Why Do Cave Stalactites Take Thousands of Years to Grow?
― The Vast Slowness of Rainwater Dissolving Stone and Setting It Again
Step into a limestone cave on a tour and you'll see stone hanging from the ceiling like icicles, and other stone rising from the floor. A guide may tell you, "This took 10,000 years to form." But the idea of stone "growing" is odd when you think about it. Stone is normally something that weathers and wears away.
On a cave tour, you look up at an icicle-shaped stone hanging from the ceiling. Water is still dripping from its tip, drop after drop. On the floor below, where the drops land, another stone pushes up from the ground.
They say these two stones may one day meet and become a single pillar. What does it mean for stone to "build up and grow"?
It grows slowly because two chemical reactions work as a pair
Rainwater takes in carbon dioxide from the air and becomes very weakly acidic. As this water passes through the limestone layers underground, it dissolves the rock, a tiny bit at a time.
When the water seeps into the cave, the carbon dioxide dissolved in it escapes into the air. The dissolved lime then reappears as a solid and builds up little by little.
"Dissolving" and "solidifying" are opposite reactions. A stalactite is what you get when they happen as a pair, one above ground and one below. Let's take them in turn.
Reason 1: Underground, rainwater becomes "stone-dissolving water"
Rain itself is almost pure water. But when it hits the ground and passes through the soil, it picks up far more carbon dioxide than the surrounding water holds. That gas comes from microbes in the soil and fallen leaves. Water with carbon dioxide in it becomes very slightly acidic.
The rock layers around caves contain limestone. Limestone is made of the shells of ancient shellfish and corals piled up over time, and its main ingredient is calcium carbonate. Limestone barely dissolves in ordinary water. But it does dissolve, bit by bit, in weakly acidic water that contains carbon dioxide.
As rainwater travels underground over a long time, it keeps dissolving limestone little by little. This is also how the hollow spaces we call caves form underground.
Reason 2: Inside the cave, the material turns back into stone
The water has dissolved limestone all the way down. It finally reaches the cave's open space and hangs from the ceiling as drops. Here an important change happens.
The air in a cave holds less carbon dioxide than the air in the soil the water has just passed through. So the carbon dioxide dissolved in the water tries to escape into the cave air. Once it leaves, the water can no longer keep all its lime dissolved. Some of the dissolved lime reappears as a solid (calcium carbonate).
This solid builds up in tiny amounts around the rim of a drop on the ceiling, and forms a stalactite hanging down. When a drop falls to the floor, the same reaction happens there, and builds a stalagmite rising from below.
The reaction that dissolves limestone and the one that sets it into stalactites are thought to be the same chemical reaction, running forward or backward depending on how much carbon dioxide is around. Where carbon dioxide is plentiful, as in soil, it runs toward "dissolving". Where carbon dioxide is scarce, as in a cave, it runs toward "solidifying".
What you can check at home
- Put a piece of eggshell into a glass of vinegar
- Watch the tiny bubbles stream from the surface of the shell
Like seashells, eggshells are mostly calcium carbonate. The acid in vinegar reacts with it, giving off bubbles of carbon dioxide as the shell dissolves. The reaction that dissolves limestone in rainwater inside a cave differs in acid strength and speed, but it is the same reaction between calcium carbonate and an acid. For an example of dissolved material slowly turning back into a solid and taking shape, see also how alum crystals grow.
Summary
Stalactites take thousands of years to grow because two reactions work as a pair. (1) Rainwater slightly dissolves limestone underground, and (2) inside the cave, that material turns solid again and builds up. This cycle goes on, unimaginably slowly.
Stone is not just something that gets worn away.
Given the right conditions, water can build it up again.
For those who want to know more ― terms, formulas, and links to textbooksFrom middle-school science to topics under active research, with each level clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school "Chemistry", or advanced material and sidebars in textbooks
- UniversityUniversity-level specialist subjects (geochemistry, hydrology) not taught in high school
- ResearchNot taught as settled fact even at university; researchers are still investigating it
Middle schoolTerms: words around caves
- Limestone: Rock made of the shells of ancient shellfish and corals piled up over time. Its main ingredient is calcium carbonate.
- Stalactite: Stone that grows down from a cave ceiling in an icicle shape.
- Stalagmite: Stone that builds up from a cave floor, from the bottom upward.
