Kartchner Caverns was formed by the dissolution of limestone. Limestone is composed of the mineral calcite (CaCO 3 ). Limestone is formed by the accumulation of calcite, which is inorganically precipitated as well as contained in the skeletons of once living sea animals and plants. Pieces of the animal skeletons and plants fell to the bottom of the sea and were later pressed down and cemented into hard rock. Finally, mountain-building forces uplifted these sedimentary rocks from the sea and exposed them to air and the dissolving power of fresh water. Water seeps and percolates into the soil.

Limestone caves are formed when a mild acid reacts with calcite. This acid is carbonic acid (H 2 CO 3 ), produced when carbon dioxide combines with water. Most of the carbon dioxide responsible for the acid that dissolves limestone comes from the soil.

The cave forming process may continue for thousands of years. It can be stopped by only two things: a lowering of the water table, or the introduction of outside air in the cave system by wearing away of the surface. As outside air enters the cave system, the water quickly becomes supersaturated with calcite, ending the dissolving process. This change usually marks the beginning of the deposition of calcite in the form of stalactites, stalagmites and other deposits, know as speleothems.

The limestone rock in which Kartchner Caverns formed was faulted and created cracks and fractures which allowed water to seep in and begin the dissolution process. The water drains slowly downward and sideways towards a drainage point. As the water dissolved the limestone along the fractures it created passageways that are surrounded by undissolved rock.

SUMMARY:

1. Organize sugar cubes into a half pyramid, about 3 cubes wide, that fits against the inside of a glass aquarium. The sugar cubes represent the limestone.
2. Seal the cubes tightly with approximately 1/8 inch layer of modeling clay, making sure there are no gaps. The clay represents the top soil. This simulates the limestone hill above Kartchner Caverns.
3. Poke holes through the top of the clay with the tooth pick, making sure the holes go all the way through to the sugar cubes.
4. Have the students make a drawing of the model.
5. Next have the students predict what will happen when water is sprayed over the top of the clay. The water represents rainfall.
6. Use the spray bottle with water and begin spraying the top of the clay. The holes may need to be made a bit bigger to get the water percolating.
7. As the water penetrates the sugar, have the students describe what they are observing.
8. Have the students write what happened as the water began to seep into the sugar cubes and make a drawing of what the cave looked like when finished.
9. Use the transparency of the cave diagram and have the students explain how this process is similar to the dissolution of limestone which created Kartchner Caverns.

ASSESSMENT:

If this activity is being conducted as a teacher demonstration, discuss the contents of the transparency with the students and have them draw a picture of the process. If using with older students use the worksheet to assess their understanding.

EXTENSIONS: