Lecture 22 - Alaska Geology and Denali National Park

Outline

I. Alaska and its Parklands

II. Microplates and Accretionary Tectonics

III. Wrangell-St. Elias National Park

IV. Denali National Park

I. Alaska and its Parklands

Alaska reaches from sea level in its fjords to over 20,000 feet at the top of Mt. McKinley. With its high mountains, volcanoes, glaciers and fjords, Alaska has spectacular geology and scenery and is a natural for many parklands that are set aside to preserve these features. Alaska contains eight national parks, all with notable geologic features. Most of these parks were declared national monuments in 1978 by President Carter and then in 1980 Congress passed the Alaska National Interest Lands Conservation Act, which upgraded most of these monuments to National Parks. Geologic highlights of these parks are given below:

1. Aleutian- Alaskan Islands- Subduction of the Pacific Plate beneath North America generates a string of andesitic volcanoes that is known as the Aleutian- Alaskan Islands. Two national parks (Katmai and Lake Clark) and one national monument (Aniakchak) are located in this region. Redoubt and Mt. Spurr volcanoes are located within Lake Clark NP and both of these have erupted in the last few years. The summer 1992 eruption of Mt. Spurr closed the Anchorage International airport and sent ash clouds into the atmosphere that circled the globe. Redoubt erupted last in 1989-1990. Katmai National Park contains 15 active volcanoes including Novarupta, which erupted in 1912 in the largest eruption of the century. This eruption was about 10 times as large as Mt. St. Helens (~ 7 km3 of pyroclastic material compared with Mt. St. Helens .7). The valley of ten thousand smokes was covered with about 700 feet of pyroclastic material, ash circled the globe for over 2 years reducing solar radiation in the northern hemisphere lowering temperatures. Aniakchak National monument is located in an old volcanic caldera that formed about 3500 years ago and was last active in 1931.
2. Arctic Rockies- Kobuk Valley and Gates of the Arctic National Parks are basically the northward extension of the Rocky Mountains that extend through the western U.S. and Canada. The Brooks Range is exposed in Gates of the Arctic National Park and consists primarily of Paleozoic and Mesozoic sedimentary and igneous rocks that were folded and faulting during late Mesozoic to early Cenozoic mountain building. The mountain building was primarily due to the continual collision of small exotic microplates with the continental margin. These various microplates are exposed in the parks to the southeast, which are part of the Pacific Border Province. Denali National Park is one of these parks.
3. Pacific Border Province- Glacier Bay, Kenai Fjords, Wrangell St. Elias and Denali National Parks are within this geologic province which extends from Alaska to southern California and is characterized by fragments of crust that have been transported from somewhere else and attached to North America through tectonic accretion. The Salinian block on which UCSC's campus is located is one of these "exotic terranes" within the Pacific Border Province. In addition to the Alaskan Parks in this province being made up of an amalgamation of exotic rocks from other locations, Glacier Bay and Kenai Fjords National Parks are covered with extensive glaciers that have carved beautiful fjords. To understand the geology exposed in any of these Parks, we must first discuss microplates and accretionary tectonics.

II. Microplates and Accretionary Tectonics

As geologists began to study rocks in many mountain belts around the world it was discovered that often very different rock types with different geologic histories and fossil assemblages would be juxtaposed with a fault between them. Large regions that share similar rocks and therefore a common geologic history are called terranes. A clear example of a region that contains many different terranes bounded by faults is along the coast in northern Canada and Alaska. For a long time the relationship between these terranes remained a mystery however once plate tectonics became widely accepted it provided a mechanism to explain the joining of these disparate fragments. All of these pieces represent small microplates that originated in other locations and were carried along with subducting oceanic lithosphere to collide and accrete (attach) to the west coast of North American. These microplates are often referred to as "accreted terranes". Many of these accreted terranes collided with North America south of their present location and traveled north along the many strike-slip faults in the region. Some of these terranes traveled thousands of miles before accreting to the North American margin.

We can determine the travel path of some of these terranes by measuring the direction of magnetization that was recorded in rocks of different ages when they were formed. This technique is called paleomagnetism. Paleomagnetic studies of the Yakutat Terrane exposed in Wrangell-St. Elias National Park indicate that this microplate traveled from somewhere off the California coast to its present location in Alaska. Paleomagnetic studies of the large Wrangelia terrane indicate that it originated as a volcanic arc near the equator and traveled north at least 3000 km since the beginning of the Mesozoic It appears to have collided with North America south of where it is seen in either Wrangell-St. Elias and Denali Parks and broken into slivers that were carried north along strike-slip faults.

III. Wrangell-St. Elias National Park

Wrangell-St. Elias and its adjacent park in the Yukon Territory of Canada is the world's largest parkland, in fact, it is larger than the combined area of all national parks in the other 49 states combined! It contains three different mountain ranges, the St. Elias Mountains and Chugach Mountains which are composed of dominantly metamorphic and sedimentary rocks and the Wrangell Mountains which consist of andesitic volcanic rock. The St. Elias Mountains are the world’s highest coastal mountain range with the Yukon's Mt. Logan at19,850 feet (second highest mountain in North America) and St. Elias Mountain at 18,008 feet. The height of these mountains and their proximity to the coast where rainfall is very high allows them to maintain the largest assemblage of glaciers in North America.

The geologic history of this park is as follows:

1. The rocks of the 5 major exotic terranes that make up Wrangell-St. Elias National Park were formed at different distant locations south of Alaska.
2. Accretion of these microplates began in early Cenozoic time. Associated with microplate collision was volcanic activity, metamorphism, faulting, folding and uplift of rock units into the present mountains.
3. The last microplate, the Yakutat terrane is still in the process of accreting to North America and the force of this ongoing collision is responsible for many large earthquakes in this region and for continued uplift of the mountains.
4. Recent andesitic volcanism in the Wrangell range is due to ongoing subduction of the Pacific Plate beneath North America.

IV. Denali National Park

Denali shares a similar geologic history to Wrangell-St. Elias National Park. It too consists of several exotic terranes, many are the same as seen in Wrangell-St. Elias, that accreted to North America in early Cenozoic time. The Alaskan Range which comprises much of Denali and which contains Mt. McKinley at over 20,000 feet, is primarily granitic and represents a batholith that was intruded into the various terranes during their collisions with North America. The uplift of this batholith resulted from continued collision and the high altitude of Mt. McKinley is due to its unique position at the boundary between the Pacific and North American plates where subduction and strike-slip motion are both occurring and pushing Mt. McKinley up. As in Wrangell-St. Elias, continued tectonic motions generate abundant earthquakes in this area. Of course much of the scenery in Denali is presently being shaped by a number of active glaciers.

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