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Mount Rushmore

 

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Overview
Over 2.5 million people a year visit Mount Rushmore in the Black Hills of South Dakota, where the world's largest presidential statues gaze at them through eyes 11 feet across. The sculpted heads of Washington, Jefferson, Lincoln, and Theodore Roosevelt measure 60 feet from chin to hairline. Noses are 20 feet long, and mouths stretch 18 feet. Had the presidents been carved full-body, they would stand 465 feet tall. (The Statue of Liberty stands 151 feet tall; the statue of President Lincoln in the Lincoln Memorial is 20 feet tall.) In 1927, South Dakota commissioned sculptor Gutzon Borglum to create a massive tourist attraction, "...colossal art, in a scale with the people whose life it expresses." Borglum chose to create his work from Mount Rushmore, knowing that it was a solid mass of granite. Granite is a tough, hard rock because it consists of interlocking crystals of the minerals quartz, feldspar, and mica. This granite formed at great depth, about 13 kilometers (7.8 miles), when a body of molten rock (magma) rose and cooled slowly, pushing up a dome-shaped structure now known as the Black Hills. This dome measures about 200 kilometers (120 miles) long and 95 kilometers (57 miles) wide. Weathering breaks rock down into smaller pieces. Granite resists weathering more than layered and bedded sedimentary rocks do. Rock outcrops are constantly subjected to the elements, which gradually cause chemical changes in the rock's minerals. Oxygen and carbon dioxide in runoff waters produce chemical weathering. Physical weathering occurs when water, lodged in fractures in the rock, freezes and expands in all directions, forcing the sides of the fractures apart. Tree roots have the same effect. Rocks exposed to long periods of alternating heat and freezing disintegrate into sand and clay. To preserve the Mount Rushmore sculpture, experts needed to predict which of the granite blocks would shift. They conducted a $250,000 high-tech checkup, which included photogrammetry and 3-D, AutoCAD imaging. First, they shot a series of 300 overlapping photographs of the monument from precision cameras mounted on an airplane and a helicopter. From data provided by these photographs, a computer created 3-D projections of the internal fracture system. With these images, preservationists can estimate the potential for damage to the monument. They can determine which fractures are stable, which will need to be filled with silicone, and which will eventually have to be held together with steel pins to prevent movement.

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