Caves and caverns

Caves—large natural holes in the ground—are part of the Earth's plumbing system. Groundwater passes through most caves at some point in time, creating many unusual features.

Formation

Caves, or caverns, are natural cavities in rock large enough for a person to enter. Most caves develop by groundwater dissolving limestone, a common rock deposited on ocean floors. Gypsum, dolomite, and marble (metamorphosed limestone) are other rocks that dissolve readily to form caves. Rain and snow pick up a trace of carbon dioxide as they travel through the atmosphere. Where the ground has a thick layer of decaying vegetation, more carbon dioxide combines with the water, and a dilute solution of carbonic acid forms. (Carbonic acid—carbon dioxide dissolved in water—is also present in soda pop.) The water soaks into the ground and finds its way into cracks in the soluble rock (limestone, dolomite, gypsum, or marble). The acid dissolves the rock in a process similar to water dissolving table salt. Groundwater removes what was previously solid rock, and a hole, or cave, remains.

The longest cave in the world is the Mammoth-Flint Ridge Cave System, a solutional cave near Bowling Green, Kentucky. More than 630 kilometers (km) of cave passage has been surveyed and mapped. Other long caves found throughout the world include Sistema Ox Bel Ha in Quintana Roo, Mexico at 496 km, Shuanghedong Cave Network in Guizhou, China at 417 km, Sistema Sac Actun/Sistema Dos Ojos in Quintana Roo, Mexico at 386 km, and Jewel Cave in South Dakota, United States at 349 km.

In mountainous areas such as the Alps in Europe or the Sierra Madre Oriental in Mexico, groundwater moves hundreds or even thousands of meters downward through cracks in the rocks. The resulting passageways are mostly vertical, with deep shafts. The deepest explored cave in the world is the 2,212-meter-deep Voronya cave in Abkhazia. This was the first cave to be explored at depths greater than 2 kilometers. Scientists have shown that water passes through a 2,525-meter-deep cave in southern Mexico, but explorers have not yet successfully followed much of its course.

Streams flow into or out of the entrances to many actively growing caves. Scientists refer to entrances where water flows into caves as “insurgences.” Entrances that have streams or rivers flowing out are called “resurgences.” Insurgences and resurgences often mark the boundary between soluble and insoluble rocks. Where water reaches insoluble rock, it flows onto the surface, and the cave ends. Similarly, streams flowing over insoluble rocks commonly sink into caves upon reaching limestone terrain. In some places, acid-charged water comes from deep in the earth and not from rainwater. Pockets of carbon dioxide and hydrogen sulfide in the Earth's crust can combine with deep-flowing water to form carbonic acid and sulfuric acid, respectively. Caves form when these deep waters rise through cracks in soluble rocks. Water charged with hydrogen sulfide rose through limestone and dissolved the spectacular Carlsbad Caverns in New Mexico.

Lava flows on the flank of a volcano can create another type of cave, commonly called a “lava tube.” As lava flows down the slope of a volcano, the surface cools and solidifies while liquid lava continues to flow under the crust. As the flow cools, self-constructed pipelines under the crust pass fast-moving, hot lava down the slope. Tubes drain when no more lava passes through, and a cave remains.

A few caves are found in insoluble rocks such as granite, sandstone, and volcanic tuff. These features are generally small and have varied histories leading to their development. In many cases, groundwater has carried individual grains of sand, one at a time, from the base of a cliff where a spring emerges. With time, the resulting hole at the cliff base is deep enough to be called a cave. Other caves have formed under blocks of rocks that fell or slid down adjacent hillsides. Pounding waves excavate caves in cliffs along some ocean shorelines. These caves are usually referred to as sea caves.

Ice caves sometimes form under glaciers near their toes. Meltwater flowing under a glacier during summer may enlarge a passageway large enough to form a cave. Ice caves, however, are usually short-lived and constantly change in size and shape.

Solutional caves are often divided into three categories: phreatic caves, vadose caves, and dry caves. Most phreatic caves are still actively forming. Their passages are below the water table and filled with water. Vadose caves are above the water table, but water passes through them as rivers and streams. Some caves form under vadose conditions. Other caves in the vadose zone were saturated when they formed but now provide convenient paths for water to flow through the unsaturated zone. Dry caves are no longer actively enlarging because water that formed them has withdrawn. The air in a dry cave is usually humid, but the cave does not act as a conduit for water.