- Limestone cave: An underground hollow made when rainwater dissolves limestone.
- Calcium carbonate: The substance in limestone, seashells and eggshells (chemical formula CaCO₃).
Middle schoolHigh schoolChecking with a formula: just how slowly does a stalactite grow?
Hearing "thousands of years" doesn't tell you much about the actual speed. Here the arithmetic gives clear numbers.
Stalactite length ＝ growth per year × number of years
| Growth per year | A commonly used rule of thumb is around 0.1 [mm/year] |
| Number of years | The length of time you want to know [years] |
This rate is said to vary a great deal with how fast the water drips, its mineral content, and the cave environment, so it is only a representative guide. In reality, some stalactites grow much faster than this and some much slower.
| Years for a 10 cm (100 mm) stalactite | 100 ÷ 0.1 ＝ 1000 [years] |
| Years for a 1 m (1000 mm) stalactite | 1000 ÷ 0.1 ＝ 10000 [years] |
Even a stalactite about 10 cm long works out to roughly 1000 years. A large one over 1 m is on the order of 10,000 years.
If a human life is 80 years, 1000 years is about 12 human lifetimes. So while a stalactite the size of a fingertip grows, about 12 generations of people are born and grow old. The "drop growing right now" that you see on a single tour is only a brief moment in that immense stretch of time.
High school+UniversityThe reaction equations: how dissolving and solidifying work
Everything so far can be written as chemical equations, as follows.
CO₂ + H₂O ⇌ H₂CO₃ (carbon dioxide dissolves in water and becomes carbonic acid, a weak acid)
CaCO₃ + H₂CO₃ ⇌ Ca²⁺ + 2HCO₃⁻ (calcium carbonate turns into ions that dissolve easily in water)
The higher the carbon dioxide concentration, the further this reaction runs to the right (the dissolving side). The lower the concentration, the further it runs to the left (the solidifying side). Soil has a high carbon dioxide concentration from microbes' respiration, so the reaction runs right and dissolves limestone. A cave has a lower concentration than soil, so when CO₂ escapes from a drop into the air, the reaction runs left and calcium carbonate appears as a solid (the stalactite). The key point is that the same equation runs back and forth depending on the surroundings.
ResearchWhat is still not well understood
- Predicting the exact growth rate of a stalactite, cave by cave, is still said to be difficult. Many factors are tangled together: how fast and how much water drips, its mineral content, and the cave's temperature, humidity and carbon dioxide level.
- Stalactites and stalagmites can hold very thin growth bands inside, like tree rings, and a research field called "speleothem science" uses them to read past climate (changes in rainfall and temperature). Techniques that estimate the past environment from things like the isotope ratios in the bands are still being refined.
- Parts of the microscopic crystal-growth mechanism are also said to be not fully understood: how drops falling in nearly the same spot for a long time grow into regular shapes.
Even the stalactites you admire under tourist lighting are still being studied in their finer details of growth. Being familiar is not the same as being understood.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of solutions / rocks | Limestone, the names stalactite and stalagmite, the eggshell observation |
| High school | Basic Chemistry: amount of substance and basic reactions | Growth-rate calculation |
| High school+ | Chemistry: chemical equilibrium (advanced) | How CO₂ concentration changes the direction of the reaction |
| University | Geochemistry and hydrology (karst terrain) | Dissolution equilibrium of calcium carbonate, groundwater chemistry |
| Research | Speleothem science and paleoclimatology (unsolved) | Predicting growth rate, reconstructing past climate from growth bands, crystal-growth mechanism |
| ― | Earth science: karst terrain | Relation to how caves form |
- Explanatory materials on limestone caves and tourist geological resources across Japan (general descriptions of limestone dissolution and stalactite growth).
- White, W.B., Geomorphology and Hydrology of Karst Terrains (a basic text on limestone terrain and groundwater chemistry).
- Fairchild, I.J. & Baker, A., Speleothem Science: From Process to Past Environments (a text on reconstructing past climate from stalactites and stalagmites).
- General descriptions in earth science and chemistry textbooks of the relationship between the dissolution equilibrium of calcium carbonate and carbon dioxide concentration.
※ Figures such as growth rates vary widely with the conditions in each cave. This article gives commonly used rules of thumb.
※This article is a general-audience science explainer. Figures such as stalactite growth rates vary widely with cave conditions, and are given as rough guides to help you understand the mechanism.