Features such as stalagmites are technically called Speleothems. The word "Speleothem" is derived from the Greek words "spelaion" (cave) and "thema" (deposit). The process by which Speleothems are formed is the reverse of that by which limestone is dissolved to produce caves.
Speleothems consist mainly of calcite, the same mineral that makes up limestone, in its crystallized form.
Conditions are right for the process to begin when the water table lowers and air enters the cave. Calcite is dissolved from the limestone above the cave by slightly acidic water as it percolates downward through the soil.
In the soil, where plant and animal remains are decaying, the carbon dioxide content is about 300 times that of the outside atmosphere. The carbon dioxide combines with the water and produces carbonic acid, which in turn dissolves some of the limestone it passes through as it moves downward toward the cave. When the acidic water reaches the cave, the carbon dioxide is released and calcite is precipitated (redeposited) on cave walls, ceilings and floors.
Speleothems form at varying rates as calcite crystals build up, one upon the other. Several factors can determine the rate of growth. Two important factors are the temperature outside, which affects the rate plants and animals decay(amount of carbon dioxide in the soil), and the amount of rainfall. The shape of Speleothems is determined by how the acidic water enters the cave (by dripping, seeping or splashing) and how the water stands or flows after entering the cave. Stalactites are the most common Speleothems.
Please note: This is an excerpt from the Cave and Karst Curriculum and Resource Guide.