Convergent Plate Boundaries—Accreted Terranes

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NPS Landscapes Developed on Accreted Terranes

Many National Park Service sites, particularly in Alaska and other western states, contain rocks that were formed far from North America—on islands or even on other continents. Plate tectonic forces ripped the continents apart and moved the continental fragments and oceanic islands over great distances, where they eventually collided with the edge of North America. NPS sites in southeast Alaska represent some of the latest terrane accretion that has added almost the entire landmass of Alaska to the North American continent over the past 200 million years. Similarly, parks in northern Washington State are just a few of the many NPS sites around the country that have rocks and landscapes that tell the story of how the continent has grown outward over the past two billion years.

map of us with parks in accreted terranes marked and labeled
Shaded relief map of United States, highlighting National Park Service sites on Accreted Terranes. Letters are abbreviations for NPS sites listed below.

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.

NPS Sites on Accreted Terranes

National Park Service sites throughout the country lie on accreted terranes. The table below shows those in southeastern Alaska and northern Washington State that are on terranes that were added (accreted) to the North American continent in relatively recent geologic time.

glaciated mountain
Denali National Park, Alaska. Denali (also formerly known as Mt. McKinley) and other high peaks in the Alaska Range are built on crust that thickened as terranes came crashing in from the south.

NPS Photo.

mountain range
North Cascades National Park, Washington. The landscape is similar to Alaska. It consists of rocks that formed elsewhere, and then were deformed, metamorphosed, and accreted to North America.

Photo by Robert D. Lawrence.

Introduction

A plate capped by thin oceanic crust will subduct beneath one with much thicker continental crust. But when continental fragments or oceanic islands approach a subduction zone, their crusts may be too thick to subduct. Instead, they crash into the edge of the continent and become permanently attached. This addition, or “accretion,” is one of the ways that continents tend to grow in size over geologic time.

Continents Grow Outward by Terrane Accretion

In the digram below, an oceanic island or continental fragment (incoming terrane) approaches a subduction zone, where it will eventually attach (accrete) to the edge of the continent. An active volcanic arc develops on crust of an older accreted terrane. Extinct volcanic arcs on still-older accreted terranes reflect the positions of earlier subduction zones. Suture zones mark the boundaries between different terranes.

illustration of upper layers of the earth where two plates are coming together

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

Grocery Store Tectonics

grocery store conveyor belt with several products laid out

A grocery store checkout counter can be used to demonstrate the gradual growth of a continent via terrane accretion. The conveyer belt represents the oceanic plate subducting beneath the continental plate. The groceries are like volcanic islands and continental fragments (terranes) that progressively crash into the edge of the continent, adding to its mass.

2 photos of groceries moving down the conveyor belt at a check-out stand
2 photos of groceries piling up at the end of the conveyor belt at a check-out stand

Photos above modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

Growth of North America

Accreted terranes are the blocks of continental fragments and oceanic islands that have collided with a continent and are now permanently attached. All continents, including North America, tend to be older in their interiors and grow outward over geologic time, as terranes are added to the edges of the continent.

map of north america showing age of rocks
North America has been growing outward over time as terranes are added along the edges. Numbers are the ages (in billions of years) of the igneous and metamorphic rocks that are exposed at the surface or lie beneath younger sedimentary layers.

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

When the Earth cooled enough for a shell of lithosphere to form, the crust that formed on top of the plates was thin like today’s oceanic crust. Where plates converged, thicker crust formed at island arcs as magma poured out on the surface as lava flows, and was added to the base of the crust as igneous intrusions. When island arcs collided with other island arcs, rock and sediment were scraped off the top of subducting plates. Those blocks of coalesced material formed the nuclei of the continents. Over time, the small continents grew outward as material was added to their edges through sedimentation, subduction, and the collision of oceanic islands and other continental fragments. Continents are thus older in their interior regions, known as cratons, and get progressively younger outward, as seen in the above map of North America.

There are many NPS sites across the country that have rocks that are out of place—manufactured elsewhere then carried long distances before being affixed to the North American continent. The Appalachian Mountains contain oceanic islands, continental fragments, and sedimentary, volcanic, and metamorphic rocks that were added to North America as an ancient ocean closed. Thus sites such as Shenandoah National Park and the Blue Ridge Parkway, discussed elsewhere in the “Collisional Mountain Range” part of these web pages, could also included here. Regions in the western part of the country, such as the Olympic Mountains in Washington and the Klamath Mountains in Oregon, were also affixed to North America in fairly recent times. Olympic National Park and Oregon Caves National Monument could therefore also be part of this “accreted terrane” discussion, as could numerous other NPS sites. Information on four parks is the focus of this chapter, three in Alaska (Denali, Wrangell–St. Elias and Glacier Bay) and another in Washington State (North Cascades). The parks are within the North American Cordillera, a broad region that has been the site of terrane accretion and other tectonic activity over the past 200 million years.

