Inside Passage, Alaska
In the late-summer of 2006, Becky (my wife) and I booked a cruise to explore and enjoy the “Inside Passage” to southeast Alaska. The “Inside Passage” is the cruise to see forest-covered islands, tidewater glaciers, and ice-scoured fjords. With the exceptions of Haines, Skagway, and Hyder, you cannot reach this Alexander Archipelago by road. We took the “Golden Princess” red route from Seattle, Washington.
Southeast Alaska (the Inside Passage), and coastal British Columbia are primarily made up of accreted terranes, (the Alexander, Wrangellia, Chugach, and Yakutat). In the diagram 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. Remnants of the Taku and Yukon-Tanana terranes, whose origins are closer to the continental edge of North America, were smashed up and joined to North America on the inland side of these later arriving rock packages. The mainland Coast Range is made of massive amounts of hard crystalline igneous rock of the Coast Plutonic Complex, (imaged below).
Imaged above, some of the magma entered the region when the Alexander and Wrangellia terranes were accreted, and additional magma was intruded during the active subduction zone of Late Cretaceous to Early Paleocene time. Extreme uplift throughout Cenozoic time led to the erosion of the overlying rocks. So much overburden rock was removed that intrusive igneous rock that cooled deep within the Earth’s crust is now exposed at the surface. These once deep rocks have recently experienced repeated glaciations, and much of the Coast Plutonic Complex is still covered by snow and icefields.
Imaged above; 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.
Southeast Alaska is a geologically active. Current geological activity is manifest in earthquakes, glacial movement, and the less perceptible uplift and erosion of the earth’s crust. Towering mountains throughout the region continue to grow by tectonic uplift, (rates of 1.3 – 3.8 cm/yr). Large-scale erosion by the flow of glaciers and rivers, which occurs at a rate similar to uplift, has removed most of the original mountains that began to grow more than 90 million years ago. These active processes of mountain building and erosion in Southeast Alaska have created the vast network of waterways that provide an ideal route for viewing the diverse geology and biology of this region.
Our first cruise stop was Skagway. Taiya Inlet, at the northern end of Lynn Canal, is the passageway for cruise ships bound for Skagway, which means “home of the north wind” in the Tlingit language. Klondike gold seekers discovered this north wind in 1898 when they descended on the area, swelling Skagway’s population to 20,000. Becky and I chose to book a tour that took us along the Klondike Highway for a few hours.
The modern Klondike Highway provides access into Canada through the deeply glaciated Burro Creek granodiorite and quartz monzonite that forms the bedrock of the Coast Mountains. No gold was ever found in the Skagway Valley. The gold fields were located approximately 900 km to the north, near the junction of the Klondike and Yukon Rivers at today’s Dawson City. Skagway became internationally known solely as a gateway to the Klondike gold fields. Today’s modern Klondike Highway follows the old Chilkoot Trail trading route (pictured below).
Pictured above, The Klondike gold seekers climbed 1,140 m in 27 km to cross the coastal mountains northward into the Yukon River valley drainage basin. This well-traveled trail follows the narrow Taiya River valley past the Irene Glacier. The valley is fringed by many 1,830 m peaks of Cretaceous to Early Tertiary granodiorite and culminates in the infamous scree slope know as the “Scales”. Here treacherous piles of loose granodiorite talus provide the only footing. Miners typically hauled 900 kg of equipment and a year’s supply of food up the “Scales” and over Chilkoot Pass into Canada. East of the divide, miners had a relatively easy descent to the south end of Bennett Lake, where they could next risk life and limb on a homemade raft to travel the rest of the way to Whitehorse and on down the Yukon River to Dawson.
Gold Rush stampeders used the Skagway area for the same reason rivers and glaciers did: it was a point of geologic weakness. The faults throughout the area allowed glaciers to carve more deeply here than elsewhere. As a result, the White and Chilkoot Passes had lower elevations and were passable year-round. They were the most used breaks in the Coast Mountains for hundreds of miles. Once over the passes it was a short journey down the Yukon River to Dawson City.
Our next cruise-stop was Juneau. The capital city of Alaska is situated on either side of the narrow and linear water-filled Gastineau Channel, between the mountains of Douglas Island on the west and the mainland on the east. Imaged below, high mountains east of the city form a rock wall between Gastineau Channel and the Juneau Icefield, one of the largest icefields in the Pacific Northwest with an area of over 3,070 square kilometers. The glaciers that flow from the Juneau Icefield have retreated dramatically since Juneau was established.
We took a tour to the northern part of Juneau. It occupies the flat-bottomed Mendenhall Valley, which was vacated as the Mendenhall Glacier retreated to the east. Pictured below, the valley is floored by post-ice age marine sediments overlain by glacial outwash and mud, and in the upper valley glacial moraines from the Little Ice Age retreat of Mendenhall Glacier. The glacier has retreated about 4.8 km since 1767, providing space for a small lake.
