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Caves have long been places of mystery and intrigue. Prehistoric people used them for shelter and decorated them with some of the earliest-known works of art. Pirates used them to hide their ill-gotten booty. By definition, a cave is simply a natural open space found underground. Sometimes caves form when large rocks get stacked up after a landslide, but most often they're the result of the chemical solution of limestone by subsurface water. In pure water, limestone is actually quite stable. However, when water gets a little acidic, the limestone dissolves. When water percolates through the soil, it picks up carbon dioxide from rotting plants. This forms carbonic acid, the same acid found in soda pop. As this acid water flows through the soil, it eats away at the underlying limestone rock and a cave is formed. Some larger caves are formed by sulfuric acid -- the same acid found in battery acid. Sulfuric acid is created when the percolating water mixes with hydrogen sulfide from underground oil and gas and from acid rain. Over time, small caves can grow to become elaborate systems of caverns containing large, needlelike structures that seem to grow down from the ceiling (stalactites) and up from the floor (stalagmites). These structures are formed by the slow, steady accumulation of calcium carbonate precipitating from the chemical-rich water dripping off the stones. Sometimes, these two "dripstones" meet, creating large pillars with spectacular colors. Very few plants can grow in a cave because of the lack of light. Instead, fungi, algae, and even some simple mosses dominate caves. Insects, reptiles, and bats also are uniquely adapted to the cave environment. One of the most unusual cave dwellers is the "cave fish." Totally blind, this colorless creature navigates by touch, using tactile sensors on its body. Sometimes, so much solution takes place inside a cave that the caprock can no longer support the weight of the surface above. The roof caves in and a feature known as a sinkhole forms. Each year, there are news stories about people returning home from work to find a large hole in the ground where their house once stood. By understanding the dynamics of what's happening inside a cave, scientists hope to minimize the risk of sinkholes in populated areas.


Many caves form because weakly acidic groundwater flows through cracks in limestone and slowly dissolves the rock. Caves form particularly well in areas where there is a resistant caprock like sandstone over the limestone below. In this activity, you will compare the solution rates of different minerals in liquids of differing pH to discover for yourself how some caves form. Note: It may take a week or longer to complete this activity. Materials
  • 9 large disposable plastic cups (12 oz or larger)
  • 3 pieces of chalk (limestone)
  • 3 quartz pebbles
  • 3 small pieces of broken concrete
  • bottle of vinegar
  • bottle of clear ammonia
  • pure water
  • 9 self-stick labels or
  • masking tape
  • marking pen
  • pen and paper
1. Fill three cups half full with vinegar and label them "acid." Fill three cups half full with pure water and label them "neutral." Fill three cups with 1cm of ammonia and 5 cm of water. Label these "base." (Note: Ammonia from the bottle is too concentrated to use without dilution. Be careful because even diluted, it can irritate skin and eyes.) 2. Examine and describe each of the rock samples in as much detail as possible. Set up a data sheet for each sample and record your first observations under the heading "time zero." Make sure to include things like color, texture, size, and shape. 3. Place one piece of chalk in a cup with vinegar, a second in a cup with water, and the last in a cup with ammonia. Add the word "chalk" to each label. Repeat the procedure with the concrete and quartz pebble so that each cup now has a rock sample in it. 4. Place all nine cups in a dark corner and observe them the next day. Record any changes you see for each sample under the heading "day 1." Continue to make observations for each day until a full week has passed. Questions
  1. Based on your observations, what can you say about the behavior of limestone in the three different chemical environments?
  2. Did any of the other samples show the same type of behavior? How might these similarities and differences help to explain how caves are formed?
  3. Based on your observations, what material is used to help hold concrete together? What effect would acid rain have on structures made from concrete?


    Bush, L. (1994, Apr) Caving beneath the Tongass. Bioscience,
    pp. 215?218.
    Coch, N. & Ludman, A. (1991) Physical geology (2nd ed). New York: Macmillan
    Publishing Co.
    Gurnee, R. (1980) A guide to American caves. Teaneck, NJ: Zephyrus Press.
    Stone, W.C. (1995, Sept) Cave quest. National Geographic, pp. 78?93.
    Reading Rainbow: The Magic School Bus inside the Earth. GPN: (800) 228-4630.
    U.S. Geological Survey
    419 National Center
    Reston, VA 22092
    The National Speleological Society operates "Grottoes" (clubs) in many areas
    with caves:

    The Potomac Speleological Club Web page: