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Denali National Park and Preserve Geologic Formations

First and foremost, there's the Mountainall 20,320 feet of it, and still growing, at about 1 millimeter per year. In the process of plate tectonics, (the Pacific plate is diving beneath Alaska, or the North American Plate) land surfaces in Alaska are continually compressed and folded, which pushes up Mt. McKinley, as well as the rest of the mountains in the Alaska Range. Although surrounded by many glacier-clad mountains of similar grandeur, Mt. McKinley (or Denali, as the Athabascans call it), is primarily made of granite, which is very hard and resists weathering much better than its sedimentary (shale, limestone and sandstone) neighbors. As Denali is pushed up, it remains, like a resistant sentinel, above the others, which wear down faster through the eons of freezing, thawing, and glaciers grinding and scraping. At 56 million years old, Denali rock is also much younger than most of its sedimentary neighbors, which varies from 100 to 400 or more million years old.

That sedimentary neighborhood is a long history of geologic puzzle pieces thrown together to form the park, and most of Alaska, as we know it. The Pacific plate has been acting like a conveyor belt for hundreds of millions of years, bringing bits of islands, ocean floor, and slivers of other continents northward to form accretionary terranes, which are pieced together like a jigsaw puzzle. This accretion of land is an ongoing process today, as the Pacific plate moves northward, colliding with Alaska at about 5 centimeters per year. New "additions" to Alaska are, of course, a very slow process, and to most people, even the most recent addition (the Yakutat block, along the southeast coast of Alaska) does not show any obvious evidence of collision and accretion. However, most of the terranes are identified as packages of rocks that are surrounded by faults, and have different rock types, fossils, and other physical properties, unlike their neighbor terranes.

The oldest terrane and rocks in the park are found near the park entrance, and are called the Yukon-Tanana rocks. These are shallow sediments with volcanic flows and intrusions (molten injections of rock) that formed in a very young Alaska, about 400 million years ago. These rocks have been buried very deep for a long time, and subjected to heat and pressure that changes the rocks into metamorphic kinds (schists, gneiss, phyllites).

For the next 300 million years, ocean environments continued to dominate the site of Denali, where marine shelf, slope and basin materials accumulated or accreted to become the shales, limestones and sandstones of the Farewell terrane. These ocean sediments hardened into rock to form many of the great mountains in the eastern portion of the park, such as Mount Pendelton and Scott Peak. Fossils found in the Farewell terrane suggest that at least some of this period of time, the climate was very tropical, and lush coral reefs and other warm water fauna flourished.

Among the exotic terranes in Denali are those referred to as island arcs (volcanic island chains, like the Japanese islands), which are identified by having volcanic and marine sedimentary rocks on top of each other. The 200 milllion year old Pingston and McKinley terranes are possibly from an island arc environment. Along the park road just west of Eielson Visitor Center, pillow basalts (lava extruded under ocean water, forming pillow shaped blobs) can be seen in the roadcut.

During the birth of Mt. McKinley 56 million years ago when molten magma solidified deep beneath central Alaska, volcanic activity (eruptions at the surface) was also occurring in the park, and produced red, yellow and brown basalts, rhyolites, and other volcanic rocks. These rocks can be seen along the park road, particularly at Polychrome Pass, named for the colorful volcanic rocks exposed there. Things again heated up at Denali about 38 million years ago, when another period of volcanic activity resulted in the basalts and andesites found exposed at Mt. Galen, and along the park road at the west end of Eielson Bluffs. Similar to Mt. McKinley, another granitic blob crytallized at depth beneath the surface to become Mt. Foraker, the second tallest peak in the park at 5303 meters (17,400 feet).

As a final tribute to the active tectonics of Alaska, a series of faults have fractured the park and most of the state in the last 100 million years. In Denali, this group of faults is known as the Denali fault system, which arcs East-West through the park (and most of the state) for 1200 kilometers (720 miles). Portions of the fault trace are visible within the park at Bull River divide, Easy Pass, and other locations. As geologic mapping in Denali is primarily at a reconnaissance level, much remains to be understood regarding the many facets of geology.

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