How stalactites and stalagmites form is one of the most common questions asked by visitors to the limestone caves of the National Parks. The answer lies in a fascinating bit of basic chemistry that transforms dripping mineral-rich water into spectacular cave formations over thousands of years.
Stalactites and stalagmites form when surface water dissolves carbon dioxide from the atmosphere or soil to form carbonic acid. Percolating downward through limestone strata, this weak acid dissolves small amounts of calcium carbonate. When it reaches a cavern roof and contacts cavern air, the acid begins to lose its dissolved carbon dioxide. This is just like a can of soda.
This loss of carbon dioxide decreases acidity, causing the dissolved calcium carbonate to precipitate out of the solution. As water drips from cavern ceilings, this precipitation of calcium carbonate forms stalactites, which hang from cave ceilings like icicles. At the drip point directly beneath the growing stalactites, the same process causes stalagmites to build upward from the cave floor.
How Stalactites and Stalagmites Form: Key Takeaways
- Stalactites and stalagmites form when mineral-rich water deposits calcium carbonate inside limestone caves.
- Carbon dioxide loss inside caves causes dissolved minerals to harden into cave formations over time.
- Stalactites hang from cave ceilings, while stalagmites grow upward from cave floors.
- Growth rings inside stalactites and stalagmites can reveal ancient climate and rainfall patterns.
- Scientists use uranium-thorium radiometric dating to determine the ages of cave formations and cave systems.
- Some stalactites and stalagmites take thousands of years to grow only a few inches.
How Stalactites and Stalagmites Form Through Growth Rings
Cave visitors admire stalactites and stalagmites for their beauty, colors and fantastical shapes. Scientists are more interested in their concentric growth rings, which are similar to those of trees. This sequence of growth rings is visible when stalagmites and stalactites are cut in cross-sections and polished.
Thanks to a new ability to “read” these growth rings, stalactites and stalagmites are revealing their ages; they are also providing insight into the geological and climatic conditions that existed when they formed.
Because of variations in seasonal and drought cycles that control surface water, growth rates are often uneven and erratic. Maximum growth occurs during peak availability of surface water in rainy seasons, periods of snowmelt or wet climatic cycles. But when water is in short supply or unavailable, growth rings may not develop at all.
Because of their faster growth rates, stalagmitic growth rings are thicker and more distinct than those of stalactites. They also have a greater scientific value. When water drips onto a stalagmite, the impact instantly “degasses” the water, sharply reducing acidity, accelerating the precipitation of calcium carbonate and creating large, well-defined growth rings. In the past, scientists visually counted annual growth rings to roughly estimate the age of stalactites and stalagmites. But researchers now employ a far more accurate radiometric dating technique.
Radiometric Dating
Uranium, which has a chemical affinity for calcium, is present in trace amounts in virtually all rocks, including stalactites and stalagmites. Being radioactive, uranium disintegrates continuously into an orderly “decay chain” of elements and isotopes, one of which is thorium. Unlike uranium compounds, those of thorium are not water-soluble. The thorium in stalactites and stalagmites, therefore, formed after calcium carbonate was deposited. Measuring the relative amounts of thorium and uranium present in growth rings can reveal the age of stalactites, stalagmites, and entire cave systems such as Carlsbad Caverns, to an accuracy of just a few hundred years.
Frequently Asked Questions
What is the difference between a stalactite and a stalagmite?
Stalactites hang from cave ceilings like icicles, while stalagmites grow upward from the cave floor. A simple memory trick is that stalactites hold “tight” to the ceiling.
How do stalactites and stalagmites form in caves?
Stalactites and stalagmites form when slightly acidic groundwater dissolves limestone and carries calcium carbonate into caves. As the water loses carbon dioxide, calcium carbonate hardens into mineral deposits that slowly build cave formations.
How long does it take stalactites and stalagmites to grow?
Growth rates vary depending on water supply, temperature and mineral content, but many cave formations grow only a fraction of an inch per century.
What are stalactites and stalagmites made of?
Most stalactites and stalagmites are made of calcium carbonate, the same mineral found in limestone, marble and chalk.
Why are cave growth rings important to scientists?
Growth rings in stalactites and stalagmites can record ancient rainfall, drought cycles and climate changes, making them valuable tools for geological and climate research.
Can stalactites and stalagmites meet?
Yes. When a stalactite growing downward and a stalagmite growing upward eventually connect, they form a single column or pillar.
Final Thoughts
While the chemistry behind how stalactites and stalagmites form is relatively simple, these remarkable cave formations preserve thousands of years of geological and climate history.
This story about how stalactites and stalagmites form in caves previously appeared in Rock & Gem magazine. Click here to subscribe! Story by Steve Voynick.
