Roadside Geology of Alaska

CATHY CONNOR

2014

Mountain Press Publishing Company

Missoula, Montana

© 2014 by Cathy Connor

First Printing, September 2014

All rights reserved

Cover image and maps constructed by Mountain Press Publishing Company Photos © 2014 by Cathy Connor unless otherwise credited

BACK COVER PHOTOS

Top: A braided delta in Kachemak Bay of Cook Inlet. —Mandy Lindeberg photo, NOAA, Alaska ShoreZone Program

Middle: Redoubt Volcano erupting in 2009. —U.S. Geological Survey photo

Bottom: Ice-smoothed surface of young basaltic columns at Punchbowl Cove in Misty Fjords National Monument. —Jim Baichtal photo, U.S. Forest Service

Geology maps for individual road logs constructed from the Database of the Geologic Map of North America; adapted by Garrity and Soller (2009) from the map by J. C. Reed, Jr., and others (2005)

Roadside Geology is a registered trademark of Mountain Press Publishing Company.

Library of Congress Cataloging-in-Publication Data

Connor, Cathy, 1952-

Roadside geology of Alaska. — Second edition / Cathy Connor.

pages cm. — (Roadside geology series)

Includes bibliographical references and index.

ISBN 978-0-87842-619-5 (pbk. : alk. paper)

1. Geology—Alaska—Guidebooks. 2. Alaska—Guidebooks. I. Title.

QE83.C74 2014

557.98—dc23

2014021836

Printed in Hong Kong by Mantec Production Company

P.O. Box 2399 • Missoula, MT 59806 • 406-728-1900

800-234-5308 • info@mtnpress.com

www.mountain-press.com

To the earliest human residents of the North Country and their descendants, who are Alaska Natives and Canada’s First Nation people. Your ancestors were the first geoscientists in the North Country more than 10,000 years ago. You have kept their observations and experiences alive for the twenty-first-century generations through your oral histories, traditions, and cultures.

To the hardy and dedicated geoscience researchers who have devoted your lifetime to the understanding of Earth from its core to the tops of Denali and Mt. Logan, and on up into the ionosphere. You have tracked and deciphered the surface processes powered by solar energy and the tectonic and seismic activity powered by internal planetary energy from the core and mantle. Your publications, maps, and thoughtful observations have made this book possible.

Finally, to all students in universities who are studying Earth and planetary sciences and to K–12 students and your teachers in math and science classrooms. You are our future.

Roads and sections covered in Roadside Geology of Alaska.

CONTENTS

Preface

Geologic History of Alaska

Understanding the Earth

Alaska’s Modern Tectonic Environment

Terranes of the North Country

PROTEROZOIC EON

PALEOZOIC ERA

Supercontinent Pangaea and the Permian Extinction

MESOZOIC ERA

Rotation of Arctic Alaska

Mesozoic Basins and the Accretion of the Insular Superterrane

Cretaceous Inland Sea and Arctic Dinosaurs

CENOZOIC ERA

Wandering Resurrection Ridge

Dextral Strike-Slip Faults

PLEISTOCENE ICE AGES

Bering Sea Shelf

HOLOCENE AND ANTHROPOCENE

Glacier Movement and Erosion

Permafrost and the Warming Climate

Natural Resources

Alaska’s Marine Transportation

A Record of Human Culture

Southeast Alaska and Coastal British Columbia

Terranes of Southeast Alaska

Alexander

Terrane

Taku Terrane

Gravina Belt

Coast Plutonic Complex and Coast Shear Zone

Queen Charlotte–Fairweather Transform Fault

Glaciation and Sea Level

GUIDES TO SOUTHEAST ALASKA AND COASTAL BRITISH COLUMBIA

North Vancouver Island—Prince Rupert

Haida Gwaii (Queen Charlotte Islands)

Ketchikan Area

Misty Fjords National Monument

Prince of Wales Island

Baranof Island, Kruzof Island, and Sitka

Wrangell and Petersburg Areas

Mt. Edziza Provincial Park

Juneau and Haines Areas

Haines Highway: Haines—Haines Junction

Tatshenshini-Alsek Provincial Park

Glacier Bay National Park and Preserve

South-Central Alaska and the Aleutians

Yakutat Plate

Aleutian Subduction Zone and Its Volcanoes

The Great Alaska Earthquake of 1964

Terranes of South-Central Alaska

Wrangellia Terrane

Peninsular Terrane

Chugach and Prince William Terranes

Yakutat Terrane

Glacial Lake Ahtna

GUIDES TO SOUTH-CENTRAL ALASKA

Seward Highway: Anchorage—Seward

Kenai Fjords National Park

Sterling Highway: Tern Lake—Homer

Glenn Highway: Anchorage—Glennallen—Tok

Richardson Highway: Delta Junction—Valdez

Edgerton Highway and McCarthy Road (Wrangell–St. Elias National Park and Preserve)

