Aerial Geology

PREFACE

As a young girl, I overturned a rock in a stream high in the mountains of West Virginia and found it covered with seashells. I knew something about fossils then and had a vague understanding about Earth’s age, but finding a slice of an ancient ocean floor on a mountaintop forever changed the way I saw the world. My fascination was not fleeting and I am now a geology writer, an avid traveler, and a mountaineer.

In many ways, geology is best understood from the air. Altitude grants a greater perspective of the land and helps us begin to visualize the extraordinary forces that have shaped our planet over the last 4.5 billion years. Mountaineering is one way to gain that perspective—the higher you go, the more you see and the more you see, the more you learn. If mountaintops are fantastic classrooms, airplane window seats are even better.

This book highlights one hundred of North America’s most distinctive geologic features and describes how they came to look the way they do from a bird’s-eye view—or an astronaut’s, or a satellite’s. On the ground, deserts appear devoid of moisture but from the air, large-scale features of the landscape reveal that even the most arid places are often shaped by water. Southwest expanses of sandstone—often relics of ancient inland seas—have been sculpted into magnificent canyons by rare rainwater over many millennia.

Follow me from the shores of Alaska, down the West Coast, through the desert Southwest, over the high Rockies, across the patchwork Great Plains, and up the ancient, fossil-rich mountains of my childhood, to the edge of the East. This book is for everyone who ever wondered how seashells end up on mountaintops, and for the high flyers who are transfixed by the view 30,000 feet above the planet. I hope this book changes the way you see the world and inspires you to get out and explore more of it.

100 GEOLOGICAL WONDERS

ALEUTIAN ISLANDS

Alaska

The Aleutian Islands stretch in a 1200-mile-long arc from Russia’s Kamchatka Peninsula eastward to the Alaska Peninsula, marking the division between the Bering Sea to the north and the North Pacific Ocean to the south. The island arc is made up of fourteen large volcanic islands and fifty-five smaller ones, the product of ongoing plate tectonic collisions along the Pacific Ring of Fire (a string of volcanic and seismic activity around the Pacific Ocean rim). From the air, the easternmost islands, near the Alaska Peninsula, are clearly the largest, with the tallest volcanoes.

These islands were formed by the same geologic processes (along the interface between two major tectonic plates) that created the Aleutian Mountains on the Alaska Peninsula. The landscape here continues to be active, as the Pacific Plate dives under the North American Plate, forming a convergent plate boundary. The Aleutian Trench, which runs along this convergence, has been measured to depths over 25,000 feet. As the heavy, waterlogged Pacific Plate is subducted, increasing heat and pressure release the stored water in the downward-heading plate, partially melting the descending and overriding plates. This low-density melt rises to the surface and erupts along a chain of volcanoes on the North American side of the plate boundary. Earthquakes along this trench generate shaking up to magnitude 8.7 and tsunamis that travel as far as Hawaii and South America.

As a reminder that geology isn’t just ancient history, the Aleutian volcanoes continue to erupt and present significant threats, including ash clouds that can damage aircraft and cause jet engine failure. In 1988, the Alaska Volcano Observatory was formed in Anchorage to monitor the Aleutian Islands as well as the state’s other active volcanoes. In recent years, Mount Cleveland and Mount Pavlof have been the most active in the Aleutians, erupting every five to ten years.

What is now the eastern end of the Aleutian Islands also helped set the scene for the first migration of humans to North America. During the last ice age—which reached its peak about 20,000 years ago and ended approximately 11,500 years ago—the Bering Land Bridge connected Russia and Alaska. Back then, huge quantities of the planet’s water were frozen in mammoth continental ice sheets, lowering sea levels and exposing the land bridge for a period of a few thousand years. Sometime between 20,000 and 15,000 years ago, the first people made their way into North America via this bridge, migrating from Asia into the New World. At that time, the modern-day Alaska Peninsula and eastern Aleutian Islands were part of the southern edge of this exposed passageway.

