How Caves Are Formed: The Stages and Science of Cave Formation

What Is Cave Formation? Understanding Speleogenesis

Speleogenesis is the origin and development of caves, and a cave is a natural underground void large enough for a human to enter. In North America the working definition usually requires a passage at least long enough or deep enough to admit a person and extend beyond the reach of daylight, while European cave classification standards and surveying bodies such as the British Geological Survey apply broadly similar size and dimensional criteria. The study of caves and the processes that form them is called speleology.

Caves are classified by how they form rather than only by how they look. The major categories are solutional caves (dissolved out of soluble rock), erosional caves (cut by mechanical erosion from water or waves), fracture caves (opened along joints and faults), talus caves (voids between fallen boulders), glacier caves (melted within ice), and lava tubes (drained volcanic conduits). Each category produces its own cave morphology, passage patterns, and characteristic features.

Solutional caves in karst landscapes are by far the most common and the largest. Karst topography forms where soluble bedrock — chiefly limestone, dolomite, and gypsum — is dissolved by groundwater, producing sinkholes, dolines, disappearing streams, and underground drainage. The surface features and the underground passages develop together, so the karst landscape above a cave records the same dissolution that hollowed out the cave below.

The Chemistry Behind Cave Formation

The chemistry of solutional cave formation comes down to a weak acid slowly dissolving calcium carbonate. Rainwater is naturally mildly acidic, and as it moves through soil and rock it dissolves limestone along cracks, widening them over geological time into passages large enough to walk through. This same chemistry, run in reverse where water loses its dissolved load, builds the mineral decorations inside caves.

Carbonic Acid Formation in Caves

Carbonic acid forms when rainwater absorbs carbon dioxide. Falling rain dissolves a small amount of atmospheric carbon dioxide, and far more as it percolates through soil, where decaying organic matter makes the soil air rich in CO₂. The water and carbon dioxide combine to form carbonic acid, a weak acid that is the primary agent of limestone dissolution. The more carbon dioxide the water carries, the more aggressively it can attack carbonate rock.

Acidic Water Dissolution of Limestone

Carbonic acid dissolves limestone by converting insoluble calcium carbonate into soluble calcium bicarbonate, which is carried away in solution. The acidified water seeps into bedding planes and fractures in the limestone, dissolving the rock walls and gradually enlarging the openings. The limestone dissolution process is slow — a passage may take tens of thousands of years to develop — but it is relentless wherever water keeps moving through soluble rock. In some caves a far more aggressive route dominates: sulfuric acid, generated when hydrogen sulfide rising from depth oxidizes, dissolves limestone from below. The vast passages of Lechuguilla Cave and Carlsbad Cavern in the United States were excavated largely by sulfuric acid rather than ordinary carbonic acid.

Carbon Dioxide Equilibrium in Cave Systems

The balance of carbon dioxide controls whether a cave is being dissolved or decorated. When acidic groundwater carrying dissolved calcium bicarbonate reaches an open cave passage, the air there usually holds less carbon dioxide than the water does, so CO₂ escapes from the water — degassing — much like bubbles leaving an opened bottle. As the water loses carbon dioxide, it can no longer hold all its dissolved mineral, and calcium carbonate precipitates out as calcite. This carbon dioxide equilibrium is the hinge of the whole system: a shift toward more CO₂ dissolves rock, a shift toward less builds speleothems.

Historical Theories of Cave Origin

Modern understanding of cave formation grew out of a sequence of theoretical models proposed across the twentieth century, each refining how groundwater zones and uplift control where caves develop.

W. M. Davis and the Double-Cycle Cave Model (1930)

The theory of the origin of caves in limestone karst developing in rocks with horizontal bedding of layers was developed by W. M. Davis (1930). For the evolution of so-called double-cycle caves, formed by the double uplift of a limestone massif, Davis distinguished five main stages:

• rudimentary channels, which form in a zone of complete saturation of slow-moving phreatic pressurized waters;
• mature galleries, when mechanical erosion (corrasion) begins to dominate under the conditions of the propagation of unpressurized vadose flows;
• dry galleries resulting from the escape of water deep into the massif due to local uplift of the area;
• natelic-accumulative, characterized by filling of galleries with natelic-drip and other cave sediments;
• destruction of underground galleries.

Phreatic and Vadose Stages of Cave Development (Bretz, 1942)

Building on Davis's views, the idea of phreatic and vadose stages of cave development was formalized by Bretz in 1942. In the phreatic stage, cave galleries are developed by pressurized groundwater below the water table, where water fills the passage completely. In the vadose stage, groundwater moves freely and not under pressure through the galleries toward drainage systems above the water table. This phreatic–vadose framework remains the standard way of reading cave passages today, since the rounded tube shape of a phreatic passage and the canyon shape of a vadose one record which condition prevailed.

Soviet Contributions: Maksimovich and Maruashvili

The issues of the evolution of underground cavities were most fully developed by Soviet researchers G. A. Maksimovich (1963, 1969) and L. I. Maruashvili (1969), who identified several stages in the formation of horizontal karst caves. Their staged model, set out in the section below, traces a cave from a hairline fissure through an active underground river to eventual collapse and the formation of a karst valley.

Aquifer Structure and Water Table Zones

Cave development is governed by where it sits relative to the water table within an aquifer, the body of water-bearing rock beneath the surface. An aquifer is divided into an upper zone of aeration, where pore spaces hold both air and water, and a lower zone of saturation, where every void is filled with water. The boundary between them is the water table, and caves behave very differently above and below it.

Phreatic Zone and Pressurized Groundwater

The phreatic zone is the zone of saturation, below the water table, where groundwater fills all available space and moves under hydrostatic pressure. Phreatic cave passages dissolve in all directions at once because the water is in contact with the entire passage wall, which is why they form smooth, rounded tubes and looping profiles. The earliest, rudimentary cave channels develop here in slow-moving pressurized water before any uplift or drainage exposes them.

Vadose Zone and Free-Flowing Water

The vadose zone is the zone of aeration, above the water table, where water drains downward under gravity through unsaturated rock. Once a passage is lifted into the vadose zone, free-flowing underground streams cut downward into the floor, carving canyon-shaped passages and leaving the rounded phreatic tube as a ceiling above. Spring resurgence points, swallets, and slockers — the places where surface streams sink underground and where cave water re-emerges — mark the inputs and outputs of this free-draining vadose plumbing.

Capillary Fringe in Aquifers

The capillary fringe is the thin transitional band just above the water table where water is drawn upward into otherwise air-filled pores by capillary action. Although saturated, this fringe is held under negative pressure rather than the positive pressure of the phreatic zone below. The capillary fringe matters to cave formation because dissolution and mineral deposition can both occur within it as the water table rises and falls, blurring the sharp line between the zone of aeration and the zone of saturation.

Stages of Cave Formation

Maksimovich and Maruashvili described horizontal karst cave development as a sequence of stages, each with its own hydrodynamics, morphology, and physical-chemical processes. The duration of individual stages of the cave-forming cycle is measured in tens and hundreds of millennia; the dry-gallery stage of the Kudaro cave in the Caucasus, for instance, has lasted some 200,000 to 300,000 years.

Fissure and Crevice Stage

The first stage is the fissure stage, then the crevice stage. As the width of cracks and crevices increases, more and more water penetrates into them, activating karst processes especially in areas of pure rock differences. This is where bedding planes and fractures first guide the developing void.

Channel Stage

The cave then passes into the channel stage. When the channels expand, underground streams acquire turbulent movement, which favours even more intensification of corrosion and erosion processes and accelerates the enlargement of the passage.

Underground River (Vokluzovaya) Stage

This is the underground river stage, or vokluzovaya stage. It is characterized by significant filling of the underground channel with water flow and its exit in the form of a vokluzhny (resurgence) source at the day surface, as well as the formation of organ pipes, collapse of vaults, and the growth of grottoes. Through erosion of the bottom of the underground channel, water seeps through cracks into the depth of carbonate and halogen strata, where it develops new cavities at a lower level, forming a lower floor of the cave.

Corridor-Grotto Nathechno-Sedimentary Stage

Gradually the underground channels widen, the water flow partially and then completely goes into the lower horizons of the massif, and the cave becomes dry, with only infiltration water penetrating through cracks in the roof. This is the corridor-grotto nathechno-sedimentary stage (the water-gallery stage, according to L. I. Maruashvili). It is characterized by widespread chemical and mechanical accumulation; in gypsum caves the stage of natetic accumulation is absent. The ceiling and walls become covered with various calcite deposits, stone and earth shards form (the latter located mainly under the organ pipes), and river and lake sediments accumulate. With the departure of the watercourse, further enlargement of the cavity slows sharply, although corrosive activity continues at the expense of infiltration and condensation waters.