The Gastineau Channel is one of the most impressive of the linear glacially carved channels that parallel the Coast Mountains between Lynn Canal on the north and Prince Rupert on the south. These long depressions mark the location of numerous faults collectively called the Coast shear zone. Most of the displacements probably occurred between 90 – 50 million years ago however, some movement on the Gastineau Channel fault is more recent. Faulting along Gastineau Channel juxtaposed older Taku terrane and younger Gravina belt rocks. Pictured below, the steep-sided channel, which provided us with a dramatic setting for Juneau, formed when glaciers preferentially carved the less resistant faulted and sheared rocks of the Coast shear zone.
Our next visit on the cruise-ship was the “Tracy Arm Fjord”. (Imaged below), the “Tracy Arm” is a narrow fjord leading into Holkahm Bay about 80 km southeast of Juneau. The fjord extends more than 32 km from the western edge of the Coast Mountains eastward to tidewater glaciers surrounded by imposing mountains that rise about 2,100 m above the sea. Numerous steep valley on either side of this fjord was once filled with ice feeding into the larger glaciers that carved the fjord. The steep mountainous region from the eastern shoreline of Holkham Bay to the Canadian border has been protected as the Tracy Arm-Fords Terror Wilderness.
Pictured below, spectacular evidence for the glacial processes that carved the fjords can be found throughout the Holkham Bay and Tracy Arm area. The shallow water that extends across the middle of Holkham Bay barely covers the end moraines of the glaciers that carved the fjord. On the north side of the bay, till has been breached by meltwater or tides to form a narrow channel into the entrance of Tracy Arm. Water rushing through this channel created large standing waves as the tide pours in and out.
Pictured above, a dense forest of spruce, hemlock, and cedar cling to the steep slopes underlain by the metamorphic rocks at the entrance to Tracy Arm. The nearly vertical bare rock faces east of the entrance are carved into igneous plutons and high-temperature metamorphic gneiss that is more resistant to the forces of erosion.
Pictured above, spectacular horizontal glacial grooves have been cut into the solid rock along the walls of the fjord. U-shaped valleys can be observed at waterline or up the steep slopes along both sides. These valleys are all hanging valleys with respect to the bottom of the fjord, which is more than 300 m below sea level. On a clear day, pointed mountain peaks or horns can also be seen; these mountains were formed at the head of valley glaciers that were flowing in divergent directions away from the peak.
One of the most remarkable aspects of the steep glacially carved fjords in Southeast Alaska is the ability of plants to rapidly populate the cliffs that are initially devoid of soil. The succession of plants typically begins with moss, small flowers, and alders that can grow without much soil in small fractures in the rock or where small pockets of glacial till were left behind by the retreating ice. These plants rapidly break down some of the rock and add nutrients to remarkable “gardens” of additional plants clinging to the near-vertical walls of Tracy Arm.
Pictured above and below, Tracy Arm terminates in two looming walls of ice that extend about 150 m from the bottom of the fjord to the top of the glaciers: Sawyer and South Sawyers. These glaciers are very active and calve frequently, often choking the fjord with everything from small bits to large icebergs. Although few records exist for the position of the ice in the pat, chars show that Sawyer Glacier may not have retreated more than about 8 km in the 160 years.
Tidewater glacier ice extends all the way to the bottom of the fjords. The glaciers initially calve ice from the upper portion of the terminus face. Imaged below, the remaining and formerly over-pressured buoyant submarine basal ice may sometime later shoot up to the surface from the fjord bottom, unannounced by sound effects.
In 2006, all the glaciers we saw were actively thinning, retreating, and producing much berg ice. Pictured below, some icebergs were spotted floating about 130 km to the Stephens Passage.
Pictured above, in lower Tracy Arm, polished and extensive fjord walls expose the Great Tonalite Sill, a series of tabular, quartz-rich, dioritic plutons that follow the Coast Shear Zone between Prince Rupert and Skagway. In Eocene time, metal-bearing fluids deposited gold and quartz in veins in the Gravina Belt, Taku terrane, Yukon-Tanana terrane, and Great Tonalite Sill.
Pictured above, the Clarence Strait is a long body of water that separates Prince of Wales Island on the west from smaller islands and the mainland on the east. Although there are no significant towns along the shores, the strait serves as a major artery of the Inside Passage. Pictured above, the eastern shore of Clarence Strait is formed by Paleozoic Alexander terrane rocks south of Ernest Sound, and Mesozoic Bravina belt rocks to the north.
Imaged above, Ketchikan is situated in the southernmost part of Southeast Alaska and is often considered a gateway city for travelers traveling north from the lower forty-eight states on the Inside Passage. The city lies along the eastern shore of Tongass Narrows between Revillagigedo and Gravina islands and is about 32 km west of Misty Fjords National Monument.
Ketchikan is underlain by rocks of the western metamorphic belt and is situated along the eastern boundary of the Gravina belt. However, unlike its northern counterparts, Ketchikan is now far south of the remaining tidewater glaciers in the Inside Passage. Perhaps the most striking thing I noticed on arrival in Ketchikan was the predominance of green rocks. The bright coloration is largely derived from chlorite and epidote that grew during low-to medium-temperature metamorphism.
The Ketchikan area has a long mining history with deposits of copper, molybdenum, uranium, and iron found nearby. Mines and known mineral deposits stretch from the Coast Mountains north and east of town to the Prince of Wales islands. The Cleveland Peninsula was extensively explored for gold during the nineteenth and early twentieth centuries.