Stalactites vs Stalagmites
Stalactites grow from the ceiling, stalagmites from the floor. The mnemonic — "stalactite hangs Tite from the ceiling, stalagmite might reach the ceiling someday" — is the easy way to remember. The geology is more interesting.
Stalactites
Form when CO2-saturated water seeps through a cave ceiling. As the water drips and contacts air, CO2 outgasses; the loss of acidity forces calcium carbonate to precipitate. Each drop leaves a tiny ring of calcite at the dripping point; thousands of years of drips build the conical stalactite. The fastest grow about 0.13 mm/year — a 1-metre stalactite is approximately 7,700 years old.
Soda straw stalactites
The earliest stage of stalactite growth. A single drop of water emerging from a fissure leaves a calcite ring at its edge; the ring extends downward as a hollow tube — a soda straw. Soda straws can reach several metres before clogging or collapsing. Lechuguilla and several Brazilian caves have spectacular soda-straw forests.
Stalagmites
Form on cave floors below stalactites, from the splash of drips. The drop that lands has spent its CO2 in the descent; the impact spreads water in a small puddle that immediately begins to deposit calcium carbonate. Stalagmites grow slower and broader than stalactites because the deposition area is larger.
Columns and pillars
When a stalactite and stalagmite grow together long enough, they fuse into a column. The Pillar of Hercules at Postojna, the Christmas Tree at Abukuma-do, and the Tower of Babel at Mitchelstown are famous examples.
Other speleothems
Helictites: irregular, gravity-defying calcite formations that grow against gravity due to capillary action. Flowstone: sheet-like calcite accumulation on cave walls and floors where water sheets rather than drips. Cave pearls: rounded calcite spheres formed by water dripping into a small pool and rotating loose calcite grains. Curtains: thin, wavy sheets where water has flowed along an overhanging surface.
Growth rate factors
Climate (warmer = faster CO2 outgassing), drip rate, dissolved calcium content, cave humidity, and cave airflow all affect formation growth. Modern stalagmite formations can be used to reconstruct past climate.
Explore on the map
See speleothem-rich caves on the interactive map.