What Are the Rocks in Caves Called and How Infiltration Water Forms Them

The rocks and formations in caves are called speleothems, a term covering every mineral deposit that water builds inside a cave over thousands of years. Water not only hollows out the cave itself but also decorates it, leaving behind stalactites, stalagmites, columns, draperies, and dozens of other shapes. Most of these are made of calcite — a crystalline form of calcium carbonate — deposited drop by drop as mineral-rich water seeps through the surrounding rock.

What Are the Rocks in Caves Called?

The decorative rock formations inside a cave are collectively known as speleothems, sometimes called cave formations or, in older literature, "drips" or "natelic formations." The word speleothem comes from the Greek spēlaion ("cave") and thema ("deposit"). Speleothems are secondary mineral deposits — they form after the cave cavity already exists, built up by water that carries dissolved minerals into the open space.

Speleothems are not the same as speleogens. Speleogens are features carved out of the existing bedrock by erosion — such as scallops, rock fins, and blades — whereas speleothems are added to the cave by mineral deposition. Both shape the appearance of a cave, but one is sculpted by removal and the other built up by accumulation.

The main agent in speleothem formation is infiltration water seeping through carbonate rock and dripping from the ceiling of karst caves. In the past these forms were divided into "upper drip" and "lower drip."

The upper drop is similar in everything to ice icicles. It hangs on the vaults of natural galleries. Through the icicles, which are sometimes a lot of different lengths and thicknesses together fused, pass from above vertical wells of different widths, from which mountain water drips, their longitude builds up and produces the lower drop, which grows from falling drops from the upper icicles. The color of kapi, and especially the upper, is mostly, like scale, white, grayish; sometimes, like a good yar, green, or completely wohryanoy.

Speleothems: The General Term for Cave Formations

A speleothem is any secondary mineral deposit formed in a cave, and the category includes far more than the familiar stalactites and stalagmites. Speleothems are defined by how they form — through the precipitation of minerals from water — rather than by their shape. Their composition is most often calcium carbonate in the form of calcite, but they can also be built from aragonite, gypsum, dolomite, and other minerals depending on the local geology and water chemistry.

Speleothems form mostly after the underground cavity already exists, a process called epigenetic formation. Far more rarely they form at the same time as the cavity (syngenetic), which is essentially never seen in true karst caves. This distinction matters because it tells geologists that a cave's decorations record a later chapter of its history than its excavation.

Beyond their beauty, speleothems are valuable scientific archives. Because calcite layers grow incrementally and trap chemical signatures from the water that fed them, speleothems can be dated and analysed to reconstruct past climates. Layered stalagmites in particular provide paleoclimate data stretching back tens of thousands of years, making them one of speleology's most important research tools.

How Cave Formations Are Created by Water

Cave formations are created by water that dissolves rock in one place and redeposits the dissolved mineral in another. The process begins in a karst landscape — a terrain of soluble bedrock such as limestone — where slightly acidic groundwater slowly enlarges cracks into passages and chambers. Once an air-filled cavity exists, the same water that carved it begins to build speleothems as it drips, flows, and seeps across the cave surfaces.

Water plays a dual role: it is both the excavator that creates solution caves and the mason that decorates them. The position of the water table, the saturation zones within the rock, and the rate of dripping all govern which formations appear and how quickly they grow. Most limestone speleothems grow on the order of a fraction of a millimetre to a few centimetres per century, meaning the largest formations represent tens of thousands of years of patient deposition.

The Chemistry Behind Cave Formations

The chemistry of cave formations is a two-stage cycle of dissolution and re-deposition driven by carbon dioxide. Rainwater absorbs carbon dioxide from the air and soil, becomes mildly acidic, dissolves limestone underground, and then releases that mineral again when conditions change inside the cave. Understanding this cycle explains why nearly all the great limestone caves form the same kinds of speleothems.

Acidic Water and the Dissolution Process

The dissolution process starts when rainwater picks up carbon dioxide and turns into a weak carbonic acid. As this acidic water percolates through soil and into limestone, it attacks the calcium carbonate of the rock, dissolving it and carrying the minerals away in solution. Over long periods this dissolution widens joints and bedding planes in the limestone into the open voids that become caves.

Carbonic Acid and Calcite Crystallization

Carbonic acid is the key reactant that both dissolves limestone and, in reverse, allows calcite to crystallize. The reaction is reversible: water plus carbon dioxide forms carbonic acid, which reacts with calcium carbonate to produce soluble calcium bicarbonate. When that bicarbonate-rich water later loses carbon dioxide, the reaction runs backward and solid calcite crystallizes out onto the cave surface.

Calcium Carbonate Deposition

Calcium carbonate deposition happens when a drop of saturated water reaches the cave and gives up its dissolved load. As the drop hangs from the ceiling or splashes on the floor, carbon dioxide escapes into the cave air, the water becomes supersaturated, and a tiny ring of calcite is left behind. Each drop adds another microscopic layer, and the accumulation of countless layers over millennia builds the visible speleothem.

