The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Describe the process leading to the formation of caves and the speleothems within caves. (G)

Speleothems are fascinating mineral formations within caves, commonly known as cave decorations. These formations grow from the accumulation of minerals deposited by water as it drips or flows through a cave system.

Despite their varied shapes and sizes, speleothems generally form through a similar process: As water travels through soil and into the underlying limestone bedrock, it dissolves calcium carbonate along the way. When this water then enters a cave and comes into contact with the air, the water starts to release the dissolved calcite, slowly building up the striking features we observe—such as stalactites, stalagmites, and flowstones.

Types of Speleothems

Stalactites cling tight to the ceiling and grow downwards, whereas stalagmites rise mightily from the cave floor. Other forms include draperies, which resemble curtains, and helictites, which defy gravity by growing in unusual directions. These quiet, stone structures provide a record of past environmental conditions and thus are invaluable to scientists studying the history of Earth's climate.

Cave formation often begins with the process of limestone dissolution, which is the initial step in developing underground voids that can grow into caves. Limestone is a sedimentary rock typically rich in the mineral calcite (calcium carbonate), which is soluble in weakly acidic solutions.

Rainwater, which absorbs carbon dioxide from the air and the soil, becomes a weak carbonic acid solution. As it seeps through cracks in the limestone, it reacts with the calcite, dissolving it and enlarging these fissures over time. Areas with large volumes of limestone and frequent rainfall are therefore prime locations for cave development. In addition to the dissolution by acidic water, the presence of plants and soil organisms can produce organic acids that also contribute to the dissolution process, further enhancing its effects.

Erosion is the naturally occurring process that shapes and enlarges cave systems after limestone dissolution has initiated their formation. Running water acts as the primary force, physically eroding rock and carrying away the particles dissolved in water.

As water flows through these underground channels, it can sculpt intricate features such as ripple marks on the floors, or scallops, which are curved indentations on the walls of the cave, indicating the direction of water flow. Erosion creates not just the large passageways but also the delicate features like shelves, niches, and anastomoses—small, maze-like patterns on the cave ceiling or walls.

The formation of speleothems is inextricably linked to the deposition of calcite crystals. After limestone is dissolved by acidic water, the resulting solution can deposit calcite as it travels through a cave system. This occurs because, as the water drips and comes into contact with the air in the cave, carbon dioxide is released from the water, decreasing its acidity and causing calcite to precipitate out of solution.

This deposition of calcite crystals, layer by layer, is what forms the variety of speleothems observed in caves. This is a slow and steady process, with some speleothems growing at a rate of just a few millimeters per year. These deposits can range in texture from smooth, like in flowstones, to fibrous, as seen in certain stalactites.

Understanding the formation of caves and speleothems requires a grasp of the geological time scale—a system used by geologists to describe the timing and relationships between events that have occurred during Earth's history. Caves and speleothems are the products of geological processes that occur over extremely long periods.

The formation of vast cave systems and the intricate speleothems within can span anywhere from thousands to millions of years. Small changes, such as the growth of speleothems, reflect incremental alterations in the geological record, while larger changes represent significant climatic and environmental transformations. Hence, the geological time scale helps scientists date these formations and understand the Earth's climatic history tied to them.