ridge Parkway. As the ocean widened, the edge of ancient North America subsided, and a blanket of sedimentary layers buried the rift valleys and eroded mountain ranges. Examples of these ancient passive continental margin layers are found today along the Blue Ridge Parkway and in Great Smoky Mountains National Park.

From about 400 to 300 million years ago the Iapetus Ocean gradually closed during subduction. In the process volcanic islands, continental fragments and the supercontinent of Gondwanaland collided with the ancient continental margin of North America. In the western part of the Appalachian Mountains, the rocks were originally part of North America. The Valley and Ridge Province is folded and faulted sedimentary strata of the ancient continental margin, and the Blue Ridge Province is a piece of the deeper, hard crust that was uplifted and shoved westward. Farther east, the rocks formed elsewhere and were attached (accreted) to the edge of North America as the ocean closed. The Piedmont Province is an array of accreted terranes, metamorphosed sedimentary layers and crust of the Iapetus Ocean that were caught up in the collision. In places the Atlantic Ocean opened some distance east of the zone of “suturing” between the continents. Rocks beneath young sediments on the Coastal Plain of Florida and areas offshore from Georgia and the Carolinas are stranded pieces of Africa, left behind when the Atlantic opened.

Tectonic Provinces of the Southern Appalachian Mountains

Modified from “Earth: Portrait of a Planet, by S. Marshak, 2001, W. W. Norton & Comp., New York.

National Park Service sites in the Appalachian Mountains have rocks and landscapes resulting from collision of North America with accreted terranes and the ancient continent of Gondwanaland.

• The Piedmont Province contains islands and continental fragments (accreted terranes) that were in the ancient Iapetus Ocean and were caught up in the collision between the continents.
• The Blue Ridge Province has hard igneous and metamorphic rocks that we uplifted along the edge of North America as the continents collided.
• The Valley and Ridge Province is composed of sedimentary layers of the ancient continental margin that were folded and faulted during the collision.
• The 184 mile long C&O Canal National Historical Park spans all of the tectonic provinces.
• Great Falls Park lies on the eastern edge of the Piedmont Province, while the Blue Ridge Parkway and Shenandoah National Park lie entirely within the Blue Ridge Province.
• Great Smoky Mountains National Park is mostly within the Blue Ridge Province, with a portion extending into the Valley and Ridge Province.
• New River Gorge National River is in the Valley and Ridge Province.
• Elsewhere along the collision zone, the Ouachita and Marathon mountains are foreland fold-and-thrust belts similar to the Valley and Ridge Province, and include Hot Springs National Park in Arkansas and a small part of Big Bend National Park in Texas.
  

The final events in the Appalachian story have been occurring over the past 250 million years, as Pangea ripped apart, the Atlantic Ocean opened, and the Appalachian Mountains have been wearing down by erosion. Sedimentary and volcanic strata are found in rift valleys in the eastern part of the Piedmont Province, relics of the rifting of Pangea about 200 million years ago. The Atlantic Coastal Plain is covered by sedimentary layers of the modern passive continental margin.

The deeper you go into the Earth the higher the temperature and pressure. Some of the rocks of the Appalachian Mountains were metamorphosed to such a degree that they must have been 5 to 15 miles (10 to 25 kilometers) below the surface. The deep metamorphic rocks were brought back to the surface via two tectonic processes: thrust faulting and isostatic rebound. By the first process, the rocks were compressed and shoved (thrust) upward along fault lines as the ocean closed and the continents collided. The second process (isostasy) results in the wholesale uplift of a broad region as the crust thickens. But as the weight of the high mountains is reduced by erosion, the thick, buoyant crust bobs back upward (rebounds), creating topography not quite as high as it was before. As much as 15 miles (25 kilometers) of rock has been removed since the Appalachians initially formed, exposing the metamorphic rocks.

Blue Ridge Parkway

National Park Service sites in the Appalachian Mountains offer an opportunity for visitors to experience the overall landscape and local geology of an ancient continental collision zone. The Blue Ridge Parkway stretches 469 miles (755 kilometers), from Shenandoah National Park on the northeast to Great Smoky Mountains National Park on the southwest. Although those parklands are mostly confined to one physiographic province, the Blue Ridge, their heights provide vistas of other parts of the collision zone: the Valley and Ridge Province to the west and the Piedmont to the east. Outcroppings along road cuts and valley walls reveal deformed and uplifted rocks that formed along the edge of ancient North America. Some of those rocks were buried more than 10 miles (15 kilometers) during the continental collision, then uplifted and exposed by a combination of thrust faulting, erosion, and isostatic rebound.

Blue Ridge Parkway, North Carolina and Virginia. Linville Falls flows over 1.1 billion-year-old gneiss, a high-grade metamorphic rock. The rock in the foreground is 550 million-year-old sandstone of the Chilhowee Formation, now metamorphosed to quartzite. These rocks are part of the continental margin of ancient North America that was uplifted as part of the Blue Ridge Province during the last phases of continental collision that formed the Appalachian Mountains.

Photo by Robert J. Lillie.

Great Smoky Mountains

Great Smoky Mountains National Park is the most visited unit in the entire National Park System, attracting more than 11 million people per year. After North Carolina’s Mt. Mitchell, the park contains the highest peaks in the Appalachian Mountains, including Clingmans Dome (6,642 feet; 2,024 meters) and Mt. Guyot (6,621 feet; 2,018 meters). The spectacular scenery of the park lies within the Blue Ridge Province, with a small portion on the northwest in the Valley and Ridge Province.

NPS photo.