Parks Highway: Wasilla—Fairbanks

Denali National Park and Preserve

Kodiak Island

Katmai National Park and Preserve

Lake Clark National Park and Preserve

Interior Alaska and Adjacent Parts of the Yukon

Northern Cordilleran Volcanic Province

Terranes in the Interior

Cassiar Terrane

Yukon-Tanana Terrane

Stikine, Quesnel, and Cache Creek Terranes

Seventymile (Slide Mountain) Terrane

GUIDES TO INTERIOR ALASKA AND ADJACENT PARTS OF THE YUKON

Taylor Highway: Tetlin Junction—Eagle

Yukon-Charley Rivers National Preserve

Fairbanks Area

Klondike Highway: Skagway—Whitehorse—Dawson

Alaska Highway: Whitehorse—Delta Junction

Kluane National Park

Arctic Alaska

Formation of the Brooks Range

Brooks Range Glaciations

Northern Alaska Coal

Oil on the North Slope

Avak Crater

Guides to Arctic Alaska

Dalton Highway: Livengood—Deadhorse (Prudhoe Bay)

Gates of the Arctic National Park and Preserve

Arctic National Wildlife Refuge

Glossary

References

Index

Redoubt Volcano erupting in 2009. —U.S. Geological Survey photo

PREFACE

I wrote the first edition of Roadside Geology of Alaska in the mid-1980s with coauthor Dan O’Haire. Since that time, while based at the University Alaska Southeast in Juneau, I have worked on many different and interesting Alaskan geology projects and in Earth science education for Alaska’s K–12 community, with colleagues from across the state. Dan left geology and Alaska long ago, relocating to Colorado, where he has been climbing the numerous Rocky Mountain fourteeners, teaching, and developing his science fiction writing, perhaps inspired by Alaska’s mysterious collection of rocks!

Soloing for this 2014 edition, my intention was to attempt to pull together, from ever-increasing sources of twenty-first-century research, an understandable summary of Alaska’s geologic experience over the eons. I have tried to find interesting developments in geologic research to share with Roadside Geology of Alaska readers; many new and interesting discoveries have been made in this region since 1988. I also hope to introduce new generations of inquiring minds to the wild and often unexpected geologic stories from Alaska. The references listed at the end of the book enable enthusiasts to dig deeper into the original literature and get a glimpse of where I have been foraging within Earth science research.

The rocks haven’t changed too much over the last thirty years, but our understanding of them has. New technologies and a lot of hard work by numerous researchers have clarified and amplified Alaska’s geologic history. Thirty more years of boots on the ground have generated new geology maps using high-precision GPS and GIS techniques. More rock samples, analyzed with higher-precision mass spectrometry and dated with better radiometric and zircon dating techniques, have cleared up some formerly murky geologic history.

The global seismic network has provided lots of data for earthquake modelers, and many satellites are collecting daily imagery from space, offering time-sequenced bird’s-eye views of our changing northern landscapes. The precision of differential GPS provides near real-time measurements of plate motion both horizontally and vertically, and distills our understanding about how active tectonic processes are rearranging and moving Alaska’s rocks and people around. Refined information about how the land is transforming beneath our feet helps Alaska municipal, state, and federal planners better cope with the big changes coming to our cities and villages as the North warms up and Alaskan plate boundaries continue to rumble.

The geologic maps included with the road guides are intended to provide readers with a general idea of terrane boundaries, major faults, and the ages and types of rocks. The area covered in each map is large, and the scale required simplification. The actual bedrock is much more complex than depicted in the maps. Detailed geologic maps are available for many regions of Alaska.

Geologic time scale and geologic events in Alaska’s history.

