Wikipedia Deep Dive

Speleothem

11 min read

The Stone That Grows

Deep beneath the surface of the earth, in the perpetual darkness of caves, something remarkable happens. Stone grows. Not in the geological sense of mountains slowly rising over millions of years, but in a visible, almost organic way—drip by drip, layer by layer, forming shapes so beautiful and varied that early cave explorers gave them names borrowed from the everyday world: bacon, popcorn, fried eggs, soda straws.

These formations are called speleothems, from the Greek words for "cave" and "deposit." And while you've almost certainly heard of stalactites and stalagmites—perhaps remembering the mnemonic that stalactites hold "tight" to the ceiling while stalagmites "might" reach up from the ground—the world of cave formations extends far beyond these familiar shapes. There are curtains of stone so thin that light passes through them. There are gravity-defying helictites that twist and spiral in impossible directions. There are cave pearls, nearly perfect spheres formed by the constant tumbling of mineral grains in dripping water.

But speleothems are more than geological curiosities. Hidden within their layers lies a record of Earth's climate stretching back half a million years—longer and more detailed than almost any other natural archive.

How Water Builds Stone

The process that creates speleothems begins long before water ever reaches a cave. It starts in the soil.

When rain falls on the ground above a limestone cave, it percolates through layers of soil rich in decaying plant matter. This organic material produces carbon dioxide—the same gas you exhale—which dissolves into the water, making it slightly acidic. Not dramatically so, not like battery acid, but enough to slowly eat away at rock.

Limestone and similar rocks are made primarily of calcium carbonate, the same material found in seashells, eggshells, and antacid tablets. When acidic water meets calcium carbonate, a chemical reaction occurs. The rock dissolves. The water, now carrying dissolved calcium and carbonate, continues its journey downward through cracks and fissures in the bedrock.

Eventually, this mineral-laden water reaches the ceiling of a cave. And here, something changes.

The air inside a cave contains far less carbon dioxide than the soil above. When the water emerges into this different environment, the dissolved carbon dioxide begins to escape—like the fizz leaving an opened bottle of soda. As the carbon dioxide departs, the water can no longer hold all that dissolved calcium carbonate in solution. The minerals precipitate out. They become solid again.

This is the moment of creation. A microscopic layer of calcium carbonate is deposited on the cave ceiling. Then another. Then another. Over thousands of years, these layers accumulate into the formations we call stalactites.

A Catalog of Wonders

The variety of speleothem forms defies easy categorization. Scientists have identified more than three hundred distinct types, each shaped by the specific conditions of its formation: how fast the water flows, what path it takes, the temperature and humidity of the cave air, even the vegetation growing on the surface far above.

The most familiar formations are the dripstones. Stalactites begin as hollow tubes called soda straws—delicate structures so thin you could thread them onto a necklace. Water travels down the inside of these tubes, depositing minerals at the tip. When the central channel becomes blocked, water begins flowing down the outside, and the stalactite takes on its characteristic icicle shape.

Below, where the drops land, stalagmites grow upward. The splash pattern matters enormously. A gentle drip from a short height produces a tall, thin formation—what cavers call a broomstick stalagmite. A heavy flow falling a greater distance spreads wide on impact, building broader, stubbier mounds. Some stalagmites are wider than they are tall, looking rather like fried eggs on the cave floor.

When a stalactite and stalagmite meet, they fuse into a column called a stalagnate. These can grow to enormous sizes. Some famous examples are taller than a ten-story building and continue growing to this day, adding perhaps a fraction of a millimeter each year.

The Rebels: Formations That Ignore Gravity

Not all speleothems follow the simple logic of water dripping down. Helictites are the rebels of the cave formation world. They grow sideways, upward, in spirals and curlicues, in shapes that seem to mock the very concept of gravity.

How do they manage this? The answer lies in capillary action and the pressure of water forcing its way through microscopic pores in the rock. Rather than dripping, the water is pushed through tiny channels, depositing minerals at whatever angle the crack happens to run. The result can resemble anything from delicate butterfly wings to, as cave explorers have colorfully described them, "clumps of worms."

Chandeliers are complex clusters of these formations hanging from cave ceilings. Ribbon stalactites twist and fold like fabric caught in a freeze-frame. The names become increasingly whimsical: bacon (striped draperies), cave popcorn (bumpy clusters), moonmilk (which has the texture of cream cheese when wet and powder when dry).

The Pool Dwellers

Some of the most remarkable speleothems form underwater. Cave pearls are perhaps the most magical—nearly perfect spheres of calcium carbonate that develop when water drips into a cave pool from a significant height. The constant disturbance keeps small mineral grains rotating, building up layer upon layer in all directions equally. The result is a stone pearl, sometimes as smooth and round as anything from an oyster.

Calcite rafts are paper-thin sheets of mineral that float on the surface of still cave pools, riding on surface tension like tiny boats. When something disturbs the water—a drop falling from above, perhaps—these rafts sink to the bottom, where they can pile up into formations called cave cones.

In the El Zapote cenote of Mexico's Yucatan Peninsula, divers have discovered formations called Hells Bells—bell-shaped structures that grow underwater, their origin still not fully understood. They hang in the dark water like the architecture of some alien cathedral.

Time Capsules in Stone

Here's where speleothems become genuinely important to science: they are time machines.

