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How are stalactites and stalagmites formed?

stalagmites and stalactites
(Image credit: Getty)


Stalactites and stalagmites decorate caves the world over. Stalactites hang down from the ceiling, while stalagmites rise up from the ground. They grow incredibly slowly, and some are so ancient that they predate modern humans, Live Science previously reported

These tooth-like rock formations grow when dripping water comes into contact with the cave air, according to the National Park Service website. The water carries dissolved minerals, picked up on its journey from Earth's surface. As it passes through the cave, it leaves tiny traces of those minerals behind, building each stalactite drip by drip.

What shape are stalactites?

Most stalactites are cone-shaped: thick at the top and tapered to a point at the bottom. But some are hollow. Shaped like straws, these stalactites grow when water trickles down their centre. As each drip evaporates, it leaves another shell of minerals at the bottom of the tube. 

Cave straws are incredibly fragile and often crumble at the slightest touch, making them a rare find in well-trodden caves, according to the Journal of Cave and Karst Studies

Some straw-shaped stalactites seem to defy gravity. Known as helictites, these structures have twists, spurs, and knobbles that tilt off in all directions. Scientists aren't sure exactly how they form, but they think it might be down to a combination of capillary action and wind, according to the Universities Space Research Association

Slight changes in the air currents through a cave, or in the orientation of the crystals in a growing stalactite, can draw tiny water droplets off in new directions. Rather than dripping towards the floor under the force of gravity, they travel sideways or even upwards, leaving their minerals behind as they go.

What do stalactites and stalagmites contain?

Cave ceiling

Each drop of water contains dissolved limestone particles. They harden when they hit air. (Image credit: Getty)

Most of the stalactites you see in caves are made from calcium carbonate, according to the Royal Society of Chemistry. It forms two main types of crystals: calcite and aragonite. They have the chemical formula CaCO3.

For this reason, stalactites only tend to appear in caves where the surrounding rocks contain calcium in the form of limestone or dolomite.

Stalactites can also carry traces of other chemicals, which give them different colors and textures. These chemicals include carbonates, sulphides, and even opal.

Limestone caves often contain stalagmites as well as stalactites. These structures grow on the floor, with a thick base and a point that looks up towards the cave ceiling. Some are flat like fried eggs, while others are long and thin, like broomsticks, according to the Encyclopedia of Caves (Third Edition, 2019).

Stalagmites often grow directly beneath stalactites, mopping up any minerals from water droplets that splash down onto the cave floor. However, the two types of cave decoration don't always come in pairs: either one can appear on its own.

Inside a cave

Other cave features

Limestone caves can also contain other kinds of cave decoration. According to the journal Transactions of the Royal Society of South Africa, stalactites and stalagmites are both types of dripstones, named because they form from dripping water. But you might also see flowstones and cave popcorn.

Flowstones appear when water comes down a cave wall in sheets, according to Yorkshire Dales National Park. They look like curtains of stalactites, hanging together like a waterfall frozen in time. Sometimes flowstones contain layers of color from the minerals left behind by the water, earning them the name 'cave bacon', according to the American Geophysical Union (AGU).

Cave popcorn forms where water comes through pores in the rock, forming bumps and lumps that look like berries.

Cave popcorn

The bumps on the wall of this cave are called cave popcorn. (Image credit: Getty)

The chemistry of limestone stalactites

Stalactites and stalagmites form when rainwater drips through limestone rock. Along the way, it picks up carbon dioxide, from the air and from any organic matter it passes as it dribbles down, according to the National Park Service. The carbon dioxide reacts with the water to make a weak acid called carbonic acid. This acid can dissolve limestone, reacting with the mineral calcite and drawing it into the water as calcium bicarbonate. 

As the water drips into the cave, it comes into contact with the air again. There, it lets go of the carbon dioxide, and the calcium comes out of solution, forming rock-hard calcite again.

How they grow

Strange stalactites

Did you know, stalagmites and stalactites aren't always found in caves? You can see them under concrete buildings, in lava tubes, and even hanging off the side of your garage in the winter. This is because stalagmites and stalactites aren't always made from limestone.

Ice stalactites are probably the most common type of stalactite. They form when it's cold enough for water to freeze, but sunny enough for it to melt again, according to a 2019 article in the Encyclopaedia of Caves. The melted water trickles towards the ground and re-freezes before it hits the floor.

Scientists from the University of Cambridge and the University of Arizona used photographs, maths, and physics to work out why icicles are pointy. They noticed that, as the water trickles down, it blends together to form a sheet. That sheet gives off heat, making a warm pocket of air around the icicle. The warm air rises up, which means that water freezes faster at the bottom of the icicle, making them grow long and thin at the tip.

Giant birdcage cave

Nohoch Nah Chich, Mexico– These stalactites and stalagmites form part of the longest underwater cave system in the world. (Image credit: Getty)

Another kind of stalactite you might see in your everyday life is a concrete stalactite, according to the Royal Society of Chemistry. You can find them in car parks and even on the pipes in your home. Concrete contains calcium oxide, which dissolves when alkaline liquid passes through it. When that liquid hits the air, the calcium comes out of solution, forming a hard substance called calthemite. If the drips come fast enough, calthemite stalagmites can start to form too.

Other strange stalactites are a bit harder to find. Lava stalactites form inside tunnels called lava tubes, which carry molten rock beneath the Earth, according to the International Journal of Speleology. When the roof of a lava tube starts to cool, it gets a skin, a bit like a bowl of custard. Underneath, hot gases keep expanding, pushing on the skin and stretching it out to form hollow tubes that harden into solid rock.

Additional resources

As well as providing stunning features, the geology of caves can teach us about Earth’s climate. You can read about these clues at the NOAA's National Center for Environmental Information. To find out more about underwater stalactites, explore the research carried out by Heidelberg University.

Bibliography

Journal of Cave and Karst Studies of the National Speleological Society (2005). https://caves.org/pub/journal/PDF/V67/cave_67-01-fullr.pdf

"Analysis and optimization of process parameters for in vitro biomineralization of CaCO3 by Klebsiella pneumoniae, isolated from a stalactite from the Sahastradhara cave". Royal Society of Chemistry (2020). https://pubs.rsc.org/en/content/articlehtml/2020/ra/d0ra00090f

Encyclopedia of Caves, Third Edition, (2019).  https://www.sciencedirect.com/topics/earth-and-planetary-sciences/stalagmite

The Geology of the Makapan and other Caves. Transactions of the Royal Society of South Africa (1951). https://www.tandfonline.com/doi/abs/10.1080/00359195109519881?journalCode=ttrs20

The origin of tubular lava stalactites and other related forms. International Journal of Speleology (1998). https://digitalcommons.usf.edu/ijs/vol27/iss1/14/

Laura is a biologist who left the confines of the lab to the rigours of an office desk as a keen science writer and full-time software engineer. She has previously written for magazines How It Works and T3. Laura's main interests include science, technology and video games.