Limestone caves hold structures of immense age and delicate beauty, collectively known as speleothems. These unique formations are the result of geological processes occurring over vast stretches of time, built drop by drop across millennia through the interaction between rock, water, and air.
Stalactites and Stalagmites Defined
Speleothems are classified based on their shape and orientation within the cave environment. The two most recognized types are stalactites and stalagmites.
The structures that descend from the cave ceiling are called stalactites. A simple way to remember their position is that stalactites hold “tight” to the ceiling. These formations often start as delicate, hollow tubes called soda straws before gradually widening into a cone shape.
Formations known as stalagmites grow up from the cave floor. To distinguish them, one can remember that stalagmites “might” reach the ceiling. Unlike stalactites, stalagmites never form as hollow soda straws, instead immediately building a solid, cone-shaped mass upon the floor.
Stalagmites are typically wider and flatter because the water spreads out upon impact with the ground. The relationship between a stalactite and the stalagmite below it is often direct, as the mineral-rich water dripping from the ceiling feeds the growth of the formation on the floor.
The Column Formation
The question of what happens when these two separate formations finally meet has a definitive answer in speleology. When a stalactite growing downward and a stalagmite growing upward converge, the resulting continuous structure is called a column or a pillar. The formation is also sometimes referred to by the technical term, stalagnate.
This column represents the successful completion of a growth journey that began at two different points. It creates a solid pillar connecting the cave floor to the ceiling, becoming a single, integrated speleothem. The convergence requires precise alignment and a stable environment over an immense period of time for the two structures to bridge the gap.
Once the two pieces have fused, the growth process changes slightly. Water no longer drips from the ceiling to the floor at that spot, but instead runs down the outside surface of the newly formed column. This continuous flow down the exterior can eventually give the column a more cylindrical and uniform appearance, obscuring the original point of connection.
The Chemical Process of Growth
The development of stalactites, stalagmites, and columns is driven by a chemical process involving water and calcium carbonate. The process begins when rainwater absorbs carbon dioxide (CO2) from the air and soil, forming a weak acid known as carbonic acid. This acidic water then seeps through the earth and into the limestone bedrock.
As the water moves through the rock, the carbonic acid dissolves the calcium carbonate (CaCO3) in the limestone, carrying it along as soluble calcium bicarbonate. When this mineral-rich solution reaches the cave ceiling and drips, it encounters the cave air, which contains less CO2 than the soil above.
This difference in CO2 concentration causes the dissolved gas to escape from the water, much like the fizz leaving an opened soda. The loss of CO2 reverses the chemical reaction, forcing the calcium carbonate out of the solution to precipitate, or deposit, as the mineral calcite. This deposition of calcite builds the formations.
The rate of this mineral deposition is exceedingly slow, averaging about 0.13 millimeters per year for these limestone structures. While some formations can grow faster, the timescale for a column to form is generally measured in hundreds of thousands of years.