Caves and Dripstone

Carbon Dioxide from the Soil

Rain reaching the ground first passes through soil. Roots breathe there, so do countless microbes, and dead matter decays, so the soil’s air holds far more carbon dioxide than the atmosphere does.

Today’s air holds about 0.043 % carbon dioxide (425.6 ppm, the global mean for 2025). The soil air in karst country typically holds 0.1⁠–⁠1 %, a few times to twenty-odd times as much; in the wet tropics the growing-season mean can reach about 3.5 %.

Water takes this carbon dioxide in and becomes carbonic acid, a weak acid: the same compound that gives fizzy drinks their tang.

Air: about 0.043 % · karst soil air: typically 0.1⁠–⁠1 % · wet tropics, growing-season mean: up to about 3.5 %

A Weak Acid Dissolves Stone

Limestone is mostly calcite, calcium carbonate. Water carrying carbonic acid dissolves it into calcium and bicarbonate ions: CaCO₃ + H₂O + CO₂ ⇌ Ca²⁺ + 2 HCO₃⁻.

The more carbon dioxide in the water, the more it dissolves. With the standard constants, water at 10 °C in equilibrium with 1 % carbon dioxide holds about 212 mg of calcium carbonate a litre; in equilibrium with today’s air, only about 70 mg, a third as much.

Cold water holds more carbon dioxide, and at the same carbon dioxide it also dissolves more calcite: with 1 %, about 231 mg a litre at 5 °C and about 165 mg at 25 °C.

At 10 °C: about 212 mg/L with 1 % carbon dioxide, about 70 mg/L with air (computed with the constants of Plummer and Busenberg, 1982)

Down the Joints

Limestone was laid down in beds; the partings between them are bedding planes, and the cracks that cut down through them are joints. Water travels mainly along these openings, and dissolution starts there, strongest where the water first meets the rock.

Near the surface the cracks are widened and filled with soil: the epikarst. It lets water through more easily than the rock below, so rain seeping down pauses at its base and leaks on slowly through the cracks; that is why many stalactites in a cave drip all year. In pure crystalline limestone it is typically about 10 m thick.

At first water only seeps along the cracks. Whichever crack carries more water dissolves faster and draws still more water, while the others hardly grow; at some point one crack breaks through and becomes a conduit.

Bedding planes and joints · epikarst: cracks widened and filled with soil · the crack with the most water grows fastest

Funnels in the Ground

As surface water drains into the cracks, the ground sinks with it into funnel-shaped hollows: dolines, or sinkholes. The USGS sorts them into three kinds: rock dissolved at the surface, a soil cover slowly settling, and a soil cover collapsing suddenly into a void below.

A stream that reaches such an opening vanishes underground: a sinking stream. The small stream on the model ends in a doline this way.

When the roof of a great cave chamber falls in and an underground river carries the rubble away, a huge pit is left at the surface, more than 100 m wide and deep: a tiankeng. Xiaozhai Tiankeng in Fengjie, Chongqing, is 662 m deep measured from its highest cliff top, 511 m from the lowest point of its rim.

Doline (sinkhole) · sinking stream · tiankeng: more than 100 m wide and deep

A River Underground

Conduits join into a network that gathers a whole district’s water: an underground river. Above the water table, falling water cuts tall, narrow canyons; at the water table, water fills the passage and dissolves round tubes.

Underground rivers return to the surface at springs. On the model the cave’s stream runs into a conduit and comes out as a spring at the foot of the cliff on the right.

Most caves take ten to a hundred thousand years to grow from a crack to a size a person can pass; a cave wall retreats at most about 1 mm a year. As a river cuts down, the water finds lower paths and old passages are left high and dry. Mammoth Cave in Kentucky, the longest cave system known, reached 426 miles (about 686 km) of survey in 2022.

Canyons above the water table, tubes at it · most caves: 10,000⁠–⁠100,000 years · walls retreat at most about 1 mm a year

The Drip Lets Its Gas Go

Water full of dissolved calcium carbonate drips from the cave roof. The cave’s air holds far less carbon dioxide than the soil’s did, so the gas comes out of the water, like a bottle of soda being opened.

Without the carbon dioxide the calcium carbonate can’t stay in solution: the same reaction runs backwards and calcite comes out again. With the standard constants, water saturated under 1 % carbon dioxide in the soil can give back about 119 mg a litre in a cave at 0.1 %. A drop comes into balance with the cave’s air in a few seconds.

It is mostly the gas going, not the water evaporating. A study of three American caves in 1964 found the magnesium in the water unchanged as calcite came out; evaporation would have concentrated it. The carbon dioxide in a cave’s air also rises and falls with the seasons as the cave breathes, in some caves highest in winter, in others the other way round.