Types of Chemogenic Cave Deposits

Chemogenic cave deposits are the formations precipitated chemically from mineral-laden water, and researchers have long worked to classify their many shapes. Early Russian work by V. I. Stepanov (1971) subdivided cave mineral aggregates into three types: the stalactite–stalagmite crust (products of crystallization from freely flowing solutions — stalactites, stalagmites, stalagnates, draperies, and wall and floor concretions); corallites (aggregates arising from capillary water films on cave surfaces); and antholites (twisting, splitting, parallel-fibre aggregates of easily soluble minerals such as gypsum and halite).

Building on the studies of G. A. Maksimovich (1963) and Z. K. Tintilozov (1968), chemogenic formations can be grouped into three principal types: natonic, colomorphic, and crystallite. The natonic (dripstone) formations, by far the most widespread, divide into two great groups by form and origin — stalactite formations, built from calcareous matter released by drops hanging on the ceiling, and stalagmite formations, built from matter released by drops that have fallen to the floor.

Stalactites: Definition and Etymology

A stalactite is a speleothem that hangs from the ceiling of a cave, formed where mineral-rich water drips downward and deposits calcite. The word comes from the Greek stalaktos, meaning "dripping," and the formations were noted in antiquity — Pliny the Elder described cave deposits, and the term itself was coined by the Danish physician Ole Worm in the 17th century. A useful memory aid is that stalactites hold "tight" to the ceiling.

Stalactites grow downward as each drip leaves a ring of calcite at its tip, gradually extending the formation toward the floor. Their internal structure often contains a central hollow canal through which water travels. The materials are typically calcite or aragonite, both forms of calcium carbonate, though stalactites can also be made of other deposited minerals. Some develop a broad, pointed "shark tooth" profile as layers build unevenly along the length.

Stalagmites

A stalagmite is a speleothem that rises from the floor of a cave, built up where water dripping from above splashes down and deposits its minerals. Stalagmites mirror stalactites but grow upward and lack a central canal, so they tend to be broader and more rounded, with shapes ranging from cones to stacked-plate columns depending on the drip rate and splash pattern. The memory aid here is that stalagmites "might" reach the ceiling one day.

Stalagmites and stalactites are intimately related: the same dripping water that loses a little mineral on the ceiling delivers the rest to the floor below, so a stalagmite very often forms directly beneath its parent stalactite. The shape of a stalagmite records the height of the fall and the volume of water — tall thin pillars form under steady slow drips, while broad mounds form where water splashes widely.

How Columns Form When Stalactites Meet Stalagmites

A column, or pillar, forms when a downward-growing stalactite and an upward-growing stalagmite eventually meet and fuse into a single continuous shaft of calcite. Once joined, the column can continue to thicken as water flows over its surface, sometimes growing into a massive structural-looking pillar. Columns are among the most dramatic speleothems precisely because they represent the union of two formations that may each have taken tens of thousands of years to grow.

Draperies and Curtains

Draperies, also called curtains, form when water runs down a sloping or overhanging ceiling and deposits calcite along the line of flow rather than at a single drip point. The result is a thin, wavy sheet of mineral that hangs like a folded curtain, often translucent and banded with colour from trace impurities such as iron. When the banding produces alternating light and dark stripes, these draperies are sometimes called "cave bacon."

Soda Straws

A soda straw is a thin, hollow, tubular stalactite — essentially the earliest stage of stalactite growth, before the central canal becomes blocked. Each drop of water emerging at the tip leaves a delicate ring of calcite exactly the diameter of the drop, so the straw grows downward as a fragile tube of uniform width, typically only a few millimetres across.

Soda straws can reach remarkable lengths when conditions stay stable; one of the longest known soda straw stalactites, in Australia's Chillagoe-Mungana area, exceeds nine metres. Because they are hollow and extremely thin-walled, soda straws are among the most fragile speleothems and break at the slightest touch, which is why cave managers fence them off and ask visitors never to touch cave formations.

Cave Popcorn (Coralloids)

Cave popcorn, also known as coralloids, consists of small knobby clusters of calcite that resemble popcorn or coral growing on walls, floors, and other speleothems. It forms from capillary water seeping through the rock and from water that splashes or evaporates on cave surfaces, depositing minerals in small bulbous nodes rather than along a single drip line. Cave popcorn often marks places where air movement and evaporation are significant, and it can grow over older formations as conditions in the cave change.

Flowstone and Crusts

Flowstone is a sheet-like speleothem deposited by thin films of water flowing over walls and floors, building up smooth, layered crusts of calcite rather than discrete drips. Because the water spreads across a broad surface, flowstone coats the rock in rippled, cascading forms that can look like frozen waterfalls. One of the most famous examples is Frozen Niagara in Mammoth Cave, a vast flowstone formation resembling a waterfall turned to stone.

Flowstone is most commonly found in solution caves, where flowing mineral-rich water has long access to broad rock surfaces. Its composition is usually calcite, though other minerals can contribute, and its layered internal structure — like that of stalagmites — preserves a chemical record useful for scientific dating.