GEOLOGIC HISTORY OF ALASKA

The Alaska landscape features many superlatives. The biggest U.S. state extends across five time zones, from Hyder at the southeastern end of the Alaska Panhandle to Attu Island in the western Aleutians. Denali (Mt. McKinley) is the highest peak in North America at 20,320 feet (6,194 m) and is still rising, pushed upward as the Yakutat Plate collides from the south. The exceptional height of Alaska’s mountains is mirrored by the trench of an active subduction zone along Alaska’s southern shore. It reaches about 22,377 feet (6,821 m) below sea level. Earthquakes associated with the subduction zone shake Alaskans frequently, and the magnitude 9.2 earthquake in 1964, with its epicenter in Prince William Sound, was one of the largest seismic events ever recorded. It destroyed parts of Anchorage, Valdez, Chenega, Cordova, Seward, and Kodiak. The Yakutat collision has also created huge mountains along Alaska’s Gulf Coast, producing the Bagley Icefield, the largest subpolar icefield in North America, where humid coastal air rises over a short horizontal distance to heights of 15,000 feet (4,572m). In the southeast panhandle, Lynn Canal and Chatham Strait, which follow the Denali Fault from Haines to the end of Baranof Island, form North America’s longest fjord. The Yukon River, with a watershed 25 percent larger than the state of Texas, stretches over 1,000 miles (1,600 km) from its Juneau Icefield headwaters to the Bering Sea. The list of amazing geologic and landscape features goes on and on, and roadside geologists can add to those an extraordinary and complex array of rocks.

Caves in limestone, common features of places like Florida and Kentucky, are present in Southeast Alaska and in Wrangell–St. Elias National Park. Lava flows so young they are not yet vegetated appear across the state from the Alaska Panhandle to the western Aleutians. Pillow basalts from ancient ocean floors outcrop along the coast, and granite tors poke upward through otherwise flat tundra surfaces. In Interior Alaska, thick layers of windblown glacial silt, frozen as permafrost, encase bones of camels, bison, and woolly mammoths that once grazed the Beringian steppe tundra grassland and were hunted by North America’s first human inhabitants. Gold miners found these bones as they excavated the silt to reach gold-bearing gravels below. On the North Slope, folded sedimentary rocks trap oil and natural gas, as does the convoluted structure of a meteor impact crater under the surface near Barrow.

Understanding the Earth

The meteor crater near Barrow is not visible on the surface. It was discovered by petroleum geologists who mapped its structure in deep bedrock. You may wonder why the surface of Earth doesn’t look more like the Moon—cratered by ancient meteor impacts. The Earth has been bombarded by meteorites just as frequently as the Moon, its satellite. The difference is that Earth has two mechanisms to resurface itself that the Moon doesn’t have: surface erosion and plate tectonics. The first mechanism that makes impact craters hard to find here has to do with Earth’s unique blue marble appearance, first noted by Apollo astronauts as they gazed homeward from the Moon. This blue view is provided by our atmosphere and hydrosphere, or Earth’s outer layers of gases and oceans, which make Earth unlike all other planets in our solar system. Warming and cooling of the atmosphere generates Earth’s weather and climate. Water evaporated from the oceans or land surfaces into the atmosphere is delivered back to Earth as rain or snow. This water in liquid or frozen form continuously sculpts mountain slopes and stream valleys and also reacts with and chemically changes the minerals in rocks. Water-saturated steep slopes, facing into storm systems, generate landslides along fractures and weathered rock zones. Mountain ranges create rain shadows and, on their leeward sides, arid landscapes. Winds in the lower atmosphere carry fine-grained sediments as they circulate around the planet, relocating volcanic ash, glacial silt, topsoil particles, and dust-riding microbes.

The second mechanism for resurfacing Earth is plate tectonics. Earth is a layered planet with a solid iron core, a molten outer core, a solid but in some regions plastic mantle, and a brittle surface crust. Circulation within the mantle, with less dense molten rock rising and cooler denser rock sinking, generates movement of the crust above. The surface crust is broken into large slabs, called plates, which move relative to each other. New crust is formed by eruptions beneath the sea along seafloor spreading ridges and along some continental rifts. Older, sediment-covered seafloor sinks along subduction zones and is dragged back into the mantle where plates converge. When continental crustal plates collide, such as where the Yakutat Plate is colliding with southern Alaska, mountains like the St. Elias Range are uplifted.

Seafloor spreading drives the birth and growth of Earth’s crust and is powered by Earth’s internal battery, or geothermal heat. This energy is created primarily by the decay of radioactive elements in mantle minerals. Thorium and uranium generate about half of our planet’s geothermal heat. Another radiogenic element, potassium, generates another 10 percent of our Earth’s internal heat. The other sources of geothermal energy include the constant gravitational pulling and planetary wrenching between the Earth and Moon and the remnant heat from when Earth was molten during the formation of our solar system.