Alaska


The bulk of Alaska is material that was added to the North American continent in fairly recent geologic time. The motion between the Pacific and North American plates, coupled with the shape of the western border of the continent, explains how Alaska tends to be the collecting area for wayward terranes. Remarkably, just about all of Alaska has been assembled through terrane accretion over the past 200 million years. Only a small piece of the state, on its eastern border with the Yukon Territory of Canada, is a bona fide part of the ancient North American craton.

NPS Sites in Alaska

The rugged topography and rocks at National Park Service sites in Alaska display the outward growth of the northwestern prong of North America over the past 200 million years. The mountains of Denali and Wrangell–St. Elias national parks are so high because the crust thickened as terranes came crashing in from the south. Kenai Fjords and Glacier Bay national parks lie on terranes that docked with North America more recently. Volcanoes in Aniakchak, Katmai, Lake Clark, and Wrangell–St. Elias are due to the continuing northward movement and subduction of the Pacific Plate that results in magma intruding through older terranes. The park lands in the northern part of the state lie in the Brooks Range, composed of terranes added to the continent during collision with a continental fragment about 100 million years ago.

shaded relief map of nps sites in alaska
NPS Sites in Alaska. NP = National Park; NM = National Monument; NPres = National Preserve. Letters in ovals refer to Accreted Terrane parks listed at top of page. C–C’ is the line of the simplified cross section of terrane accretion shown in the next illustration.

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.

Terrane Accretion in Southern Alaska

Southern Alaska has grown over the past 200 million years as terranes were added to the continent progressively from north to south. The terranes follow an arching pattern, paralleling the Gulf of Alaska. Because they came crashing in from the south or southwest, the terranes are generally older toward Alaska’s interior and get progressively younger southward. Kenai Fjords National Park and Glacier Bay National Park & Preserve lie on relatively young terranes. Aniakchak National Monument & Preserve, Katmai National Park & Preserve, Lake Clark National Park & Preserve, and Wrangell–St. Elias National Park & Preserve contain active volcanoes caused by the ongoing subduction of the Pacific Plate beneath North America. The thickened crust beneath Denali National Park & Preserve and Wrangell–St. Elias National Park & Preserve contributes to the extremely high topography of the Alaska and St. Elias mountain ranges. Yukon–Charley Rivers National Preserve lies on the small prong of the North American Craton that extends into Alaska.

diagram of terrane accretion in southern alaska
Lines with double arrows show approximate extent of National Park Service sites with respect to the various terranes. NP = National Park; NM = National Monument; NPres = National Preserve. The line of cross section C-C’ is shown on the map in the previous illustration.

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 Yukon–Tanana Terrane began to collide with North America about 225 million years ago, and was firmly attached to the continent by 180 million years ago. The Wrangellia, Alexander and Peninsular terranes slammed into North America from 110 to 85 million years ago, followed by the Chugach Terrane about 67 million years ago, and then the Prince William Terrane by 50 million years ago. The Yakutat Terrane started to collide with North America by 25 million years ago; it is still attached to the Pacific Plate, so it continues to smash into the continent.

The Landscapes of Denali National Park Reveal Different Accreted Terranes

Denali

glaciated mountain
The highest mountain in North  America, Denali (also known as Mt. McKinley), consists of granite that intruded through metamorphic rocks of older accreted terranes.

Photo by Mike Green.

Toklat River

wide river with mountains in the distance
The Toklat River is fed by glaciers coming off the Alaska Range, and flows across the Yukon–Tanana Terrane on the north side of the park.

Photo by Mike Green.

Pillow Basalt

rock outcrop with bumpy surface
The blobs of lava formed as fluid lava poured out on an ancient seafloor about 200 million years ago. They have since been accreted to North America and uplifted as part of the McKinley Terrane. These features are on Eielson Bluffs near milepost 68 of Denali National Park Road.

NPS photo.