Parks in the Washington DC area reveal a lot about the geology and tectonic evolution of the Appalachian Mountains. The Chesapeake and Ohio (C and O) Canal National Historical Park runs for 185 miles (298 kilometers) from Cumberland, Maryland to Washington DC. It follows the valley that the Potomac River has carved across all of the Appalachian tectonic provinces, including the Valley and Ridge, Blue Ridge, Piedmont and Coastal Plain. Near DC it parallels Great Falls Park in Virginia, where rapids form as the Potomac River flows over the steep edge of the hard metamorphic rocks of the Piedmont Province to the softer sedimentary layers of the Atlantic Coastal Plain.

Photo by Robert J. Lillie.

The Ouachita Mountains of western Arkansas and southeastern Oklahoma are a classic example of a soft continental collision. Plate convergence ceased soon after the ocean separating ancient North America from Gondwanaland closed. The crust beneath the growing mountains remained relatively thin. Peaks never attained heights anywhere near those of the modern Himalayas and Alps, nor of the ancient Appalachians. In their prime, Ouachita peaks probably rose to no more than the modest height (~9,000 feet; 2,750 meters) of the Carpathian Mountains in central Europe. The low buoyancy of thin crust allowed the accumulation of thick sedimentary layers as the ancient ocean closed about 280 million years ago. Without thick crust and high mountains, there has been only small amounts of erosion and isostatic rebound, so that a pile of sedimentary layers as much as 12 miles (20 kilometers) thick still lies beneath the Ouachita Mountains.

Tectonic Evolution of the Ouachita Mountains

400 Million Years Ago

Illustrations above modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Hot Springs National Park in Arkansas preserves evidence of the soft continental collision that formed the Ouachita Mountains. Rocks are not the hard igneous and metamorphic variety found in the Appalachian Mountains, but rather softer sedimentary layers scraped off the seafloor and thrust over the edge of the continent as the ocean between North and South America closed. The originally horizontal rocks are folded and faulted, much as the strata in the Valley and Ridge Province of the Appalachians.

Ouachita Mountains, Arkansas

The sandstone, shale and chert layers are part of a thick pile of sediments that were deformed and uplifted as Gondwanaland collided with the southern edge of ancient North America 280 million years ago.

Photo courtesy of Robert J. Lillie.

Congress established Hot Springs National Reservation in 1832 to preserve and protect the region of thermal waters around Hot Springs Mountain in Arkansas. In its mission and administration, it might arguably be considered the first site functioning as a “national park.” The first formal national park, Yellowstone, was established by act of Congress 40 years later. The thermal waters for which Hot Springs National Park is named are not due to the presence of shallow magma beneath the area, as at Yellowstone. Rather, the hot springs occur because there are cracks in chert layers outside the park that allow rainwater to circulate slowly downward, where it is heated due to the normal increase in temperature with depth in the Earth. More prominent systems of faults and cracks directly beneath the park allow the water to return back to the surface much quicker, while it is still about 140 °F (60 °C)—more than enough for a refreshing, therapeutic soak!

Marathon Mountains

The area of southwest Texas where the Rio Grande River takes a sharp turn to the north displays the effects of three superimposed tectonic episodes. The latest is the ongoing continental rifting forming the Rio Grande Rift arm of the Basin and Range Province. Earlier compression of the North American continent from 80 to 40 million years ago formed the Laramide Uplifts, which include the frontal ranges of the Rocky Mountains. And before that, the soft continental collision that formed the Ouachita Mountains 280 million years also formed the Marathon Mountains. The northernmost part of Big Bend National Park, around Persimmon Gap, lies in the Marathon Mountains and includes rocks and structures similar to those found in the Ouachitas. The park thus preserves the westernmost vestiges of the oceanic closure and continental collision found not only in the Appalachians of the U. S. and Canada, but also in widely-separated regions of Africa and northern Europe.

Big Bend National Park, Texas

NPS photo.

Northern Alaska was the site of continental collision that progressed to a stage somewhere between the soft collision seen in the Ouachita and Marathon mountains, and the hard collision observed in the Appalachians and Himalayas. In its prime, about 100 million years ago, the Brooks Range probably had mountains as high as the Alps of Europe. Gates of the Arctic National Park and Preserve reveals mountains much higher than those seen in the Appalachians, because the Brooks Range is still young—erosion has not worn the landscape down quite as much. Other NPS sites farther west in the Brooks Range include Cape Krusenstern National Monument, Kobuk Valley National Park, and Noatak National Preserve.

NPS Sites in Alaska

Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

200 Million Years Ago
Ocean separating northern Alaska and a continental fragment begins to close.

150 Million Years Ago
Continents collide but continue to converge.

120 Million Years Ago
The northern continent extends laterally for more than 60 miles (100 kilometers) beneath northern Alaska, thickening the crust.

100 Million Years Ago
The mountains reach heights similar to those of the modern-day Alps, as the hard crust of northern Alaska breaks and uplifts.

Today
The Arctic Ocean opens on the north. After eroding away up to 10 miles (16 kilometers) of rock, there is still a small crustal root that maintains mountain heights much greater than those seen today in the Appalachians.

Illustrations above adapted from “Crustal structure and kinematic evolution of the Brooks Range, Alaska, from gravity and isostatic considerations,” by Jonathon Williams, M. S. Thesis, Oregon State University, 2000.

Gates of the Arctic National Park and Preserve, Alaska

Photo courtesy of Robert J. Lillie.

Related Links

• NPS—Geoscience Concepts
• NPS—Be Geohazard Aware

Plate Tectonics and Our National Parks (2020)

• Text and Illustrations by Robert J. Lillie, Emeritus Professor of Geosciences, Oregon State University [E-mail]

• Produced under a Cooperative Agreement for earth science education between the National Park Service's Geologic Resources Division and the American Geosciences Institute.