Corridor-Grotto Collapse-Cementation Stage

As the cave develops further it passes into the corridor-grotto collapse-cementation stage (the dry-gallery stage, according to L. I. Maruashvili). At this stage, roof collapse over underground cavities can open some parts of the cave to the surface. If the thickness of the roof exceeds 100–200 m, as a rule no surface openings form, and the underground cavities fill with blocks of rock collapsed from the ceiling and with sandy-clay sediments, which break the cave into separate isolated cavities. In this case the development of the cave ends at the collapse-cementation stage.

Final Destruction and Karst Valley Formation

Gradual collapse of the cave vault leads to its complete destruction, which is especially characteristic of the upper parts where the roof is thin. Only karst bridges and narrow arches remain in the surviving sections, and at complete destruction of the cave a karst valley is formed. You can read about the formation of karst caves in a separate article.

Tiered Caves — Caves With Floors

Complex cave systems usually consist of areas at different stages of development. In the Ischeevskaya cave in the Southern Urals, for example, there are sections ranging from the channel stage all the way to the karst valley.

The distance between two adjacent levels of multistory caves varies from several metres to several dozen. The collapse of the vaults separating cave floors leads to the formation of giant grottoes, sometimes reaching a height of 50–60 m, as seen in the Red and Novoafon caves.

G. A. Maksimovich connects the appearance of a new floor with tectonic uplift of the area where the cave is located. N. A. Gvozdetsky attributes the main role in the development of multistory caves, in conditions of large thickness of karst rocks, to upward movements, which he considers not a disturbing factor but the general background of karst evolution. According to L. I. Maruashvili, multistory development can be driven not only by tectonic uplift of the karst massif but also by a general lowering of ocean level, which causes intensive deepening of river valleys and a rapid drop in the level of horizontal circulation of karst waters. During cave formation, the axis of cave galleries is sometimes displaced from its original vertical plane because of the inclination of the tectonic fractures to which the cavities are confined.

Cave Formations and Speleothems

Speleothems are the mineral deposits that decorate the inside of a cave, built up over millennia as water deposits calcium carbonate. Most speleothems are made of calcite precipitated from dripping or flowing water, and their many named forms — stalactites, stalagmites, columns, flowstone, draperies, helictites, and cave popcorn — depend on how the water arrives at the surface where it deposits its load. Speleothems should not be confused with speleogens, which are features carved into the bedrock itself rather than added to it.

Stalactite Formation

A stalactite is a speleothem that hangs from a cave ceiling, growing downward as mineral-laden water drips from its tip. Each drop hanging at the end deposits a tiny ring of calcite before it falls, and successive rings extend the formation downward. The thinnest, most delicate stalactites begin as hollow soda straws, single tubes the width of a water drop; when the central channel clogs, water runs down the outside and thickens the straw into a classic carrot-shaped stalactite.

Stalagmite Formation

A stalagmite is a speleothem that builds upward from a cave floor where dripping water lands and deposits its calcite. Because each drop splashes on impact, stalagmites grow with rounded or broad tops rather than pointed tips, and their shape varies with the drip rate and the height of the fall — fast, high drips spread the mineral into squat, wide mounds, while slow, steady drips build slender pillars. A simple way to keep the two straight: stalactites cling "tight" to the ceiling, while stalagmites "might" reach the ceiling one day.

Column Formation When Stalactites Meet Stalagmites

A column forms when a downward-growing stalactite and an upward-growing stalagmite meet and fuse into a single continuous pillar of calcite spanning floor to ceiling. Once joined, the column can keep thickening as water films flow down its surface, and large columns are among the oldest visible features in a cave, since both halves had to grow for tens of thousands of years before meeting.

