Can you break a rock you can't even lift? Can you carve a rock? Can rocks crumble?
Peggy takes a hard-nosed look at how erosion is making it difficult to "save face" on Mount Rushmore.
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.
Water and wind, the two most powerful agents of erosion, can destroy the grandest human-created structures. What other examples of the erosive powers of these agents can you think of? What role does time play in this process?
Additional sources of information
When substances change from liquid to solid, they usually decrease in volume. Water, however, increases in volume when it becomes a solid. To demonstrate what happens when water turns to i ce, you'll need access to a freezer.
Examine physical and chemical weathering in your environment. What examples do you find in your school yard? Observe such things as slabs of sidewalks that have been uplifted by roots of trees, plants growing between cracks, moss on rocks, and areas that are rusting. Look particularly for cliffs or rocky walls that have small rocks at their base. What about your roads and streets? What causes cracks and pot holes in them?
The ratio of Borglum's models to the actual figures is 1:12. If you grew 12 times as big as you are now, how tall would you be? Photocopy a small picture of yourself and on it draw a grid of 1-cm (.4") squares. On a large piece of paper, draw a similar grid of squares 12 cm (4.8") on a side. Using your picture as a guide, copy the details of each small square into the corresponding large square. Arrange drawings of your family or classmates on the wall for your own Mount Rushmore!
Take a trip to a local cemetery to see a variety of granite colors and textures. Are some monuments easier to read than others? Compare the rock types of the weathered ones with the nonweathered ones. Make gravestone rubbings. If there is a monument company in your city, ask for chips or chunks of granite or marble. What could these tell you about carving out of granite or marble?
Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.