The energy creates movement in the tectonic plates. Plate motion creates friction at fault boundaries and builds up stresses and strains in rocks that can ultimately be released as seismic energy and earthquakes. Tectonic plates move slowly, a few inches per year, but over millions of years the plates have moved across the globe. Rocks that originally formed in tropical climates, such as limestone of carbonate reefs, are found in Alaska, transported there on the tectonic conveyor belt.

The study of Earth’s fossil magnetic fields, trapped in the iron minerals of cooling lava, has allowed geoscientists to track plate motion through time. At divergent plate boundaries, undersea ridge systems produce annual increments of new basalt as magma rises to the surface. Ancient seafloor basalts store information about the intensity and direction of the Earth’s changing magnetic field as they cool and solidify. This geomagnetic barcode can be scanned from extracted basalt cores and decoded in a magnetometer. Researchers use this fossil magnetic field information, along with rock age information, as a sort of paleo-GPS system to track the position of continents through time.

Earth’s iron core is surrounded by the mantle, which circulates very slowly and generates the motion of tectonic plates on Earth’s surface. Plates spread apart at spreading zones, where new ocean crust is generated. Subduction zones form where oceanic plates collide with continental plates. —Mountain Press artwork

Processes that take place at a subduction zone. —Mountain Press artwork

Aurora borealis viewed from Joint Base Elmendorf-Richardson near Anchorage. —U.S. Army photo

Magma erupted today preserves the modern magnetic field. The magnetic field is generated by the interaction of electrical currents and the molten iron in Earth’s inner and outer core. The Earth’s magnetic north pole, which marks the emergence of Earth’s magnetic field in the northern hemisphere, is wandering around in the Arctic Ocean north of Resolute Bay in the Northwest Territories. It has been moving northward about 40 miles (64 km) per year. Geophysicists require stout hearts and great endurance to track the pole’s location by driving snow machines across the Arctic sea ice. The Earth’s magnetic field is essential for deflecting the ionized atomic particles of the solar wind that would otherwise strip away the gases in our atmosphere and the water in our oceans, making our planet uninhabitable. Interaction of the solar wind with Earth’s magnetic field can result in Alaska’s famous auroral displays in the winter, attracting visitors from around the world.

Alaska’s Modern Tectonic Environment

As many as twenty-two thousand earthquakes are detected each year in Alaska, more than all other forty-nine states combined. Two of the largest recorded earthquakes in the world have occurred in Alaska, including the magnitude 9.2 quake in 1964. In addition, more than 130 volcanoes and volcanic fields in Alaska have been active in the last 2 million years, with more than 40 volcanoes actively erupting in historic times.

Major earthquakes recorded in Alaskan history. Keep in mind that all of these are larger than 6.0 magnitude. While they primarily occur along the Aleutian subduction zone, known as the Aleutian Megathrust, where the Pacific Plate is being stuffed beneath the North American Plate, many earthquakes also occur along active strike-slip faults located inland.—Modified from Haeussler and Plafker, 1995

Alaska’s active seismicity occurs because this northwesternmost edge of the North American Plate, a large tectonic plate of Earth’s crust, interacts with several other plates here. To the north, the mid-Atlantic Ridge crosses the Arctic Ocean and propagates onshore into Russia, separating northern Alaska from northern Eurasia along an onshore strike-slip fault boundary. The North American Plate collides with the Eurasian Plate in the Russian Far East along the Verkhoyansk Range. A triple junction, or three-plate union, between the Okhotsk, the North American, and the Pacific Plates occurs at the western end of the Aleutian volcanic arc. The Okhotsk Plate, between Eurasia and North America, is being tectonically extruded toward the Pacific Plate, thrusting the Kamchatka Peninsula over the Bering Plate and the Kurile Trench.

High-precision Global Positioning System studies of the region suggest the Bering Plate, which includes the Bering Shelf, the Aleutian Islands, and western Alaska, is presently rotating in a clockwise fashion between 0.12 to 0.31 inch (3 to 8 mm) per year around a pole of rotation located about 300 miles (480 km) northeast of Beijing, China. This rotation is likely slowing movement on the western Denali Fault and contributing to the compressive environment in the central Alaska Range. The Bering Plate rotation is complicating plate slip rates and slip directions between the Aleutian arc and the subducting Pacific Plate along the Aleutian Trench, especially west of Amchitka Island.

