Rocky: 2 Billion EarthCache
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This site is accessed through the North Rim entrance to Black Canyon of the Gunnison National Park. Take Hwy 92 from Delta or at the Blue Mesa Dam on Hwy 50. Look for Black Canyon signage and North Rim Road at Crawford.
Geologists classify rocks into three different major categories; Sedimentary, Igneous and Metamorphic.
Sedimentary rocks are formed from the intense pressure of many layers of sediments (e.g. sand, mud, or silt) piled on top of one another. When compressed for log periods of time they form a solid rock. Examples of sedimentary rock are sandstone, mudstone, and limestone.
Metamorphic rock is sedimentary or igneous rock that has been changed from its original state by intense heat and pressure. Limestone, a sedimentary rock, becomes marble when it is exposed to intense heat and pressure. Marble is a metamorphic rock. Most of the rock in the canyon walls of Black Canyon is metamorphic rock which was formed when the ancient, sedimentary “basement rock”, far below sea level, in this area was exposed to intense heat and pressure. Gneiss and Schist are the two main forms of metamorphic rock found at the Black Canyon. Gneiss (pronounced like nice) was buried deepest, meaning it underwent the most intense temperatures and pressures. It’s hard to tell what the original rock was, but geologists think that the rock around here was originally sandstone, a sedimentary rock. Schists weren’t buried as deeply, meaning they didn’t change nearly as dramatically as the gneisses. So they still look similar to their original rock form. Schists look like a stack of paper. The main difference between gneiss and schist is the thickness of their internal layers, or lamellae (la MEL lee). Gneiss has thick lamellae and schist have very thin, fine layers.
Igneous rocks are produced by volcanic activity. This hardened magma is classified into two types; Intrusive and Extrusive. Extrusive igneous rock is magma that has erupted above the surface of the earth and hardened. An example of an extrusive igneous rock is the lava that erupted from the West Elk and San Juan Mountain areas 26 to 35 million years ago and bonded with mud and ash to form the “breccia” surface rock in this area. Intrusive igneous rock is molten rock than hardened below the surface of the earth. An example of an intrusive igneous rock is the pegmatite in the rock walls of Black Canyon. Pegmatite is composed of the minerals feldspar, mica and quartz. Approximately 1.4 billion years ago, these minerals combined in a molten form and were forced into the cracks of the metamorphic rock that now form the canyon walls. When they hardened they formed the rock that appears as they pinkish streaks you now see on the canyon walls.
The interesting formations you see throughout the Black Canyon are caused by the differences in the rocks that form them. The basement rock, believed to have been sandstone, was subjected to intense heat and pressure changing it to gneiss and schist. Molten minerals mixed and created pegmatite which was forced through the cracks in the gneiss and schist. A huge portion of the crust of the earth containing this metamorphic and igneous rock was forced upward by volcanic activity and shifts in the earth’s plates (The Gunnison Uplift). As the Gunnison River has carved down through the hard Precambrian rock, it has revealed these different types of rock and eroded them at different rates due to their differing properties. Pegmatite is more resistant to erosion than gneiss or schist, so it is the pegmatite that forms the spires and dikes throughout the canyon as the gneiss and schist are eroded away around them.
The rock that is exposed in Black Canyon is believed to be some of the oldest rock in the world, and it is a rare and humbling experience to see this rock that was created some 2 billion years ago.
To claim this cache you must email me the answer to the following question which you will gather from your observations at the posted coordinates:
What is the name of the overlook at these coordinates and describe the geological formation (approx. dimensions) that gives it its name?
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