How Are Caves Formed?

Caves can be formed by water, lava, and the action of bacteria.

Introduction

A natural opening in the Earth that is large enough for a person to enter is a cave. Within some caves, beautiful cave formations or speleothems, structures such as stalactites and stalagmites, rise from the floor and hang from the ceilings. Stalactites are calcium carbonate formations that hang down while stalagmites build up on the floor due to an accumulation of calcium carbonate. In many cases these two meet and form tall columns (see Figure 1).

Figure 1

Caves are classified into two big groups according to how they were formed. Those that developed as the host rock was solidifying are primary caves. Primary caves are most often found in old flows of lava. Secondary caves were created after the host rock was formed. In these cases, soluble rock such as limestone, dolomite, or marble was carved out over millions of years. Most caves are of the secondary type and were formed by the action of acidic water.

Water can become acidified to form either carbonic acid or sulfuric acid. Weakly acidic carbonic acid (H2CO3) is formed when precipitation dissolves carbon dioxide (CO2) in the air. This water percolates through the ground, dissolving other compounds to form a slightly stronger acidic solution that

erodes limestone and other soluble rocks (see Figure 2). Sulfuric acid is formed when hydrogen sulfide gas from deep in the Earth moves up through cracks in the crust until it encounters the water table. A solution of hydrogen sulfide and water creates sulfuric acid. In this experiment, you will conduct research to learn the details of cave formation and you will present information on a specific cave to the class.

Figure 2

Time Required

90 minutes

Materials

• access to the Internet
• color printer or colored markers
• poster board
• science notebook

Please review and follow the safety guidelines at the beginning of this volume.

Procedure

1. Access the Internet and go to “How Caves Form” by Rick Goreau at http://www.pbs.org/wgbh/nova/caves/form.html. Click on the animation link titled “How Caves Form.” Select each of the four ways in which caves form and read the information about each.
2. Answer Analysis questions 1 through 11.
3. Go to the article on caveology on the Onondoga Cave State Park Web site at http://www.mostateparks.com/onondaga/cavesformed. htm. Read the article and answer Analysis questions 12 through 15.
4. Select a cave or cavern to research. You may use one of the famous caves listed on the data table below or choose another one you know about.
5. Create a poster about the cave you investigated. Your poster should be neat and colorful, and it should include the cave’s location and features as well as explaining how the cave was formed.

Analysis

1. What are the most numerous types of caves?

2. How is carbonic acid formed?

3. How far does rainwater seep into limestone?

4. What effect does carbonic acid have on the limestone?

5. When do stalactites and stalagmites appear in limestone caves?

6. How can moving water create caves?

7. How do waves create caves?

8. How does lava form a cave?

9. What are extremophiles?

10. What gas do extremophiles produce?

11. How does this gas contribute to cave formation?

12. How was Onondaga Cave formed?

13. What is the source of the rock that makes up Onondaga Cave?

14. Write the chemical equation that shows how carbonic acid is formed.

15. How does carbonic acid affect limestone and dolomite?

Most of the caves in the United States are clustered in specific regions. These regions have geological features in common and are classified as karst landscapes. A karst is a geographic area that is made of soluble rock such as limestone, dolomite, marble, or gypsum. About 20 percent of he U.S. landscape is comprised of karst regions. Besides caves, these regions have features such as sinkholes, disappearing streams, and pits. Karsts also contain aquifers, large reservoirs of water, beneath the surface.

Aquifers are filled by rainwater that percolates through the soil. When water reaches the aquifer, it travels through two zones. The first is the zone of aeration, an area just above the water table where most of the spaces in rocks are filled with air. The next section is the zone of saturation, where the rock is completely saturated with water. Between these two zones is the capillary fringe (see Figure 3). Cave passages that contain air are within the zone of aeration, but cave-forming processes occur in both regions.

Figure 3

Water that percolates through the soil into an aquifer is slightly acidic because it has combined with carbon dioxide in the air and with acidic compounds in the soil. This acidified water eats away at the soluble rock, especially at the region of the water table. Over time, when the water table recedes, the caves are exposed to air for the first time. At this stage, stalactites and stalagmites begin form. Stalactites hang from the ceiling like icicles. Each structure begins as a single drop of acidified water that carries dissolved calcium compounds.

As the drop hangs suspended from the cave ceiling, the water evaporates, leaving behind calcium carbonate and calcite. A second drop develops on this deposit, eventually adding its dissolved minerals. Over thousands of years, large stalactites form. Stalagmites are built from the cave floor upward. Water drips from stalactites, leaving tiny deposits of dissolved calcium compounds on the cave floor. When the water evaporates, only the minerals are left behind. These build up slowly, forming impressive structures. If the stalagmite is able to grow for long enough, it eventually merges with its parent stalactite to form a column.

Connections

Carlsbad Caverns in New Mexico is one of the largest caves in the United States. This cave is made up of an enormous tunnel of caverns and includes more than 84 large rooms, many of them featuring dramatic limestone formations. Archeological evidence shows that nomadic hunters and gatherers used the cave for shelter. The same caverns were visited by Spanish explorers in the 1500s.

About 250 million years ago, the area of these caverns was an inland sea. Near the shore, a massive reef of lime-secreting organisms like corals and sponges formed. The reef grew for millions of years until it was hundreds of feet thick and more than four miles (6.4 kilometers [km]) long. When the sea dried up, the skeletons of these organisms were deposited and pressed into limestone.

The area was uplifted by geological processes and fractures in the rock filled with water, beginning the weathering process. Much of the dissolution that produced Carlsbad Caverns was due to more than carbonic acid. Sulfuric acid, a much stronger compound, played an important role. Near this uplifted seabed are regions that are rich in oil fields. Hydrogen sulfide gas from these regions migrated up through the limestone floor toward the water table, where it combined with water to form sulfuric acid. It was the action of this strong acid that created the large rooms characteristic of Carlsbad. Eventually, the water table dropped and the caves drained, leaving the large rooms exposed.