For the more information about the geologic resources of the National Park Service, please visit https://www.nature.nps.gov/geology/.
Activity on Stalactites & Stalagmites
Grade Level: Modify for all ages
Source: National Park Service
Caves are naturally occuring underground cavities large enough for human entry. Caves contain a large number of minerals, of which calcite is one of the most common minerals. Speleothems, often called cave formations, are formed as rainwater passes through the soil and absorbs carbon dioxide. As water and carbon dioxide mix it forms a weak acid called carbonic acid, which helps to dissolve underlying rock. When water seeps into the air-filled cave it loses carbon dioxide to the cave atmosphere causing the water to precipitate calcite deposits in different forms. Calcite precipitates on ceilings, walls, and floors as speleothems.
There are many types of speleothems, but the most common that people are familiar with are stalactites and stalagmites. Stalactites are formed by mineral rich dripping water and they grow down from the ceiling like icicles. Stalagmites are formed when mineral rich water drops from stalactites and accumulates on the cave floor, growing upward like a cone. Sometimes a stalactite and stalagmite grow together to form a column.
- 1 small plate/saucer, or piece of aluminum foil
- 1 spoon
- 2 paper clips
- 2 jars of the same volume/size
- Epsom salt
- Hot water
- Thick cotton string or yarn (natural fibers)
Additional images are included on the downloadable student page below.
- Fill both jars with hot tap water ⅔ of the way full.
- Add enough Epsom salt to each jar until the salt will no longer dissolve in the hot water (approximately 8 or more ounces per jar).
- Place 2-5 drops of food coloring into each jar and stir.
- Cut string between 12-18 inches in length. You want it long enough so that each end remains submerged close to the bottom.
- Tie a paper clip to each end of the string, to act as a weight in the jar.
- Place a small plate between the two jars to catch the water as it drops.
- Wet the entire string in tap water.
- Place the ends of the strings into each jar so that the clips rest on the bottom of the jar.
- Leave enough slack between the jars so that the string sags in the middle (do not let the string touch the plate; you might need to cut the string if it is too long).
- Place your experiment next to a window, in a safe location, with minimum sunlight. Direct sunlight can prevent crystallization, make sure to reduce exposure to sunlight. Make sure not to touch or move the experiment once the stalactites start to grow, any movement of the string could cause breakage of the formation.
- Check your "cave" at least once a day and record your observations in the observation tables.
Note: Results may vary depending on the temperature and humidity of your area.
- Growing Speleothems Student Page
Includes background information, photo instructions, and conclusion questions. [13.5 MB PDF]
Explore Further| Cave & Karst Education | Cave & Karst Resources | Education & Outreach |
Last Updated: January 04, 2017