A speleothem is a mineral deposit that forms inside a cave from water that drips, flows, seeps, or pools on cave surfaces. Stalactites and stalagmites are the most familiar examples, but the term covers dozens of formation types, from thin ribbons of mineral curtains to bizarre gravity-defying twists called helictites. Most speleothems are made of calcium carbonate, the same mineral found in limestone, and they grow extremely slowly, sometimes less than a tenth of a millimeter per year.
How Speleothems Form
The process starts long before water reaches a cave. Rainwater seeps through soil, where decaying plant matter releases carbon dioxide. The water absorbs that carbon dioxide and becomes mildly acidic, forming carbonic acid. As this acidic water trickles down through cracks in limestone bedrock, it dissolves the rock and picks up calcium ions along the way.
When that mineral-laden water finally enters an open cave passage, the chemistry reverses. The cave air contains far less carbon dioxide than the soil above, so the dissolved gas escapes from the water droplet, much like carbonation fizzing out of an opened soda. Losing carbon dioxide makes the water less acidic, and the dissolved calcium can no longer stay in solution. It crystallizes out as the mineral calcite, adding a thin film to whatever surface the water touches. Repeat this process over thousands or hundreds of thousands of years, and those films build into the formations visitors see on cave tours.
Evaporation can also drive the process. In drier cave passages where airflow is stronger, water may partially evaporate before it drips away, concentrating the dissolved minerals and forcing them to crystallize.
Calcite, Aragonite, and What They’re Made Of
The vast majority of speleothems are composed of calcite, the most stable form of calcium carbonate at surface temperatures. A less common mineral called aragonite, which has the same chemical formula but a different crystal structure, forms under specific conditions. Research in caves in southern France found that aragonite crystals tend to appear when drip rates are very low and when the water carries a relatively high ratio of magnesium to calcium. Temperature and evaporation play less of a role than scientists once assumed. Aragonite in a stalagmite can actually signal drier climate conditions at the time it formed, which makes it useful for reconstructing past environments.
Over geological time, aragonite is unstable. Studies show that fossil aragonite in some caves has converted to calcite in less than a thousand years through a process of dissolution and recrystallization.
Common Types of Speleothems
Speleothems fall into two broad categories: dripstone formations, shaped by the downward flow and dripping of water, and erratic formations, which grow in unusual directions driven by capillary forces or other mechanisms.
Stalactites grow downward from a cave ceiling or ledge, built drip by drip as calcite is deposited in thin rings. They tend to be widest where they attach to the ceiling and taper toward their tips, resembling stone icicles. Stalagmites form directly below, on the cave floor, as drops land and splash, depositing minerals that build upward into mounds or pillars. When a stalactite and stalagmite eventually meet, they fuse into a column.
Where water flows in a thin sheet down a cave wall rather than dripping from a point, it deposits ribbons of calcite that can grow into translucent curtains or draperies. Flowstones are broader, slab-like deposits left by water sheeting across floors or walls. Both types can display banding that reflects changes in water chemistry or flow rate over time.
Rare and Unusual Formations
Some of the most striking speleothems are also the rarest. Helictites are contorted, twisting formations that seem to defy gravity, curling sideways or even upward. Every helictite contains a tiny central channel through which capillary water slowly seeps, feeding growth at the tip regardless of which direction it points. The result can look like a frozen tangle of worms or coral branches.
Frostwork consists of needle-like crystals, typically aragonite, that radiate outward from a surface like the spines of a cactus or thistle. The National Park Service describes frostwork as among the most exquisite, fragile, and intricate of all speleothem types. Cave flowers, usually made of gypsum rather than calcite, grow outward from cave walls as thick mineral petals. And boxwork, a grid-like lattice of thin mineral blades resembling rows of post office boxes, is found almost exclusively in Wind Cave in South Dakota. Boxwork forms when minerals fill fractures in limestone, and the surrounding rock later erodes away, leaving the tougher mineral grid behind.
Why Speleothems Have Different Colors
Pure calcite is white or translucent, so any color you see in a speleothem comes from impurities or surface staining. The orange, red, yellow, and brown tones common in many caves are typically caused by iron oxide carried in by percolating water. Even small concentrations of iron can produce vivid warm hues, from pale tan to deep reddish-brown.
Gray, dark brown, and black coloration has several possible sources. Manganese oxide minerals are a frequent culprit. Analysis of black layers in formations at Carlsbad Cavern found manganese concentrations of 1.4% and iron concentrations of 6.4%. But not all dark staining is mineral in origin. Bat guano dust, soot from historical kerosene lamps, and even carbonaceous material carried underground by ancient floods have all been documented as causes of blackened speleothem surfaces in different caves. In one New Mexico cave, dark coatings on massive stalagmites near the entrance were traced to a goat-manure fire. Sulfate speleothems, by contrast, tend to stay white or transparent, occasionally tinged light tan by trace impurities.
How Scientists Date Speleothems
Speleothems incorporate trace amounts of uranium from the water that forms them, typically in the parts-per-billion to parts-per-million range. Over time, that uranium decays into thorium and other elements at a known rate. By measuring the ratio of uranium to its decay products, scientists can pin down when a particular layer of calcite was deposited. This technique can date material as young as a few decades and as old as 600,000 years.
The precision is remarkable. For a sample around 10,000 years old containing roughly 1 part per million of uranium, the margin of error is only about 40 years. Even at 120,000 years, the uncertainty stays within about 500 years. This level of accuracy, combined with the fact that speleothems grow in sequential layers like tree rings, makes them one of the most reliable geological clocks available for the last half-million years.
Speleothems as Climate Archives
The same layered growth that allows precise dating also preserves a chemical record of past climate. Scientists measure the ratio of heavy to light oxygen atoms locked in the calcite. This ratio reflects the temperature and rainfall conditions at the time each layer formed. On seasonal and annual timescales, it captures changes in where rain came from, how intense storms were, and surface air temperature. On longer timescales spanning thousands of years, it records shifts in ocean composition, changes in storm tracks, and the growth and retreat of ice sheets.
Trace elements preserved in speleothem layers add another dimension. Variations in elements like magnesium and strontium serve as indicators of rainfall amount, vegetation changes above the cave, and how fast the speleothem was growing. High-resolution analysis of these elements can resolve seasonal patterns and even identify individual years within the record, turning a single stalagmite into a detailed climate diary stretching back millennia.
How Fast Speleothems Grow
Growth rates vary enormously depending on water supply, temperature, and carbon dioxide levels, but a global analysis of annually layered stalagmites found an average vertical growth rate of 0.093 millimeters per year. That works out to roughly one centimeter per century, or about the thickness of a fingernail every decade. A floor-to-ceiling column in a large cave passage may represent hundreds of thousands of years of continuous mineral deposition.
Growth is not constant. Speleothems can stop forming entirely during dry periods when water flow ceases, leaving visible gaps called hiatuses in their layered structure. They grow faster in warmer, wetter conditions when more water percolates through the overlying soil and rock. These growth variations are themselves a source of climate information, since periods of rapid or interrupted growth correspond to known wet and dry intervals in the regional climate record.