Rocks : LUSENET : Introductory Geology, Oswego State : One Thread

What kind of minerals is Mt Rushmore made up of and will weathering effect the surface during our lifetime.

-- Nick Duval (, March 02, 2001


The geologic formations of the heart of the Black Hills region are also evident at Mount Rushmore, including large outcrops of granite and mica schist.

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.

-- Mike Porter (, March 05, 2001.

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