Denali and Wrangell–St. Elias national parks contain many of the highest mountains in North America. The high topography of the Alaska and St. Elias ranges, which run through the two parks, is a consequence of collision of the youngest terranes from the south. The collision uplifts the region as the crust thickens and is compressed along fault lines. On the south, Glacier Bay and Kenai Fjords national parks, as well as some of Wrangell–St. Elias National Park, lie on younger terranes that are still being deformed above subducting oceanic crust of the Pacific Plate. The subduction leads to active volcanoes in Aniakchak National Monument and Katmai, Lake Clark, and Wrangell–St. Elias national parks. The volcanoes poke up through earlier accreted terranes. Northward subduction of the Pacific Plate also results in faulting, folding and uplift of oceanic materials, forming coastal mountain ranges (modern accretionary wedge).

The Landscapes of Southeast Alaska

Wrangell–St. Elias National Park

glaciated mountain
Mt. St. Elias is the second-highest peak in the United States. It consists of metamorphosed sedimentary and volcanic rocks of the Chugach Terrane.

NPS photo.

Glacier Bay National Park

mountains and tidewater glacier
Glaciers break up into icebergs (calve) as they enter the sea. Mountains in the background are parts of terranes that have recently been accreted to North America.

NPS photo.

Lower 48


The accreted terranes that form Alaska are only some of the vast number of terranes that have added to the mass of western North America over the past 200 million years. The array of terranes can be traced from National Park Service sites in Alaska southward through British Columbia, North Cascades and Olympic national parks in Washington State, and to other NPS sites in Oregon and California.

Western U. S. Terrane Accretion

map of western u.s. showing accreted terranes

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

The yellow and green shows material added (accreted) to the western edge of North America during the past 200 million years. The North American Craton refers to the continent that existed prior to that time. Rocks of the Wrangellia Terrane (green) are found in several blocks that are exposed from Denali National Park in Alaska, through British Columbia and all the way into western Idaho. Letters in ovals refer to Accreted Terrane parks listed in the Table at the top of this page.

Growth of the Pacific Northwest

Most of the Pacific Northwest did not exist 200 million years ago. The Pacific coastline at that time ran through what is now Idaho. Gradually, like groceries piling up on the conveyor belt in a supermarket checkout line, various pieces were added to the edge of the continent. Together with material erupted from volcanoes and marine sedimentary and volcanic layers scraped off the ocean floor, these added pieces have gradually formed the landscapes of Washington, Oregon and northern California so familiar to us today. The Pacific Northwest is still growing, as material is added along the coast and in the Cascades.

map showing the growth of the pacific northwest as terranes were added along the subduction zone
200 million years ago the coastline of the Pacific Northwest was near Idaho’s western border. There was no Oregon and Washington!
diagram showing the growth of the pacific northwest as terranes were added along the subduction zone
Sometimes a terrane crashes in and clogs the subduction zone. A new subduction zone develops farther west. The Cascadia Subduction Zone is just the latest of several subduction zones that developed in the western United States over the past 200 million years.

Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

Landscapes of North Cascades National Park

The Cascade Mountain Range has volcanic features forming as the Juan de Fuca plate subducts beneath the western edge of North America. But rocks of North Cascades National Park are not volcanic like the modern Cascades; they are part of much older accreted terranes that crashed into North America before the volcanoes formed. Together with the adjacent Ross Lake and Lake Chelan national recreation areas, North Cascades National Park reveals some of the history of terrane accretion that contributed to the westward expansion of the North American continent.

glaciated mountain

Photo by Robert D. Lawrence

At 9,127 feet (2,782 meters), Mt. Shuksan is the highest point in North Cascades National Park, Washington. The Alpine scenery is the result of rocks being shoved upward as terranes came crashing in.

forested mountains with snow covered peak in the distance

Photo by Robert D. Lawrence.

Snow-covered Mt. Baker, looking west from the North Cascades National Park, Washington. The active volcano results from the ongoing subduction of the Juan de Fuca Plate beneath North America. It pokes up through granite that intrudes metamorphic rocks of older accreted terranes. The two peaks in the middle ground are the Twin Sisters.

The North Cascades looks more like the Alps than other parts of the Cascade Mountain Range, with elevations over 9,000 feet (2,750 meters). Such heights result from a combination of thrust faulting and crustal thickening, both consequences of plate convergence and terrane accretion. Massive glaciers that were far more extensive than the ones currently in the park have carved up the North Cascades. The old igneous and metamorphic rocks of the accreted terranes have been eroded to sharp ridges by the glaciers, in contrast to the nearby Cascade volcanoes that are smooth cones because they continue to erupt.


Figures Used

Site Index & Credits

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.

Last updated: February 11, 2020

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