Caves in limestone form primarily by dissolution by groundwater enriched in carbon dioxide, a process occurring below the water table. Which process is responsible?

• A. Oxidation
• B. Carbonate dissolution by CO2-rich groundwater
• C. Mechanical weathering only
• D. Evaporation

Answer: (B)

Limestone caves form mainly through dissolution of calcite by acidic groundwater. When rainwater infiltrates the soil, it picks up carbon dioxide from the air and from soil organisms, forming carbonic acid (H2CO3). This weak acid reacts with the calcium carbonate in limestone (CaCO3), dissolving it and releasing calcium and bicarbonate into solution. The key chemical step can be summarized as CaCO3 + CO2 + H2O → Ca2+ + 2HCO3−. Over time, this dissolution enlarges cracks and voids into caves.

This process is most effective below the water table because continuous groundwater contact sustains the flow of CO2-rich, dissolved limestone solution and transports the dissolved material away, allowing the caves to grow. Oxidation targets metals rather than carbonate rocks, evaporation tends to concentrate solutions rather than dissolve large rock masses, and purely mechanical weathering cannot create the extensive, channelized voids typical of caves in limestone.