Stalactite

A stalactite is a mineral formation that hangs from the ceilings of caves, hot springs or man-made structures such as bridges, mines and concrete buildings. Stalactites develop wherever mineral-laden fluids drip from an overhead surface and deposit material over time. Although they are most familiar as speleothems found in limestone caves, stalactites can also form from lava, mud, peat, resin, sand, geyserite, ice and even crystallised rodent urine known as amberat. The corresponding formation on a cave floor is a stalagmite, and when the two meet they create a pillar or column.

Formation in Limestone Caves

The majority of natural stalactites occur as speleothems within limestone caves. Their formation is governed by the dissolution and precipitation of calcium carbonate. Rainwater absorbs carbon dioxide from soil and air to create a weak carbonic acid which dissolves limestone rock, forming a solution of calcium bicarbonate. As this mineralised water percolates through cave ceilings, it drips into the open air. Here carbon dioxide degasses from the droplet, reversing the dissolution reaction and causing calcium carbonate to precipitate.
The repetition of countless droplets leads to incremental growth. The first deposits form narrow hollow tubes, known as soda straws, usually 4–5 mm in diameter. Each falling drop leaves behind a thin calcite ring, and over time soda straws may extend significantly. Their fragility means that if they become blocked, water flows over the exterior instead, creating the more familiar cone-shaped stalactite.
Growth rates vary greatly. Under typical conditions stalactites may extend only a few millimetres per year. Under ideal conditions—consistent, slow dripping of water rich in calcium carbonate and carbon dioxide—they may grow more quickly. If a drip rate is too fast, however, most dissolved minerals fall to the floor before precipitation occurs, contributing instead to stalagmite formation.
Cave roof areas often contain numerous potential flow paths for mineralised water. Slight differences in dissolution can cause one channel to dominate, drawing in more water and suppressing neighbouring channels. This competitive process explains why stalactites often maintain characteristic minimum spacing from one another.

Pillars and Stalagmites

The dripping from stalactites initiates the formation of stalagmites on the cave floor below. These are rounded or conical deposits that grow upward by the same precipitation process. Stalagmites never begin as hollow straws. Over long periods, sustained growth from above and below may lead to the fusion of a stalactite and stalagmite to form a continuous pillar or column, sometimes called a stalagnate.

Lava Stalactites and Related Forms

Stalactites also form in lava tubes through volcanic processes. When molten lava flows through underground channels, it may drip from the ceiling where temperatures and viscosities allow. Because lava solidifies rapidly, lava stalactites can form within hours or days. They do not continue to grow once lava flow stops and therefore cannot regenerate if broken.
Several distinctive types occur:

• Shark-tooth stalactites: thick, tapering features formed as layers of lava coat a semisolid base during changing flow levels. Sizes range from millimetres to more than a metre.
• Splash stalactites: irregular forms produced when lava splashes onto the ceiling and hardens as it drips back down.
• Tubular lava stalactites: hollow forms analogous to soda straws, created when gases force semimolten lava through small openings. These may develop twisted or vermiform shapes influenced by crystallisation patterns or air currents within the tube.

Lava stalactites can form corresponding stalagmites from dripping lava, and like limestone forms, may eventually merge into columns.

Ice Stalactites

Ice stalactites—commonly known as icicles—occur where water seepage freezes in caves or on outdoor surfaces. They may also form from the freezing of water vapour or in marine environments where saline water meets colder ocean water, producing brinicles. Ice stalactites form rapidly and, unlike lava stalactites, can grow back when conditions remain suitable. Ice stalagmites may grow beneath them, forming ice pillars when they meet.

Stalactites on Concrete Structures

Concrete structures can develop stalactites when water percolates through cracks and dissolves calcium hydroxide from cement. Concrete contains calcium oxide, which reacts with water to form calcium hydroxide. When this alkaline solution seeps out of a ceiling or ledge, it meets atmospheric carbon dioxide and precipitates calcium carbonate, producing thin calcite straws or thicker stalactites.
These formations grow far more rapidly than natural speleothems due to higher calcium hydroxide concentrations. As they occur outside cave environments and derive from concrete chemistry rather than geological processes, they are classified as calthemites rather than speleothems. Calthemites include stalactites, stalagmites and flowstone deposits that mimic cave forms but arise from anthropogenic materials.

Additional Mineral and Organic Forms

Beyond limestone, lava and ice, stalactites may form from other materials if conditions allow:

• Mud, peat and pitch stalactites form when viscous materials drip and solidify.
• Resin and sand stalactites occur in specialised geological environments.
• Amberat stalactites are formed from crystallised urine of pack rats.
• Geyserite stalactites result from mineral deposition near geothermal features.

These unusual forms broaden the concept of stalactites beyond traditional cave settings, illustrating the wide variety of natural processes capable of producing pendant mineral or organic structures.

Significance in Geology and Environmental Science

Stalactites provide important information about climatic conditions, hydrological processes and cave development. Their internal layering records variations in mineral content, water flow and environmental chemistry. Modern analytical techniques allow the study of isotopes and trace elements within stalactites, offering insights into past climate cycles and environmental change.