Little carbon dioxide in the cave’s air · the drop gives up its gas and calcite comes out · within seconds

It Starts as a Hollow Tube

Every stalactite begins as a hollow tube: a soda straw. A drop hangs from the roof, loses carbon dioxide at its surface and grows a thin film of calcite; when the drop falls, the film is left on the roof as a ring. Drop after drop, the rings make a tube.

A soda straw is about as wide as a drop, about 5 mm at the narrowest; a drop is about 0.1 mL. Water runs down the inside and the tube grows longer.

As long as water keeps dripping, the tube slowly thickens into a stalactite, like an icicle or a carrot hanging from the roof. At its centre is a hollow tube that first carried the water; a stalactite cut in two shows it. Stalactites often hang in rows along cracks in the roof, where the water comes through.

Soda straw: about a drop wide, about 5 mm at the narrowest · a drop: about 0.1 mL · stalactites often in rows along roof cracks

Stalagmites and Columns

A drop that reaches the floor still holds some dissolved calcium carbonate; as it lands it gives off carbon dioxide and leaves a mound where it falls: a stalagmite. A stalagmite has no hollow tube; it is often thicker than the stalactite above it, with a rounded top.

The stalactite grows down, the stalagmite grows up; where they meet they make a column.

Where water doesn’t drip but runs in sheets down a wall or over a sloping floor, it lays down flowstone, like a frozen waterfall.

Stalagmite: where the drops land · column: stalactite and stalagmite joined · flowstone: water running over the walls

How Fast It Grows

Stalagmites grow slowly, and at very different speeds: in one global database, stalagmites growing at the same time averaged from about 0.00002 mm to 1 mm a year, nearly five orders of magnitude apart.

The speed depends on the extra calcium carbonate in the water, the temperature and how fast the drops come. The rate constant grows about tenfold from 0 °C to 30 °C. On this page’s model (10 °C, soil at 1 %, cave at 0.1 %): a drop every 30 seconds grows about 0.18 mm a year, a drop an hour about 0.04 mm, a drop a day about 0.002 mm.

Soda straws are hollow and look like the fastest growers. In Lehman Caves, straws broken off between 1885 and 1922 have since grown back from under an inch to about four inches.

Average growth: about 0.00002⁠–⁠1 mm a year · slower drips, slower growth (this page’s model)

A Climate Record in Stone

A stalagmite grows up layer by layer, and some have yearly layers like tree rings; the yearly layers have to be checked by radiometric dating. The usual method is uranium⁠–⁠thorium dating: in the best conditions a sample 10,000 years old is dated to about 10 years, 130,000 years to about 100 years, 600,000 years to about 12,000 years.

The oxygen-isotope records of stalagmites from Chinese caves now join up to cover the past 640,000 years. Hulu Cave near Nanjing bears a striking resemblance to the Greenland ice cores (11,000 to 75,000 years ago); Sanbao Cave in Hubei rises and falls in cycles of about 23,000 years, in step with summer sunshine in the Northern Hemisphere.

These oxygen isotopes are read as a signal of the Asian monsoon, but whether they record local rainfall, the moisture upstream or the winds is still debated. Hulu Cave’s stalagmites were dated by both radiocarbon and uranium⁠–⁠thorium and became part of IntCal20, the radiocarbon calibration curve.

Uranium–thorium: about ±10 years at 10,000 years, about ±12,000 years at 600,000 · Chinese cave records cover 640,000 years

The South China Karst

The karst of southern China covers about 550,000 km², falling from the Yunnan–Guizhou Plateau down to the Guangxi Basin. The South China Karst was inscribed as World Heritage in two parts, in 2007 and 2014: seven areas, among them the Shilin stone forest in Yunnan, Libo in Guizhou, Wulong in Chongqing and Guilin in Guangxi.

UNESCO calls it the global type site for three landforms: fenglin (tower karst), fengcong (cone karst) and shilin (stone forest), with Guilin the best-known example of continental tower karst. At the back of the model is a cluster of cones joined at their feet; the lone peak on the right is a tower.

About 15 % of the world’s ice-free land has carbonate rock at the surface, and about a sixth of all people (1.18 billion in 2015) live on it; about 680 million (in 2016), some 9 %, drink karst groundwater. That water comes fast and is barely filtered, and pollution travels just as fast.

About 550,000 km² · World Heritage in 2007 and 2014 · about 9 % of people drink karst groundwater

The Block Taken Apart

The model comes apart into five: the soil and plants, the epikarst, the bedded limestone below with the cave inside it, the cave’s stalactites and stalagmites, and the water.

Water takes stone away from under the soil and lays some of it down again in the cave. The part under the pointer lifts out alone and is named.

Soil and plants · epikarst · bedded limestone · stalactites and stalagmites · water