Resource: Virtual Cave

Underground caves, also known as solution caves, form from a chemical reaction between water and rock. Limestone rock is composed primarily of the mineral calcite, a crystallized form of calcium carbonate. Pure water by itself has little effect on calcite. Rainwater, however, which absorbs carbon dioxide (CO₂) as it passes through the atmosphere and percolates through soil, becomes a slightly acidic solution called carbonic acid.

Carbonic acid is capable of dissolving calcite. When it does, it forms a solution called calcium bicarbonate. As the calcite is slowly dissolved, cavities and passageways form in the limestone rock. Most initial cave development occurs just below the water table in the zone of saturation.

A secondary phase of development occurs after the water table falls and the cavities fill with air. As soon as calcium bicarbonate solution enters a ventilated cave through cracks in its ceiling, CO₂ gas begins to evaporate out of solution. Calcium bicarbonate only exists in solution, and the loss of CO₂ prompts calcite to be deposited and the remaining water to run off. The general term for all such secondary cave formations is speleothems, from the Greek words spelaion, meaning cave, and thema, meaning deposit.

Speleothems form at varying rates, depending on how fast the calcite residue builds up. Although it takes an average of 120 years for a cubic inch of calcite to form, environmental factors can influence the rate of growth. For instance, higher ground temperature speeds the decay rate of plants and animals. This, in turn, adds more organic material to groundwater and raises its calcium bicarbonate concentration. Because this fortified water is more acidic, the rate of speleothem growth increases.

The location and unique shapes of speleothems depend on whether the depositing water source is flowing, dripping, seeping, splashing, still, or heated, among other possibilities. The most commonly known speleothems are stalactites and stalagmites. Stalactites project down from the ceiling and resemble slowly dripping icicles. Their droplets give rise to stalagmites, which grow up from the cave floor.

A delicate soda straw represents the earliest growth phase of stalactites. Imagine a thin tube attached to the ceiling. As a drop of calcium bicarbonate solution accumulates and runs down the inside of the tube, it hangs momentarily -- and sometimes much longer -- before falling to the floor. During this hesitation, CO₂ escapes into the air, precipitating a very thin film of calcite. Over successive drops, the straw lengthens ever so slightly.

To learn more about limestone caves and management strategies, check out How Caves Form and Caves and Karst.

To learn about bacteria's role in cave formation, check out Cave Formation: Biogeochemical Cycles and Cave Formation: Kane Cave.

To learn more detailed information and images of features of another cave type, check out Virtual Lava Tube.

• Choose one cave feature and describe it. Explain how the feature forms, what material it is made of, and where you might see it.
• Describe how stalactites, stalagmites, and columns form in relation to one another.
• How does rain water become a weak carbonic acid by the time it hits the ground?
• Pick three speleothems not yet discussed and explain how they are formed.