stalaktos

Stalactite comes from Greek stalaktos (dripping, trickling), the verbal adjective of stalassein (to drip, to trickle drop by drop). The New Latin form stalactites appears in scientific writing in the seventeenth century, when natural philosophers began systematically describing cave formations. The word captures the process exactly: stalactites form when mineral-laden water drips from a cave ceiling. Rain and groundwater absorb carbon dioxide as they pass through soil, forming a weak carbonic acid that dissolves limestone along the way. When this calcium-bicarbonate solution seeps through the rock and emerges at a cave ceiling, the drop of water hanging there releases carbon dioxide into the cave air, and the dissolved calcium carbonate precipitates out — a minute fraction of solid mineral deposited as a ring at the point of the drop. Drop by drop, over centuries and millennia, the rings stack up into the hanging stone formation.

The growth rate of stalactites and stalagmites is one of the enduring fascinations of speleology. Growth varies enormously depending on water flow, temperature, and the concentration of dissolved minerals, but typical rates are around 0.1 to 10 centimeters per century. A stalactite one meter long may represent anywhere from a thousand to a hundred thousand years of dripping. This slow growth makes stalactites and stalagmites natural archives of past climate: the chemistry and isotopic composition of each growth layer records the conditions at the time it was deposited — temperature, rainfall, vegetation cover, even the occurrence of volcanic eruptions or fires. Speleothems (the collective term for cave mineral formations) are now used by paleoclimatologists as some of the most precise records of Holocene and Pleistocene climate available.

The distinction between stalactite (ceiling) and stalagmite (floor) produces one of the most reliable mnemonic challenges in all of geology, leading to generations of schoolchildren being taught various memory aids. The most common: stalactite has a c for ceiling, stalagmite has a g for ground. More colorful versions invoke the stalactite's need to hold on tight (to the ceiling) versus the stalagmite's ambition to might reach the ceiling one day. When the two grow long enough to meet, they merge into a column — a single pillar of calcium carbonate connecting ceiling to floor, the record of continuous dripping long enough to close the gap. The cave of Lechuguilla in New Mexico contains columns formed over several million years.

Caves were among the most significant sites of early human culture. Lascaux, Chauvet, Altamira, and dozens of other European caves contain Paleolithic paintings dating from 17,000 to more than 40,000 years ago. The cave environments that preserved these paintings — stable temperature and humidity, darkness protecting pigments from light degradation — also produced the stalactites and stalagmites that are their geological companions. Some of the paintings at Lascaux and other sites use the cave's three-dimensional walls, including the protrusions and recesses created by speleothem formations, as part of their composition. The cave artists knew the geological formations and incorporated them into their work. The dripping stone and the painted bison share the same underground world.