How Do Stalactites Form? Water, Acid and a Very Long Wait

Rainwater absorbs carbon dioxide from the atmosphere and much more of it from the soil, where root respiration and decay raise concentrations far above atmospheric levels, forming a weak solution of carbonic acid. That mildly acidic water percolates into limestone, which is mostly calcium carbonate and is almost insoluble in pure water, and dissolves it, carrying calcium and bicarbonate ions in solution. When the water emerges into an air-filled cave, the cave air contains much less carbon dioxide than the soil water is carrying, so carbon dioxide escapes from the droplet into the cave atmosphere. Losing that gas makes the water less able to hold calcium carbonate in solution, and the excess precipitates as calcite. Everything about cave formations follows from this single equilibrium running one way in the soil and the other way in the cave, and evaporation plays only a minor role in most caves because the air is at or near saturation.

The distinct forms come from where the water goes before it deposits its load:

• •A stalactite grows down from the ceiling, starting as a soda straw, a hollow tube exactly the diameter of a drop, which thickens into a cone once the tube blocks and water runs down the outside
• •A stalagmite grows up from the floor beneath, built by the same drips after they land, and is blunter and thicker because the water splashes; a stalactite and stalagmite that meet form a column
• •Flowstone forms where water runs as a sheet over a surface rather than dripping, producing smooth curtains and terraces
• •Draperies or curtains hang as thin wavy sheets where water trickles down a sloping ceiling along a fixed line
• •Rimstone dams build up around slow-moving pools, since turbulence at the lip drives off carbon dioxide fastest
• •Helictites twist against gravity, growing sideways and upwards, driven by capillary flow through a tiny central canal where surface tension outweighs the pull of gravity
• •Cave pearls form where dripping water keeps a grain moving in a small pool so calcite coats it evenly

Growth is slow and highly variable, depending on the drip rate, the carbon dioxide content of the soil above, the temperature and the thickness of overlying rock, with typical rates in the range of a tenth of a millimetre per year and a wide spread around that. A formation a metre long therefore represents something on the order of ten thousand years, and large chambers full of columns represent hundreds of thousands. Because the calcite is laid down in thin layers whose thickness and chemistry record the conditions above, a sawn stalagmite is a climate archive: layers can be dated precisely by uranium-thorium methods over several hundred thousand years, far longer than radiocarbon reaches, and the ratios of oxygen and carbon isotopes within them record rainfall and vegetation. Speleothem records from caves on several continents now supply some of the best-dated evidence for past monsoon strength and for abrupt climate events, which is a considerable second career for a cave decoration.

Cave formations are fragile in a way that is easy to underestimate, and the damage is effectively permanent on human timescales. A single touch leaves skin oils that repel water and stop deposition at that spot, so the formation stops growing there, and the surface discolours. Breakage cannot be repaired in any meaningful sense, since replacing a centimetre takes a century. Lighting installed for tourists creates lampenflora, growths of algae, moss and cyanobacteria that live on the artificial light and stain and etch the calcite, which is why show caves now use cold lights on timers and periodically treat affected surfaces. Changes in airflow and humidity from opening a cave up can halt deposition across whole chambers, and heavy visitor numbers raise carbon dioxide levels enough to start dissolving the formations rather than building them, which is the same reaction simply running the other way. Several major caves, including Lascaux for its paintings and Altamira, have been closed or restricted for exactly these reasons.

Rainwater picks up carbon dioxide from soil, becomes weakly acidic, and dissolves limestone. When it reaches a cave the gas escapes into the air, the water can no longer hold as much calcium carbonate, and calcite precipitates. Stalactites grow down from drips, stalagmites up from where those drips land, and flowstone, draperies and helictites come from water running, trickling or creeping by capillary action. Growth is on the order of a tenth of a millimetre per year, and the layered calcite is a precisely datable record of past climate.