Dripstones: What are Stalactites and Stalagmites?

Problem

What are stalactites and stalagmites?

Materials

• 1 cup (250 ml) Epsom salts
• two 5-ounce (150-ml) paper cups
• tap water
• spoon
• 10-inch (25-cm) piece of cotton string
• 2 paper clips
• 6-by-10-inch (15-by-25-cm) piece of cardboard
• timer

Procedure

1. Pour cup (125 ml) of Epsom salts into each paper cup.
2. Add just enough water to cover the Epsom salts in each cup.
3. Stir. Most of the Epsom salts will not dissolve.
4. Thoroughly wet the cotton string with water, then run the string between your thumb and index finger to remove the excess water from the string.
5. Tie a paper clip to each end of the string, then place 1 paper clip in each cup.
6. Set the cups on the cardboard and position them so that the center of the string hangs about 1 inch (2.5 cm) above the cardboard.
7. Place the cups in a draft-free area where they will not be disturbed.



8. Observe the string every 5 minutes for a half an hour, then every 24 hours for 7 days. Record your observations.

Results

Usually within 5 minutes, a drop of liquid forms and hangs from the center of the string. This drop falls and another drop forms. This occurs several times during the first 30 minutes. After 24 hours, the drop begins to look like ice, and icy-looking crystals are seen on the cardboard beneath the string. After 7 days, a white frosty crust has formed that hangs from the string. In addition, a mound of white crystals builds up on the cardboard beneath the string. NOTE: If this does not happen, see the variations in "Let's Explore. "

Why?

Water containing Epsom salts moves through the string. As the water evaporates, frosty crystals of Epsom salts are deposited along the string and hang down from its center. A small mound of white crystals also forms on the cardboard. The formation of these hanging crystals and the crystal mound demonstrates the way mineral deposits form in caves.

The long hanging spikes in caves form when rainwater containing carbon dioxide from the air combines with limestone (a rock made wholly or chiefly of calcium carbonate). As this liquid seeps through the roofs of caves, small particles of calcium carbonate, generally in the form of calcite, cling to the ceiling. The calcite deposits build up, eventually forming long hanging spikes called stalactites. Drops of liquid that splash to the floor of the cave leave calcite deposits over a wide area. As more drops land, a rounded deposit builds upward. Mounds of calcite projecting up from a cave floor are called stalagmites. Stalactites and stalagmites are commonly called dripstones.

Let's Explore

1. Humidity (the amount of water in the air) in caves can be high. Can humidity affect the growth of the crystals? Repeat the experiment during times of high, moderate, and low humidity. The local weather report on TV or radio or in your newspaper will give the current level of humidity.
2. Does the length of the string affect the results? Repeat the original experiment, using a string 24 inches (60 em) long.
3. Does the height from which the drops fall affect the results? Repeat the variation of the experiment that works best, but this time set the cups on boxes to raise the string at least 6 inches (15 cm) above the cardboard. Science Fair Hint: Take photographs daily of this and the previous experiments. Use the photos to prepare display posters comparing the results of each experiment. The figure shows one method of arranging the photographs.

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