DENALI

Alaska

After being called Mount McKinley for nearly a century, the original Athabaskan name of Denali, meaning “great one,” was officially restored in 2015. And a great one it is. Not only is Denali the highest mountain on the North American continent at 20,310 feet, it’s still growing taller by nearly half an inch a year. From the air, however, Denali can be hard to pick out from surrounding peaks in the Alaska Range. Its double summit is sandwiched between the white expanse of the Kahiltna Glacier to the west and the elongated Tokositna Glacier to the south.

The Denali Fault (a tectonic fault named for the mountain) is part of the greater continental-wide strike-slip fault system that runs along the west coast of North America and includes the notorious San Andreas Fault in California. Strike-slip faults, also called transform faults, occur when two adjacent plates slide past each other, sometimes moving smoothly and other times becoming stuck, eventually releasing their stored energy all at once in the form of earthquakes. This intracontinental fault system is in turn connected to—and driven by—the offshore subduction zone where the Pacific Plate is diving under the North American Plate at a rate of about an inch per year.

Over time, movement along the Denali Fault has given rise to the Alaska Range and Denali. Continued movement along the Denali Fault, which runs under the Alaska Range, accounts for Denali’s ongoing ascension. The north face of Denali, known as the Wickersham Wall, is the largest vertical mountain face on Earth—higher than any face on Mount Everest. From the base of the Wickersham Wall, at 5350 feet of elevation on the Peters Glacier, to the north summit, the face soars upward for nearly 14,000 steep, vertical feet. The avalanche-swept direct route up the face is so dangerous it has only been climbed once, in 1963, by a team of alpinists from the Harvard Mountaineering Club.

The distinction between the highest mountain and the tallest mountain is an important one in geology, and in mountaineering. The highest point on Earth is the summit of Mount Everest, at 29,029 feet, as measured from sea level. But the tallest mountain is measured from base to summit. By this metric, Denali is taller than Mount Everest, as it rises more than 18,000 feet from its base, while Everest stands only 12,000 feet above its base, which sits on the already high-altitude Himalayan Plateau. Both Everest and Denali are dwarfed by the Mauna Kea volcano on the Big Island of Hawaii, which, measured from its base on the ocean floor to the summit, is 33,476 feet tall. However, its elevation above sea level is a mere 13,808 feet.

GATES OF THE ARCTIC NATIONAL PARK & PRESERVE

Alaska

Gates of the Arctic National Park and Preserve is the northernmost national park in North America and it’s not a place for a casual visit. With no roads and no trails, this landscape is one of the most remote and untamed places in North America.

The name Gates of the Arctic is apt when the region is viewed from above: the Brooks Range, a major component of the park, presents an impressive wall separating the mountainous country of central Alaska from the rolling hills of the High Arctic tundra. The east-west-running Brooks Range straddles the continental divide for 600 miles. Water from the range’s north slopes flows into the Arctic Ocean and runoff from the south slopes finds its way to the Pacific Ocean. On the north side of the range is the Arctic foothills tundra, a hilly, deeply permafrosted region that is home to some of the largest migrating herds of caribou in the world.

The view of this landscape depends on the time of year: snowy, icy winters are long and formidable, with temperatures plunging well below zero for nine months of the year. Summers are brief and busy, as an entire ecosystem of life-forms hurries to breed and raise their young in a few short months. These annual freeze-thaw cycles, along with glacial erosion, have sculpted the Brooks Range into one of Earth’s most dramatic landscapes.

A village may seem out of place in this wilderness, but the community of Anaktuvuk Pass, in the northeast area of the park, is home to around 350 people of Nunamiut descent. In fact, people have lived in the region for more than 13,000 years, following the herds of caribou on their annual migrations. This ecozone may have hosted the first humans in North America after their journey from Asia across the Bering Land Bridge.

Just south of the Brooks Range is the Batza Tena obsidian source, an outcrop of volcanic glass that ancient people used to make tools such as knives, arrowheads, and spear points. Each obsidian source has its own unique geochemical signature, enabling geoarchaeologists to trace far-flung individual tools back to their original source. Tools made from Batza Tena obsidian have been found throughout Alaska, transported on foot for hundreds of miles by the seasonally nomadic hunter-gatherers who scraped a harsh existence from this severe environment.

Today, the only ground access into Gates of the Arctic is by way of the Dalton Highway, a primitive unpaved road that runs from Fairbanks, Alaska, up to Prudhoe Bay on the Arctic Ocean and is frequented by large trucks from the oil industry. The highway defines the eastern boundary of Gates of the Arctic National Park and Preserve as it snakes through a pass in the otherwise impenetrable Brooks Range. From the road, entry into the park is tricky, as there is no bridge over the deep, cold waters of the Middle Fork Koyukuk River, which parallels the highway. This natural barrier means that flying is the most practical way to reach Gates of the Arctic. Hardy visitors who make it to the tiny town of Bettles, just south of the park, can arrange to be dropped off by a small float plane equipped to land on one of the many lakes in the area.

BEAR GLACIER

Alaska

An impressive sight from any angle but especially from high overhead, Bear Glacier is the eastern gateway to Kenai Fjords National Park in southern Alaska. From the perspective seen here, the river of ice has flowed downhill for thirteen miles to its terminus in the Bear Glacier Lagoon, a bright blue lake of glacial meltwater separated from the open water of Resurrection Bay in the Gulf of Alaska by a terminal moraine, or ridge of rock deposited by the glacier. A dozen miles upslope to the west lies Bear Glacier’s parent, the Harding Icefield, a 300-square-mile mountainous expanse of ice that’s one of the largest ice fields in North America.

Glaciers form when snow accumulates at high elevations year after year, compressing into a thick sheet of ice. As this ice gets thicker and heavier, it begins moving downslope under its own weight, grinding and carving the rock at its base on its journey to the ocean. These slow-moving rivers of ice are powerful erosive agents, capable of chiseling entire mountain ranges into broad U-shaped valleys and grinding even the hardest rocks into fine powder.

Moraines are accumulations of debris formed by this glacial movement. From above, it’s easy to see that Bear Glacier is adorned with a number of moraines. Lateral moraines are stripes across the glacier’s surface, formed from eroded dirt and rocks that gather at the edge. When tributary glaciers join the main flow, as they do upstream in the Harding Icefield, these stripes merge together into medial moraines toward the middle of the merged ice flows. A terminal moraine is a mass of rocks carried to the end of the glacier and left as the toe of the glacier melts and retreats upslope. Terminal moraines mark the historic end of a glacier, before it began melting back to its current location. Bear Glacier’s terminal moraine dates back several hundred years, when the glacier made its way down to Resurrection Bay. During times of increased melting, the waters of the lagoon spill over into the bay, in a glacial lake outburst flood. As overflow sloshes out, the lagoon’s water level drops and chunks of ice break off from the toe of the glacier, forming car- and house-sized icebergs that can be seen floating in the lagoon. The lagoon’s bright green-blue color results from light bouncing off glacial flour—very finely ground sediments in the water.

Just upstream from the lagoon, the body of Bear Glacier is broken into deep crevasses that crack open as the glacier moves. Crevasses typically open in lines parallel to the downhill direction of glacial movement, but sometimes cracks will also appear running perpendicular to the direction of travel, making a cross-hatch pattern. These deeply crevassed areas most often occur atop rocky outcrops buried deep under the ice.

MALASPINA GLACIER

Alaska

Glaciers may be rivers of ice, but they can take many shapes based on the underlying topography. The Malaspina Glacier in southern Alaska, for example, looks almost perfectly round from above. It is a piedmont glacier, which forms when one or more glaciers spill out onto a relatively flat plain, where they spread laterally, like pancake batter on a grill. The Malaspina is the largest piedmont glacier in the world—bigger than the state of Rhode Island.

This mass of ice covers more than 1500 square miles and is fed by several other glaciers that descend from the Saint Elias Mountains onto the coastal plain between Icy Bay and Yakutat Bay. The edge of the Malaspina comes within a few miles of the Gulf of Alaska, but its terminal moraine of piled-up rocks keeps it from reaching the water.

