to Videolink arrow

Newton logo to print

Mount Rushmore

 

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.

Activity

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 ice, you'll need access to a freezer. Materials
  • water
  • glass and plastic containers with lids
  • plastic containers without lids
  • clear plastic wrap
  • rubber bands
  • trays large enough to hold containers spaced far apart
  1. Divide into three groups. Each group will conduct this demonstration using a different form of container.
  2. Two groups will use containers with lids: one group will use plastic, one glass. Fill the containers with water until they overflow, then secure the lid tightly without spilling. Wrap each glass container in aluminum foil or plastic wrap in case it shatters. Place the containers on the trays with several inches between them, and place the trays in the freezer.
  3. The third group will use water to fill plastic containers nearly full and place them on trays. Then place the trays in the freezer, add water to fill the containers to the top, and cover the containers with clear wrap secured by a rubber band. 4. Leave the containers in the freezer overnight and bring them back to the classroom for observation and comparison. When water freezes it increases in volume by 10% and exerts a great amount of pressure. Spring thawing and freezing results in repeated melting of ice and freezing of water trapped in rock fractures. This "frost wedging" promotes mechanical (physical) weathering.
    Questions
    1. How does water change when it goes from liquid to solid?
    2. Does water change in density when it goes from liquid to solid and back? How would you conduct an experiment to determine this?
    3. How could you measure the exact volume of the expansion of water in a frozen state?
    4. Why are docks removed from northern lakes every winter? Why do people insulate hot water pipes in winter?

Resources

  • Chu, D. (1991, July 22) About faces: Sioux editor Tim Giago sees Mount Rushmore as a symbol of dishonor. People Weekly, pp. 68-69.
  • Fodor, R.V. (1983) Disappearing rocks: Weathering. In Chiseling the earth: How erosion shapes the land. Hillside, NJ: Enslow Publishers.
  • Geologist Badge. In Webelos scout book (1986), pp. 177-193. Irving, TX: Boy Scouts of America.
  • Heard, A. (1991, July 15 & 22) Mount Rushmore: The real story. The New Republic, pp. 16-18.
  • Shaff, H., & Shaff, A. K. (1985) Six wars at a time: The life and times of Gutzon Borglum. Sioux Falls, SD: Augustana College Center for Western Studies.
  • Sklarewitz, N. (1991, June) Mt. Rushmore turns 50. Boys Life, pp. 40-41.
  • Smith, R.A. (1991, July/Aug) Shrine of democracy. American History Illustrated, pp. 26-41.