How Caves Form: Dissolution, Speleothems, and Karst Landscapes

The Chemistry of Cave Formation

Limestone caves form through a simple but powerful chemical reaction. Rainwater absorbs carbon dioxide from the atmosphere and, more importantly, from the soil through which it percolates (where biological activity produces concentrations of carbon dioxide 10 to 100 times higher than in the atmosphere). The water and carbon dioxide combine to form carbonic acid, a weak acid that is nonetheless capable of dissolving calcium carbonate, the mineral (calcite) that makes up limestone. As this slightly acidic water infiltrates through joints, fractures, and bedding planes in the limestone, it slowly dissolves the rock along these pathways, gradually enlarging them from hairline cracks into passages and eventually into caves.

The rate of dissolution depends on several factors: the acidity of the water (controlled by carbon dioxide concentration), the temperature (dissolution is faster in warmer water that carries more carbon dioxide), the volume of water flowing through the rock, and the purity and structure of the limestone. Highly jointed or fractured limestone dissolves faster because water has more pathways to penetrate the rock. Dissolution is most effective at and just below the water table, where rock is fully saturated and water moves laterally through the limestone under hydraulic pressure. Many of the world largest cave passages formed at or near the water table level, then were drained as the water table dropped due to regional uplift or changes in the local drainage pattern.

Karst Landscapes

Regions underlain by soluble rock, particularly limestone, develop a distinctive set of landforms collectively called karst topography. Karst landscapes are characterized by sinkholes (depressions formed when the ground surface collapses into underlying dissolved cavities), disappearing streams (rivers that flow into cave entrances and continue underground), springs (where groundwater resurfaces), dry valleys (former river channels now drained underground), and exposed, sculpted limestone pavement. Approximately 15 to 20 percent of the Earth ice-free land surface is classified as karst or has karst-like features.

Major karst regions include the Dinaric Alps along the Adriatic coast (where the word "karst" originates from the German name for the region), the Yucatan Peninsula of Mexico (site of the world longest underwater cave systems), South China (with its iconic tower karst landscape of steep limestone pillars), and the Mammoth Cave region of Kentucky (home to the world longest known cave system, with over 680 kilometers of surveyed passages). Karst presents unique challenges for human activities. Sinkholes can swallow buildings, roads, and vehicles without warning. Groundwater in karst aquifers moves rapidly through large conduits with minimal natural filtration, making karst groundwater highly vulnerable to contamination. Construction on karst terrain requires careful geological investigation to identify subsurface voids and unstable ground.

Speleothems: Cave Decorations

Once a cave passage is drained of water and filled with air, the chemical process reverses. Water seeping through the cave ceiling carries dissolved calcium carbonate from the limestone above. When this water enters the air-filled cave, carbon dioxide escapes from the water into the cave atmosphere (similar to how carbonation escapes from an opened bottle of soda), reducing the water ability to hold calcium carbonate in solution. The excess calcium carbonate precipitates as calcite crystals, gradually building up deposits called speleothems.

Stalactites grow downward from cave ceilings where water drips through cracks. They begin as hollow tubes (called soda straws) through which water flows, depositing calcite around the rim of each drop. Over time, water flows down the outside of the growing stalactite as well, thickening it into a cone shape. Stalagmites grow upward from the cave floor beneath drip points, where water splashing on the floor releases carbon dioxide and deposits calcite. When a stalactite and stalagmite meet, they form a column. Flowstone forms where water flows in sheets over cave walls or floors, depositing smooth, layered calcite curtains. Draperies (also called bacon strips for their translucent, banded appearance) form along sloping ceilings where water traces a path before dripping. Helictites are twisted, seemingly gravity-defying formations that grow in curving or spiraling patterns, driven by capillary forces and crystal growth pressures rather than gravity.

Speleothem growth rates are extremely slow, typically 0.01 to 0.1 millimeters per year, meaning that a stalactite one meter long may have taken 10,000 to 100,000 years to form. This slow growth rate makes speleothems valuable archives of past climate. The chemical composition and layering of speleothems record changes in temperature, precipitation, and vegetation cover, and because they can be precisely dated using uranium-thorium radiometric methods, they provide some of the most detailed records of past climate variability available to scientists.