Each layer of a speleothem contains a chemical fingerprint of the conditions when it formed. The ratio of certain oxygen isotopes—different versions of the oxygen atom with slightly different masses—reveals the temperature and amount of rainfall at that time. Carbon isotopes tell us about the vegetation growing above the cave. Trace elements like magnesium indicate how wet or dry conditions were.

Unlike tree rings, which record only the last few thousand years, or ice cores, which are limited to polar regions and certain glaciers, speleothems can preserve climate information from almost anywhere caves exist—and for an astonishingly long time. Scientists have extracted detailed climate records from speleothems spanning the last five hundred thousand years.

The dating techniques are remarkably precise. Speleothems incorporate tiny amounts of uranium from the surrounding rock, and uranium decays at a known rate into thorium. By measuring the ratio of these two elements, scientists can determine when a particular layer formed, often to within a few decades—even for formations hundreds of thousands of years old.

In Oklahoma, researchers discovered something extraordinary: climate data from 289 million years ago, preserved in speleothems from caves that were later filled with sediment and exposed by modern quarrying. This is climate information from the early Permian period, long before dinosaurs, when the supercontinent Pangaea was still forming. The formations themselves had survived nearly three hundred million years.

The Speed of Stone

How fast do speleothems grow? The answer varies enormously—from less than a tenth of a millimeter per year in dry conditions to several millimeters per year where water flows freely. Some of the world's largest formations are hundreds of thousands of years old.

The growth patterns contain their own information. Thin, closely spaced rings indicate dry periods when water was scarce. Wider spacing between layers suggests wetter times with heavier rainfall. Scientists can correlate these patterns with known climate events, like the El Niño-Southern Oscillation cycles that affect weather patterns across the Pacific.

Temperature matters too. The chemical reactions that form speleothems are sensitive to even small temperature changes. In general, cooler conditions and higher carbon dioxide concentrations make carbite minerals more soluble in water, which affects how much material can be transported and deposited.

A Rainbow of Stone

Pure calcium carbonate is colorless and translucent—formations made from it can transmit light like frosted glass. But pure formations are rare. Most speleothems display colors ranging from brilliant white through yellows, oranges, reds, browns, and even black.

Iron oxide produces rust-red and brown tones. Copper can add greenish or bluish tints. Manganese oxide creates the darkest colors, staining formations deep brown or black. Sometimes the coloring is less romantic: mud and silt incorporated into the growing formation produce brown tones that tell us about flooding events or changing water sources.

Some formations display banding—alternating layers of different colors that create patterns remarkably like sliced bacon. These bands often represent seasonal variations or changes in water source, each stripe a record of conditions at the time it formed.

The Distinction That Matters

Scientists draw a careful line between speleothems and their lookalikes. Formations that result from the removal of bedrock rather than the deposit of minerals are called speleogens—they're carved by water rather than built by it. The scalloped walls you see in some caves, or the delicate hanging pendants of remaining rock, are speleogens rather than speleothems.

Similarly, when limestone leaches from concrete, mortar, or other human-made structures and creates stalactite-like formations, these are called calthemites, not speleothems. You can sometimes see calthemites forming under bridges or in old tunnels—the chemistry is identical, but the term speleothem is reserved for natural cave formations.

Living Stone

Not all cave formations are purely mineral. Snottites—and yes, that's the technical term—are bacterial colonies that hang from cave ceilings with exactly the consistency their name suggests. These organisms oxidize sulfur compounds and create highly acidic environments, sometimes dripping with liquid more corrosive than battery acid. They represent a different kind of cave formation entirely, one that blurs the line between geology and biology.

The presence of bacteria and organic matter in caves reminds us that these environments, despite their apparent stillness and isolation, are connected to the living world above. The very carbon dioxide that drives speleothem formation comes largely from biological activity in the soil—the respiration of plant roots, the decomposition of organic matter, the metabolic activity of countless soil organisms.

Fragile Archives

Speleothems grow in conditions of remarkable stability. The temperature in deep caves varies by only fractions of a degree throughout the year. Humidity often approaches one hundred percent. Protected from the chaos of surface weather, these formations can preserve detailed climate records for hundreds of thousands of years.

But this stability makes them vulnerable. A broken stalactite cannot be reattached in any meaningful way. The climate record contained in a damaged formation is lost forever. Cave tourism, while it inspires wonder and appreciation for these underground worlds, inevitably brings changes: body heat from visitors can alter cave temperatures, lighting can promote algae growth, and even the best-intentioned touch transfers oils and contaminants to the formations.

The slowness of their growth makes the loss particularly poignant. A stalactite snapped off by a careless visitor may have been growing since before the pyramids were built. The formation that replaces it—if one ever does—will not reach the same size until long after our civilization is forgotten.

Messages from the Deep

Standing in a cave surrounded by speleothems is standing in a library written in stone. Each formation encodes information about the conditions of its creation—the chemistry of ancient rainwater, the temperature of vanished seasons, the vegetation that once covered the land above. The formations themselves span time scales that dwarf human history: some began growing before our species existed and will continue long after we are gone.

There is something humbling in this. We live in an age of rapid change, where the climate shifts noticeably within a single human lifetime. The speleothems offer perspective. They have recorded ice ages and interglacial warmings, catastrophic volcanic winters and millennia of stability. They will record whatever comes next.

And they will do it the same way they always have: drip by drip, layer by layer, building stone from water with the patience of geological time.