Caves formed in soluble rock like limestone (karst topography) are often adorned with geological formations called speleothems. These mineral deposits are built up over vast stretches of time, requiring consistent water movement, carbon dioxide, and the parent rock material. These slow-growing structures create unique shapes within subterranean chambers.
Identifying Stalactites and Stalagmites
These formations are primarily composed of calcite, which is a crystalline form of calcium carbonate (CaCO₃). The difference between the two main types lies entirely in their position and direction of growth within the cave. One type hangs from the ceiling, while its counterpart grows upward from the cave floor beneath the drip point.
A simple way to differentiate these structures is through common mnemonic devices used by cavers and guides. The downward-hanging formation is a stalactite, which holds “tight” to the ceiling. This formation often begins as a thin, hollow tube called a “soda straw” before thickening into a cone shape.
The upward-growing formation is a stalagmite, which “might” reach the ceiling one day. Stalagmites tend to be broader and more rounded at the base because the water droplet splatters upon impact, distributing the mineral deposit over a wider area. Another helpful trick is remembering that a stalactite has a ‘C’ (for ceiling) and a stalagmite has a ‘G’ (for ground).
The Resulting Cave Column
When the two distinct formations grow toward each other from the ceiling and the floor, they eventually bridge the gap between them. The single, continuous structure that results from this merger is called a Column or Pillar. This formation represents a completed geological process where the vertical growth of the stalactite and the stalagmite has finally fused.
The column spans the entire height of the cave passage, creating a solid connection between the roof and the floor. The resulting appearance is that of a single, monumental structure. Some scientific literature may use the less common term stalagnate to describe this fused formation.
The Slow Process of Merging
The growth of speleothems is a chemical process that begins when rainwater absorbs carbon dioxide (CO₂) from the atmosphere and soil, turning it into a weak carbonic acid. This mildly acidic water then percolates through cracks in the overlying limestone, dissolving the calcium carbonate and carrying it in solution as calcium bicarbonate.
Once this mineral-rich water reaches the cave ceiling, it comes into contact with the open air of the cavern. The lower air pressure inside the cave causes the water to “degas,” releasing the dissolved CO₂ back into the atmosphere.
This release reverses the chemical reaction, forcing the calcium carbonate out of the solution to precipitate as a solid mineral. A small fraction of the calcite precipitates immediately on the ceiling, building the stalactite downward. The remaining water falls to the cave floor, where continued degassing and evaporation cause the rest of the mineral load to precipitate, building the stalagmite upward beneath the drip point.
The column is formed over immense geological time because the growth rate is exceptionally slow. An average stalactite or stalagmite grows at a rate of approximately 0.13 millimeters per year, though a constant water source can increase that to a few millimeters annually. The fusion of the two forms requires thousands to hundreds of thousands of years of uninterrupted dripping.