stalactites

Stalactites are icicle-shaped mineral formations that hang from the ceilings of caves. They are a common type of speleothem, formed by the precipitation of minerals from dripping groundwater, most commonly calcium carbonate in the form of calcite.

Formation: In limestone or dolostone caves, water dissolves calcium carbonate as it percolates through rock. When this mineral-rich water enters a cave and drips from the ceiling, carbon dioxide degasses from the solution, reducing its capacity to keep calcium carbonate in solution. As a result, calcium carbonate precipitates at the drip site and gradually accretes downward, forming a stalactite. Over time, continued deposition can create a stalactite that extends toward the floor; if a drop falls and deposits mineral on the floor, a stalagmite may form beneath it.

Composition and variation: Most stalactites consist of calcite, but they can be composed of aragonite or gypsum in other geochemical settings. Their color is influenced by impurities such as iron or organic matter. They range from slender, needle-like forms to broad, tapering structures. Some stalactites are hollow or form delicate dangling curtains; rare forms called helictites grow in curved or twisting directions due to irregular flow of mineral-rich water.

Growth and rate: Stalactite growth is extremely slow, typically measured in fractions of a millimeter to a few millimeters per year, varying with water chemistry, saturation state, temperature, and drip rate. In drier or less saturated conditions growth can be effectively halted.

Distribution and significance: Stalactites occur in many karst caves worldwide and are commonly found with stalagmites and other speleothems. They provide records of past cave hydrology and climate and are valued for education and science, but preservation is emphasized because they are sensitive to damage from contact and changes in cave air or water flow.