A stalagmite is a geological formation that rises from the floor of a cave, forming a mound or cone shape. These structures are a type of speleothem, which is a general term for mineral deposits found in caves. Most stalagmites are found in limestone caves, where the slow action of dripping water over long periods results in the accumulation of mineral material.
Essential Geological Ingredients
The formation process begins outside the cave, requiring calcium carbonate bedrock, such as limestone. Rainwater absorbs carbon dioxide (\(\text{CO}_2\)) as it travels through the atmosphere and the organic-rich soil layer above the cave, creating weak carbonic acid (\(\text{H}_2\text{CO}_3\)). This mildly acidic water seeps through cracks in the limestone. The carbonic acid encounters the calcium carbonate (\(\text{CaCO}_3\)) rock, dissolving it and converting the insoluble calcium carbonate into soluble calcium bicarbonate (\(\text{Ca}(\text{HCO}_3)_2\)). This mineral-rich solution is then carried into the cave.
The Chemistry of Calcite Deposition
The water carrying the dissolved calcium bicarbonate eventually reaches the cave ceiling and drips into the cavern air. The air inside the cave contains a lower concentration of carbon dioxide (\(\text{CO}_2\)) compared to the soil solution, triggering degassing. As the water droplet is exposed to the air, it releases \(\text{CO}_2\) gas, which reverses the initial dissolution reaction. This causes the water to become less acidic and reduces its capacity to hold the dissolved minerals, forcing the calcium bicarbonate to precipitate as calcium carbonate (calcite). This calcite is deposited on the cave floor where the water droplet lands, and the accumulation of these microscopic deposits builds the stalagmite structure upward over thousands of years.
Dripstone: Stalagmites and Stalactites
Stalactites vs. Stalagmites
Stalagmites are frequently confused with stalactites, which hang down from the cave ceiling. Both are classified as “dripstone” formations because they are created by the same mineral-laden water. The key difference lies in the physical mechanism of growth. Stalactites form as mineral deposits build up around the rim of the drop before it falls, creating a tapering, icicle-like shape.
Growth and Fusion
A stalagmite grows upward from the floor due to the mineral content remaining in the water after it has dropped. Gravity pulls the water droplet down, and the residue it leaves behind on impact deposits the calcite. Stalagmites tend to be thicker and blunter than stalactites because the minerals are deposited across a wider area when the water splashes. If a stalactite and the stalagmite directly below it grow long enough, they will eventually meet and fuse, forming a complete column. The growth rate for limestone stalagmites is often very slow, averaging around 0.1 millimeters per year.