Materials Included In Kit

Additional Materials Required

Prelab Preparation

The paper twine should be cut into 18" pieces before this activity is performed.

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Use caution with glass containers. Use clear plastic jars if available. Please review current Safety Data Sheets for additional safety information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Flush the magnesium sulfate solution down the drain with an excess water according to Flinn Suggested Disposal Method 26b or save for future use. 

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students.
• Allow several days for the full formation of the stalactites and stalagmites.
• Be sure that the paper twine hangs low enough in the middle. Crystals will not form if the twine is not “drooped” enough.
• Food coloring may also be added to the twine to grow colorful stalactites and stalagmites.
• Materials like cotton and wool will not wick the solution away from the containers like the paper twine.

References

Gross, G.; Calangelo E.; Holzer, M. A Demo A Day—A Year of Earth Science Demonstrations™; Flinn Scientific Inc.: Batavia, IL, 2001; p 136.

Tarbuck, E.; Lutgers, F. Earth Science, MacMillan, New York, NY, 1991; pp 104–105.

Introduction

How do stalactities and stalagmites form in caves? Learn how by performing this hands-on activity.

Concepts

• Stalactites
• Stalagmites
• Caves

Background

In many regions of the world limestone (calcium carbonate) rocks make up a major portion of the rock strata. Although limestone is nearly insoluble in pure water, it is easily dissolved by water that is slightly acidic. Rainwater usually contains carbonic acid from dissolved carbon dioxide in the air. When the acidic rainwater comes in contact with limestone, the carbonic acid reacts with the calcite (calcium carbonate) in limestone to form calcium bicarbonate. This acidic groundwater erodes the limestone and can create limestone caverns or caves. Most caves that form are usually small, but some can be extremely large in size. One example of the latter can be found in Carlsbad Caverns in southeastern New Mexico. One chamber in this system of caves is equivalent to the size of fourteen football fields!

As groundwater drips through caves, elaborate formations occur. These cave formations and deposits are called dripstone and can be classified into two unique groups: stalactites and stalagmites. Stalactites are icicle-like formations that hang from the ceilings of caves (“c” for ceiling). When groundwater drips through and reaches the air in the cave, some of the dissolved carbon dioxide escapes from the drop and calcite begins to precipitate. The calcite is deposited as a ring around the water drop. Drop after drop a very small amount of calcite is left behind. Eventually a hollow tube made of limestone is created. Water continues to move through this tube and continues to add calcite to the stalactite. The stalactite can be appropriately described as a soda straw at this point. The soda straw becomes plugged and water is forced to flow outside the stalactite. Calcite deposits are then left on the outside of the stalactite and the familiar conical or icicle shape forms.

Formations that occur on the floor of a cavern are known as stalagmites (“g” for ground). The water that supplies calcite to the growing stalactite splatters on the floor of the cave. As a result, stalagmites are formed and grow upwards towards the ceiling of the cave. Stalagmites are often not uniform in shape and are more massive than stalactites because of this splattering.

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

1. Prepare a saturated magnesium sulfate solution by filling a 125-mL flask about half full with magnesium sulfate and then fill to the top with warm water. Shake well.
2. Slide the hex nuts over the ends of the twine and soak the twine in the solution.
   {13853_Procedure_Figure_1}
3. Pour the saturated magnesium sulfate solution back and forth between the original flask and the second flask. Once thoroughly mixed, add equal amounts of solution to each flask and add more water to each so that the flasks are almost filled.
4. Hang the paper twine into the two containers so the center droops but does not touch the tabletop. The center of the twine should hang below the level of the solutions in the two containers (see Figure 1).
5. Place a weighing dish below the droop in the twine to collect the drippings.
6. Capillary action will wick the magnesium sulfate solution through the twine. The magnesium sulfate will deposit from the low spot on the twine and form a stalactite. Stalagmites will form on the surface of the dish below the twine.