The Aleutian volcanic island arc is formed by the collision of the northwest-directed Pacific Plate and North America. Volcanoes do not form west of Attu Island, where the convergence becomes very oblique, creating a shear or transform fault. In the Gulf of Alaska along the subduction zone’s eastern end, the convergence ceases where a small converging plate, the Yakutat Plate, separates the Pacific and North American Plates. Near Sitka, the Queen Charlotte–Fairweather Transform Fault features right-lateral off set between the northbound Pacific Plate, and mainland Southeast Alaska on the North American Plate. A transform fault is a strike-slip fault along a plate boundary that links other types of plate boundaries. Here, it is connecting two sections of convergent boundaries, the Explorer and Aleutian subduction zones. In general ocean crust is neither created nor destroyed along a transform fault plate boundary, although there can sometimes be localized areas of extension or convergence that can allow magma to surface in limited areas.

Major tectonic plate boundaries near Alaska and their rates of movement. —Modified from Haeussler and Plafker, 1995

Near the Queen Charlotte Islands in northern British Columbia, the Pacific Plate is separated from the North American Plate by the tiny Explorer Plate north of the Juan de Fuca Plate. The northern Juan de Fuca spreading ridge generates new ocean crust, some of which subducts to the east beneath northern Vancouver Island and some of which slowly migrates by seafloor spreading to the northwest, across the North Pacific toward Kodiak Island, Alaska.

During an average North American’s lifetime of about seventy-five years, the Pacific Plate will move about 15 feet (4.6 m) to the northwest, equivalent to the lifetime growth of your fingernails. The northward progress of the Pacific Plate and Yakutat Plate has been the source of most of Alaska’s earthquakes in Late Cenozoic time. Th ese plate collisions have also folded western Cook Inlet basin, generated the Alaska Peninsula and Aleutian volcanoes, and uplift ed the St. Elias, Chugach, and Talkeetna Mountains and the Alaska Range.

Several seamount chains in the Gulf of Alaska were formed by the Pacific Plate moving over hot spots off the coast of British Columbia, Washington, and Oregon during the past 40 million years. Geographically fixed seafloor hot spots, or mantle plumes, like those under Yellowstone or Hawaii today, erupt basaltic magmas as oceanic or continental crust slides over the top of the plume. A chain of pointy-topped volcanic mountains, called seamounts, form on the seafloor. In some cases, the erupting mantle plume may produce enough volcanic rock to emerge above sea level and form an island, but over time the island is eroded by waves and changing sea level. If the island becomes completely submerged, it forms a flat-topped volcanic mountain called a guyot.

Volcanoes in the Alaska region occur above the Aleutian subduction zone, in the Wrangell volcanic field, and in the Northern Cordilleran Volcanic Province. —Mountain Press artwork

The successive ages and positions of these undersea volcanoes have been used to measure rates of plate motions across the northeastern Pacific seafloor. Southeast of Kodiak Island, the 24-million-year-old Kodiak guyot lies at the northern end of the Kodiak-Bowie (Pratt-Welker) chain. These submarine volcanoes were formed over the Bowie hot spot far to the south near the triple junction of the Pacific, North American, and Explorer Plates west of the Queen Charlotte Islands in British Columbia. The Kodiak guyot is located on the southern edge of the Aleutian Trench and is poised and ready to be accreted onto the Alaskan continental margin in a couple of million years or so. By radiometrically dating rocks from these seamounts, geologists have calculated that the Pacific Plate is spreading away from the Juan de Fuca and Explorer Ridges at about 3 inches (7.6 cm) per year. Other more precise plate motion using satellites and GPS methods peg this spreading rate at about 2.6 inches (6.6 cm) per year. The Cobb hot spot, located on the Juan de Fuca Plate boundary, has formed seamounts south of the Kodiak-Bowie chain. Deep-sea manned submersibles like ALVIN and unmanned remotely operated vehicles have been used to explore these seamounts. National marine sanctuaries have been created to protect the deep, cold-water coral ecosystems here.