Water levels in caves commonly respond quickly to rain. A river may pass through an otherwise dry passage during the spring snowmelt. A vadose passage with a small stream can become filled with water within a few minutes after a heavy rain commences. Tops of phreatic zones in caves have been observed to rise more than 50 meters within a few hours after the start of a surface deluge.

Once the void forms, nature commonly starts to fill caves. While most of the limestone (or dolomite or gypsum) dissolves, some impurities in the rock always remain as sediments on the cave floor. In addition, sand, mud, and gravel brought into the cave from outside by streams add to these sediments. Caves can become completely choked by sediments over hundreds, thousands, or even millions of years.

If too much rock is dissolved and the rock is not strong enough to support the void, the ceiling collapses. Failure can occur one small rock at a time or in massive blocks. If the cave is still actively forming, groundwater may eventually dissolve the debris, and the passage will continue to grow upward. However, if the debris is not removed, the passage can fill with rubble, ending the cave's existence.

Speleothems

Attractive deposits of minerals, called speleothems, form from supersaturated water in a cave. Supersaturated waters contain more dissolved minerals, usually calcite, than they can maintain in solution. Supersaturation can occur when water evaporates, and the dissolved minerals stay behind in the remaining liquid water. More commonly, supersaturation happens when cave water releases dissolved carbon dioxide. The less carbon dioxide dissolved in the water, the less acidic the water is, and the less minerals the water can hold in the solution. Cave water loses carbon dioxide—in the same manner that carbon dioxide bubbles escape from soda pop—when the surrounding pressure on the water drops (like opening a soda can) or when the water temperature rises. In both cases, dissolved minerals solidify—a process called precipitation—as speleothems.

The most common speleothems are soda straws, stalactites, stalagmites, and flowstone. Soda straws look like their namesakes and hang from the ceilings of caves. Water is fed from a hole at the top of the soda straw, flows down through the hollow speleothem, and hangs on the end before falling. Calcite precipitates around the edges of the water as it slowly drips from the soda straw's end. Stalactites are typically cone-shaped deposits that hang from the ceiling. Originally soda straws, they grow as water deposits calcite around the outside of the speleothem. Stalagmites grow when drops of water from the ceiling hit the ground, lose carbon dioxide when they splash (like shaking a soda can), and precipitate calcite. They look almost like upside-down stalactites, but their ends usually are more rounded. Stalactites and stalagmites that grow together result in a column. Flowstone, a sheet of calcite coating a sloping wall or floor of a cave, forms under flowing water.

Gypsum, composed of calcium sulfate, is commonly deposited within limestone, dolomite, and gypsum caves. It precipitates similarly to calcite, but the process involves dissolved hydrogen sulfide instead of carbon dioxide. Gypsum speleothem shapes differ from those of calcite speleothems. One type of speleothem, a gypsum flower, looks like clear or white rock flowers growing out of cave walls. These structures form at the base of the “petals” and extrude earlier-formed deposits away from the wall like toothpaste squeezing out of a tube.

Ice forms many of the same speleothems as calcite. Ice stalactites (icicles), stalagmites, and flowstone are displayed in cold caves, particularly in winter and spring. Some caves in the Austrian Alps have moving glaciers and massive ice columns.

Environment

The temperature of most caves is the mean (average) annual temperature of the local area above ground. Temperatures typically fluctuate slightly near entrances and usually are a constant temperature a short way from the entrance. Thus, caves usually seem cool in summer and warm in winter. The moisture in the ground makes most caves very humid. Like temperature, humidity remains nearly constant year-round away from entrances.

Caves will adjust to changes in local barometric conditions. When the outside atmosphere changes from higher barometric pressure to a lower pressure, strong winds blow out the entrances of large, air-filled caves as they also lower their atmospheric pressure. When an area changes from lower pressure to higher pressure, large caves will suck air in from the outside. Just as on the surface, the temperature and air pressure of caves increase at greater depths. The change can be substantial in caves of more than 1,000 meters in depth. “Blowing caves” have entrances at different elevations. A chimney effect causes cold air to drop through the cave and blow out the lower entrance throughout the winter. The effect reverses as air blows out the upper entrance during the summer.

Principal Terms

calcite: a common, rock-forming mineral that is soluble in carbonic and dilute hydrochloric acids

groundwater: water beneath the earth's surface

lava: molten rock extruded from a volcano

limestone: sedimentary rock, usually formed on the ocean floor and composed of calcite

phreatic: a zone in the ground below the level of complete water saturation

speleothem: a mineral deposit formed within a cave

vadose: a zone in the ground above the level of complete water saturation

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