Antholites and Crystalline Formations

Antholites and other crystalline speleothems form from easily soluble minerals that crystallize directly rather than building up drip by drip. This group includes gypsum flowers, gypsum crusts, and gypsum "snowballs," which grow as twisting, splitting, parallel-fibre aggregates pushed outward from their base. Caves can also host mirabilite and epsomite — soluble sulphate minerals that form delicate crusts and needles in dry, stable conditions.

Helictites are perhaps the most baffling crystalline speleothems, twisting and curling in seemingly random directions that defy gravity. Helictites form where capillary forces move water through tiny pores faster than gravity can pull a drop free, so the calcite or aragonite grows sideways and upward in branching, zero-gravity shapes. Their fragile, erratic structure makes them rare and highly prized, and like soda straws they are easily destroyed, so their preservation depends on visitors leaving them untouched.

Comparison of Speleothem Types

The main speleothem types differ in where they grow, how water reaches them, and what shape results. The table below summarises the most common formations.

Natural vs. Artificial Stalactites (Calthemites)

The main difference between natural and artificial stalactites is the source material: natural stalactites grow from limestone-derived calcium carbonate, while artificial ones grow from man-made concrete. Despite looking alike, the two arise from different chemistry and on vastly different timescales — natural stalactites take millennia, while their concrete imitators can appear within years.

Concrete Stalactites and Calthemites

Calthemites are secondary deposits that mimic cave speleothems but grow from concrete, mortar, or lime structures rather than natural limestone. They form on the undersides of bridges, tunnels, and buildings where water leaches calcium hydroxide from concrete; as that water meets the air, carbon dioxide reacts with it to deposit calcium carbonate, producing stalactites, stalagmites, and flowstone-like crusts. Calthemites grow far faster than natural speleothems because concrete leaches calcium much more readily than weathering limestone does.

Coastal Cave Formation by Wave Erosion

Not all caves are dissolved by acidic groundwater — coastal sea caves are carved by the mechanical force of waves. Pounding surf exploits weaknesses such as fractures and faults in a sea cliff, hammering them with water pressure and abrasive sand and pebbles until a recess deepens into a cave. Because they are excavated by erosion rather than mineral deposition, sea caves are dominated by speleogens like scallops and worn rock surfaces rather than by dripstone speleothems.

Other non-karst caves form by entirely different mechanisms. Lava tubes form when the surface of a flowing lava stream cools and crusts over while molten lava drains away beneath, leaving a hollow tube; their roofs can hang with lava stalactites, or "lavacicles," and feature distinctive lava-tube speleogens. Glacier caves form within ice rather than rock, melted out by water and warm air inside a glacier, and may grow ice stalactites and icicles; on a related note, brinicles are tube-like ice formations that grow downward beneath sea ice when super-cold brine sinks through warmer seawater.

Cave Conservation and Preservation

Cave conservation matters because speleothems are extraordinarily fragile and effectively non-renewable on a human timescale — a formation broken in a second may have taken fifty thousand years to grow. Even touching a formation transfers skin oils that block the thin water film calcite needs to keep growing, so the guiding rule in every protected cave is to look but never touch. Soda straws and helictites are especially vulnerable, snapping at the lightest contact.

Human impact on caves has a long history. Prehistoric American Indians mined cave minerals such as gypsum and mirabilite, and centuries of torch-bearing explorers left soot that darkened formations — torch smoke damage is still visible on the walls of historic caves. Today agencies such as the National Park Service in the United States and national caving associations like the NCA promote conservation, restrict access to delicate areas, and run educational programs so that students and visitors understand why specimen collection and graffiti are prohibited. Caves are also living systems; their subterranean ecosystems depend on stable, undisturbed conditions, which conservation seeks to protect.

Exploring Caves: Tourism and Safety

Caves can be explored safely as a tourist on guided tours, or more adventurously through caving, which carries real risks that demand training and equipment. Show caves offer lit pathways and guided routes through spectacular speleothem displays, while wild caving exposes explorers to hazards including falls, flooding, cold, getting lost, and falling rock — which is why caving should never be attempted alone or without proper preparation.

Many of the world's great caves are famous for specific formations and named tours. Mammoth Cave in Kentucky — the longest cave system on Earth — offers routes such as the Domes and Dripstones Tour, the Great Onyx Lantern Tour, and the Violet City Lantern Tour, passing landmarks like Frozen Niagara, the Giant's Coffin, the Snowball Room, the Snow Room, and historic passages including Broadway, Gothic Avenue, Grand Avenue, Cleveland Avenue, and Kentucky Avenue. Elsewhere, the Jeita Grotto in Lebanon holds one of the longest known stalactites, Doolin Cave in Ireland features the enormous Great Stalactite, and the Gruta Rei do Mato in Brazil is celebrated for its towering columns.

Photographers and scientists alike are drawn underground; National Geographic photographer Carsten Peter has documented extreme caves, while researchers continue to study speleothems for the paleoclimate records they preserve. Online communities such as Reddit's caving forums and educational resources from the National Park Service help newcomers learn safe practice before they venture below ground. To explore more about how caves and their formations are studied, see our Speleology section.