Metamorphic Bears EarthCache

The oldest terrane and rocks in Denali National Park are found near the park entrance. Visitors to the park are likely to first set foot on or see these rocks when they stop at the visitor’s center. The marbled rusty brown, buff to gray and white rocks underlying the park entrance are referred to as the Yukon-Tanana Crystalline Rocks. These are a series of metamorphic rocks including quartz mica schist, quartzite, phyllite, slate and marble, which originated as shallow ocean sediments ranging from 400 million years ago.

The Yukon-Tanana Crystalline Rocks are found throughout most of Alaska. In Denali they are exposed in an East-West band and are best seen here at the Savage River.

These rocks have been buried very deep for a long time, and subjected to heat and pressure that changed them into metamorphic rocks.

The Terranes of Denali National Park

According to James A. Madonna, Retired University of Alaska Fairbanks professor, molten magma deep within the earth pushed upward forming the Yukon-Tanana Terrane about 90 million years ago. The Yukon-Tanana Terrane is found in the easternmost parts of Alaska.

For the next 300 million years, ocean environments dominated the site of Denali, where marine shelf, slope and basin materials accumulated or grew to become the shales, limestones and sandstones found in the park. These ocean sediments hardened into rock to form many of the great mountains in the eastern portion of the park. Fossils found in the park 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.

This sedimentary neighborhood has a long history of geologic puzzle pieces thrown together to form Denali National Park, and most of Alaska. 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 does not show any obvious evidence of collision and accretion. 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.

GEOLOGY

There are three main classes of Rocks – Sedimentary, Igneous and Metamorphic.

Sedimentary Rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers.

Igneous Rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet's mantle or crust.

Metamorphic Rock is the result of the transformation of an existing rock type in a process called metamorphism, which means "change in form". The rock is subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. It may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust. They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior.

This area is a group of metamorphic and igneous rocks that were once buried deep within the Earth’s crust but are now exposed at the surface by a process of uplift and crustal extension. They began as original sedimentary rocks (limestone, dolomite, and shale) and have been metamorphosed to gneiss (pronounced “nice”) and marble and have been intruded by igneous rocks (granite and pegmatite).

The posted coordinates will take you to the parking lot and trailhead for the Savage River Loop. It is located at mile 15 on the park road. You can drive your car here or take the free Park Shuttle to this location. This is the farthest you can drive your own car in this National Park. Just past this parking lot is a Ranger Booth that stops all public traffic unless you have a special Park Permit to enter at this point. You will begin the 2 mile hike here. You should plan on a 1 ½ - 2 hour roundtrip. You will be heading North along the East Side of the Savage River.
When you get to the bridge at North 63° 45.084 West 149° 17.4941, you will turn and begin heading South on the West Side of the Savage River. From here you will be facing a very large example of Yukon-Tanana Crystalline Rock. We thought this was amazing, as it appears to be a mother bear with a cub on her back. The Bears are at North 63° 45.030 West 149° 17.582

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