A cavern is defined as a natural underground void or chamber large enough for a person to enter. While various processes can create underground openings, the largest and most abundant caverns are formed through a slow chemical process called dissolution. This involves slightly acidic groundwater dissolving specific rock types over immense stretches of geologic time. The resulting landscapes are known as karst topography, and the caverns formed are called solutional caves. This article focuses on the chemical and physical mechanisms by which groundwater creates these extensive solutional systems.
The Essential Ingredient: Soluble Rock
The creation of a vast cavern system begins with the presence of rock chemically susceptible to dissolution. The most common is limestone, a sedimentary rock primarily composed of the mineral calcite (\(\text{CaCO}_3\)). Other rock types, such as dolomite (calcium magnesium carbonate) and highly soluble gypsum, can also form solutional caves. These carbonate rocks are prevalent across large areas of the continental crust and are vulnerable to even very weak acids.
Limestone is largely impermeable, meaning water cannot easily pass through the solid rock matrix. Groundwater must initially follow existing structural weaknesses within the rock mass. These weaknesses include micro-fissures, bedding planes, and fractures called joints, which are present from the rock’s formation or tectonic stress. These small entry points set the stage for the slow work of the groundwater.
Preparing the Water for Dissolution
The water that carves out underground chambers starts as neutral rainwater, but must transform into a corrosive agent. The initial step occurs when raindrops absorb carbon dioxide (\(\text{CO}_2\)) gas from the atmosphere. This reaction creates a weak solution of carbonic acid (\(\text{H}_2\text{CO}_3\)), giving the rain a slightly acidic \(\text{pH}\).
This weak acidity is amplified once the water infiltrates the ground and moves through the soil layer. Decomposition of organic matter and respiration of plant roots release large amounts of \(\text{CO}_2\). The concentration of this \(\text{CO}_2\) in the soil gas can be 20 to 100 times higher than in the atmosphere. The percolating water absorbs this \(\text{CO}_2\), significantly increasing its carbonic acid concentration and corrosive power.
This slightly acidic groundwater then seeps downward, exploiting minute fractures in the underlying rock. The water’s corrosive strength is now sufficient to begin chemical weathering. The soil acts as a crucial chemical factory, concentrating the acid necessary to initiate dissolution.
The Core Chemical Process of Cavern Formation
The carving of the cavern is a chemical reaction that transforms solid rock into a dissolved substance carried away by the water. The carbonic acid (\(\text{H}_2\text{CO}_3\)) in the groundwater reacts with the calcium carbonate (\(\text{CaCO}_3\)) of the limestone. This reaction breaks down the rock, yielding soluble calcium ions (\(\text{Ca}^{2+}\)) and bicarbonate ions (\(\text{HCO}_3^{-}\)). The resulting calcium bicarbonate remains dissolved in the water, effectively removing the rock material without physical erosion.
This dissolution is a slow process, taking tens of thousands to millions of years to enlarge initial cracks into passages large enough for human entry. The water preferentially flows along existing joints and fissures, gradually widening them into conduits.
Initial cavern formation often takes place in the phreatic zone, the region below the water table where the rock is completely saturated. Here, the flow is slower, allowing chemical dissolution to occur over a large surface area. As surface river valleys erode deeper, the water table drops, shifting the saturated phreatic zone downward. The former phreatic passages, now air-filled, become the caverns we explore.
The water that continues to pass through the newly air-filled passages is now in the vadose zone, or the zone of aeration. While dissolution continues here, the flow is more like an underground stream, and the primary work of carving major chambers is often complete. The size and complexity of the system relate directly to the thickness of the soluble rock, the density of initial fractures, and the duration the water table remained stable.
Building the Interior: Secondary Formations
Once the cavern has formed and the water table has dropped, exposing the space to air, a new phase of geological activity begins. Secondary mineral deposits, collectively known as speleothems, decorate the interior and include stalactites and stalagmites. This process is the reverse of the dissolution that created the cavern.
Water still seeps through the overlying rock, carrying dissolved calcium bicarbonate. When this solution reaches the air-filled cave space, it encounters a lower concentration of \(\text{CO}_2\) than it held in the soil. This difference causes the water to “degas,” releasing excess \(\text{CO}_2\) into the cave atmosphere.
The loss of carbon dioxide causes the dissolved calcium bicarbonate to become unstable and precipitate back into solid calcium carbonate. This precipitation occurs in tiny amounts with every drop of water.
Where water drips from the ceiling, the deposit builds downward to form stalactites. When the drops hit the cave floor, they deposit calcium carbonate, building cone-shaped stalagmites that grow upward. If a stalactite and stalagmite meet, they form a solid column. Mineral-rich water flowing across the walls or floor creates broad, layered deposits known as flowstones.