Seamounts in the Gulf of Alaska formed as the Pacific Plate moved over the magma-generating Bowie and Cobb hot spots. —Image from NOAA

Terranes of the North Country

The landscape, or terrain, of Alaska and adjacent parts of the Yukon and northern British Columbia is made in large part of terranes, regions of rocks with similar histories. Terranes are bounded by major faults. Adjacent terranes often share very different geologic histories and have little in common with each other. The terranes were geologically welded together in some mountain building event or by fault movement along converging plate boundaries. Many of the terranes in Alaska originated as island arcs or ocean basins and were carried along by the tectonic conveyor process known as seafloor spreading. Eventually they crashed against and accreted to the continent along subduction zones over the past 200 million years.

This process is a bit like the results of a trip to the grocery store. You select individual food items (terranes) from widespread aisles and randomly place them into your cart, which you next roll across the store (ocean basin) to the checkout area (continental edge). You further randomize their order by placing them, one by one, onto the checkout conveyer belt. Each item stops as it collides with the end of the loading area (subduction zone) and is recombined by the bagger into your shopping bag. If you next melted your full grocery bag on a camp stove, the fused result might be a plausible model for the process and results of terrane accretion and the younger magmatic intrusions that weld the geology of the northwest coast of North America. Once terranes arrive and are amalgamated, post-arrival rearrangement by fault shearing, like a delicatessen meat slicer, can enable the offset of crustal segments along transform faults. Crustal extension in areas of high geothermal heat, often along spreading zones, can further fragment and complicate the movement of accreted terranes along fault boundaries and the types of igneous intrusions that rise up into them from below. Metamorphism from the ongoing collision of terranes further masks their identity.

Unraveling the complex history of Alaska’s terranes has tested the mettle of many a field geologist. Large regions of metamorphic rocks that underlie the Yukon and Interior Alaska were once lumped together as a single, cryptic formation, such as the Birch Creek Schist in the Fairbanks vicinity. With the invention and evolution of ion microprobe technology and refined mass spectrometry, it has been possible for researchers to determine the age and environments of the original rocks before metamorphic events masked the original history. It is also now possible to document the sequence of metamorphic events that transformed the rocks. These overprinting episodes caused by younger occurrences of structural deformation no longer obscure the rocks’ earlier histories.

Major terranes of Alaska and adjacent parts of the Yukon and northern British Columbia. —From Colpron and Nelson, 2011

The chronology that follows is a story of how Alaska’s terranes ended up as part of the modern North American continent and what happened to them once they arrived.

PROTEROZOIC EON

Roughly 1 billion years ago, seafloor spreading driven by upwelling mantle heat was the force behind the assemblage of the supercontinent Rodinia. The name Rodinia comes from the Russian word for motherland. About 750 million years ago, the rift ing, or breaking apart of Rodinia separated rocks of the future Siberia from Laurentia, or the ancestral North America. Laurentia’s western shoreline ran through northern Idaho and western Montana and into Canada. Laurentia was south of the equator at this time, but by the end of the Proterozoic Eon, it had moved northward into the equatorial regions.

The oldest rocks in Alaska are about 2 billion years old. They formed near the equator next to the ancient Laurentian continent and are now part of the Arctic Alaska terrane. Rocks of the same age occur in Chukotka, Russia, and on the Chukchi Sea and Beaufort Sea shelves. By contrast, some of Earth’s oldest rocks are the 4.28-billion-year-old Nuvvuagittuq greenstones in northern Quebec, outcropping around Hudson Bay.

PALEOZOIC ERA

The oldest parts of Alaska and the Yukon were assembled during the Paleozoic Era when Laurentia and Baltica (ancestral Europe) lay astride the equator. In Early Devonian time, Siberia and Alaska’s future Farewell terrane sat near latitude 30 north, northeast of Laurentia and Baltica, separated from them by the Uralian Sea. Some of the rocks now found in modern Arctic Alaska and in the Alexander terrane of Southeast Alaska were originally formed at the northern end of an extensive barrier reef complex in the Uralian Sea. These rocks are also found in the Ural Mountains of Russia. Throughout the last 400 million years, these ancient tropical rocks slowly moved northwestward to their present latitudes, well out of the tropics.

An Early Devonian tectonic plate, known as the Northwest Passage Plate, is imagined to occur along this seaway, shaped like the modern Caribbean or Scotia Plates, which are quite tabular. The Northwest Passage Plate separated Laurentia and Baltica from Siberia to the north. The Arctic Alaska and Alexander terranes are now thought to have been part of this Northwest Passage Plate, moving over the top of Laurentia westward into the Panthalassa