From the air, you can see wavy, circular, and zigzag patterns across the top of the glacier. The brown lines against the white ice are moraines—rocks, soil, and dust that get scraped up by the glacier as it moves, then are left on top of the ice, usually along the sides of the glacier. When two glaciers come together, these lines of debris merge to form a medial moraine close to the center of the ice.

Glaciers that flow at steady rates tend to have relatively straight moraines, while those that periodically surge because of increased melt or steep changes in topography develop wavy moraines. These are caused by the folding, shearing, and compression of the ice. The patterns of curves, zigzags, and loops on the Malaspina Glacier are the result of such surges as well as many individual rivers of ice combining into one ice mass on the flat plain.

Measuring the thickness of a glacier isn’t a perfect science, but a useful technique involves using seismic waves to create a three-dimensional picture of the interior of the ice. Such studies have shown the Malaspina to be over 2000 feet thick. The ice is so heavy that the bottom of the glacier has sunk nearly 1000 feet below sea level. Long-term studies of the health of the glacier have revealed that the Malaspina lost more than sixty feet of thickness between 1980 and 2000. The meltwater produced from this shrinkage was sufficient to raise global sea levels by half of one percent during that time period.

GLACIER BAY

Alaska

Glacier Bay’s collection of more than 1400 glaciers looks impressive from a satellite view, but 250 years ago, this inlet in the Gulf of Alaska was home to just one gigantic glacier called the Grand Pacific Glacier—a chunk of ice more than 4000 feet thick. An aerial perspective helps highlight the powerful erosive forces produced by millions of years of advancing and retreating glaciers. From the air, the main channel of the bay traces where the imposing body of the Grand Pacific Glacier used to reside. Along the shoreline, dozens of branches, inlets, and lagoons have been carved out by rivers of ice as they’ve made their way from the mountains down to the sea.

When these sluggish ice flows reach the relatively warm waters of Glacier Bay, huge chunks of ice calve—break off—from the leading edge of the glacier. These chunks then become icebergs, which can float for years, sometimes presenting a hazard to boats in Glacier Bay. After a magnitude 8.4 earthquake struck the region in 1899, Glacier Bay was closed to ships for nearly a decade in response to the hazards posed by icebergs that were calved during the shaking.

The effects of climate change on ice are undeniable, looking at Glacier Bay. The Grand Pacific Glacier that once covered the entire bay has retreated by more than sixty-five miles in the last few hundred years, to the head of the bay at Tarr Inlet, leaving dozens of separate glaciers in its wake. This retreat is driven by melting ice, as well as increased calving at the leading edge of the glacier, where it meets the open water of the bay. It’s interesting to note that despite an overall trend of retreat, many glaciers around Glacier Bay have been advancing in recent years—a result of offshore weather patterns bringing increased snowfall to the Fairweather Range, where many of the glaciers originate.

Accessible only by boat, Glacier Bay National Park is located in the southern Gulf of Alaska, near Juneau. No roads run to the park and only a few trails lead to the interior. On the water, cruise ships and kayaks are met by soaring walls of ice that spill down into the bay.

ALEXANDER ARCHIPELAGO

Alaska

The coastline of British Columbia and southeast Alaska is a maze of more than 1000 islands, inlets, and peninsulas sprinkled throughout the Gulf of Alaska, forming what is called an archipelago. In a satellite image, the deep channels you see dividing the islands from each other and from the mainland hint at a long history of separation. Many of the landmasses here were actually formed near the equator several hundred million years ago, then pushed to their current positions along the coast of North America by plate tectonics during the Jurassic Period, around 200 million years ago. Geologists refer to these as exotic terranes, because they formed elsewhere—in this case, in tropical equatorial seas—and were then transported, eventually becoming part of an existing landmass. During the last ice age, the archipelago’s submerged coastal mountains were scoured by glaciers and carved into the puzzle of heavily eroded islands we see today.

The meandering route taken by cruise ships, ferries, and fishing boats through this morass of islands is the Inside Passage, which avoids the hazards of the open ocean and links many otherwise isolated coastal and island villages. The dry land of the islands is literally just the tip of these “icebergs”; when viewed from the seafloor, the islands are actually huge mountains, with only their tops peeking above the waves.