9.10 Landforms from Groundwater Erosion and Deposition

How would you find an undiscovered cave?

Caves may be beneath your feet, especially if your feet are on limestone. In 1974 two amateur cavers found warm, moist air coming out of a crack in the ground in southern Arizona. They managed to find a way in and discovered the amazing Kartchner Caverns, 2.5 miles of pristine caves. You can see these spectacular caverns on a guided tour.

Groundwater Erosion

Rainwater absorbs carbon dioxide (CO₂) as it falls. The CO₂ combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. The flow of water underground is groundwater. Groundwater is described further in the chapter Water on Earth.

Groundwater is a strong erosional force, as it works to dissolve away solid rock (Figure above). Carbonic acid is especially good at dissolving the rock limestone.

Cave Formation

Working slowly over many years, groundwater travels along small cracks. The water dissolves and carries away the solid rock, gradually enlarging the cracks. Eventually, a cave may form (Figure below).

Sinkholes

If the roof of a cave collapses, a sinkhole could form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure below).

Groundwater Deposition

Groundwater carries dissolved minerals in solution. The minerals may then be deposited, for example, as stalagmites or stalactites (Figure below). Stalactites form as calcium carbonate drips from the ceiling of a cave, forming beautiful icicle-like formations. The word stalactite has a c, and it forms from the ceiling. Stalagmites form as calcium carbonate drips from the ceiling to the floor of a cave and then grow upwards. The g in stalagmite means it forms on the ground.

If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures. Some of the largest, and most beautiful, natural crystals can be found in the Naica mine, in Mexico. These gypsum crystals were formed over thousands of years as groundwater, rich in calcium and sulfur flowed through an underground cave. Check it out:

Summary

• Groundwater dissolves minerals, carries the ions in solution, and then deposits them.
• Groundwater erodes rock beneath the ground surface, especially carbonate rock.
• Groundwater deposits material in caves to create stalactites, stalagmites, and columns.

Review

1. How does groundwater erode rock material?
2. Describe how groundwater deposits stalactites and stalagmites.
3. Why is groundwater acidic?

Explore More

Use this resource to answer the questions that follow.

1. What rock type is the cave system formed in?
2. What was the source of the acid in the water that carved the cave?
3. Why are the rooms large at Carlsbad compared to most other caves?
4. What cave formations can be seen at Carlsbad? How do they form?
5. How does water form in and out of the caves?
6. What biological wonders are found at Carlsbad?