Cave Popcorn and Other Decorations

Beyond the dripstone forms, caves hold a wide range of decorations named for what they resemble. Cave popcorn is a cluster of small knobby calcite nodules that grow on walls and other speleothems where water seeps or evaporates from the rock surface. Other common and erratic speleothems include:

• Flowstone — sheet-like deposits laid down by water flowing over walls and floors; the famous Frozen Niagara formation in Mammoth Cave is a massive flowstone cascade.
• Curtains and draperies — thin, wavy sheets that hang like fabric where water trickles down a sloping ceiling.
• Rimstone dams — calcite ridges that build up at the edges of cave pools and terrace the water.
• Helictites — twisting, gravity-defying growths that curve in seemingly random directions because capillary forces, not dripping, move the water.
• Pool fingers — finger-like deposits that form underwater in standing cave pools.
• Gypsum formations — crusts, flowers, and snowballs that crystallize where sulfate-rich water evaporates; rarer evaporite minerals such as mirabilite and epsomite form fragile crystal growths in dry passages.

Carbonic Acid and Calcite Crystallization

Calcite crystallization in caves is the chemical reverse of the carbonic acid dissolution that opened the cave in the first place. Water that dissolved limestone underground arrives at a cave wall or ceiling still carrying dissolved calcium bicarbonate and excess carbon dioxide; when that water meets cave air, carbon dioxide degasses, the water becomes supersaturated, and calcite crystallizes out grain by grain to build the speleothem. Because each layer of calcite traps a chemical signature of the water that deposited it, speleothems serve as archives of paleoclimate data, and scientific dating and analysis of their growth bands — using uranium-series and isotope methods — let researchers reconstruct rainfall and temperature stretching back hundreds of thousands of years.

Other Types of Cave Formation

Not every cave is dissolved out of limestone. Several distinct mechanisms — wave erosion, lava drainage, ice melt, and rock fracturing — produce caves with their own forms and settings.

Coastal Cave Formation by Wave Erosion

Sea caves, also called littoral caves, form where waves pound a coastal cliff and exploit a zone of weakness in the rock. The mechanical erosion of repeated wave impact, the hydraulic force of compressed air and water driven into cracks, and the abrasion of sand and pebbles together hollow out a cavity along a fault or softer band. Unlike solutional caves, sea caves can form in almost any rock type because the work is done by physical erosion rather than chemical dissolution.

Anchialine Caves

Anchialine caves are coastal caves containing a mix of fresh and salt water that is connected to the sea underground but has no direct surface opening to the ocean. The water typically sits in layers, with lighter freshwater floating over denser seawater, and these caves host highly specialized animal life found nowhere else. Long flooded cave systems such as Sistema Ox Bel Ha on the Yucatán Peninsula are extensively explored by cave divers and rank among the longest underwater caves on Earth.

Cave Physical Patterns and Structures

Beyond the speleothems added to a cave, the bedrock surfaces preserve speleogens — erosional features cut into the rock that record how water moved. Scallops are spoon-shaped hollows scoured into passage walls; their size and asymmetry reveal the speed and direction of past water flow. Rock fins and blade-like projections, organ pipes, and ceiling channels are other speleogens that map the cave's flow history. Volcanic caves preserve their own speleogens — lava stalactites, flow ledges, and ropy floors frozen into the rock. A lava tube forms when the surface of a lava flow cools and crusts over while molten rock keeps flowing beneath; when the eruption stops and the lava drains out, a hollow tube remains. Kazumura Cave in Hawaii is the longest and deepest lava tube in the world. Glacier caves, by contrast, are melted within glacier ice by meltwater and geothermal heat — fragile, shifting passages found in features such as the Greenland ice sheet. Talus caves are simply the voids left between large fallen boulders, and fracture caves open along joints and faults where soluble layers between insoluble beds are dissolved or where rock slabs separate.

Geographic Distribution of Caves

Caves occur on every continent wherever the right rock and conditions meet, but the great solutional cave regions cluster in karst landscapes of soluble limestone. In North America, the United States holds Mammoth Cave in Kentucky — the longest known cave system in the world — along with Carlsbad Cavern and Lechuguilla Cave in the sulfuric-acid karst of New Mexico, and many show caves such as Cosmic Cavern, Talking Rocks Cavern, Lake Shasta Caverns, and Lechuguilla's neighbours. In Europe, the Carboniferous Limestone of the Mendip Hills in England hosts Wookey Hole and Gough's Cave at Cheddar, where the River Yeo and the local geology of the Avon Group, the Portishead Formation, and the surrounding Old Red Sandstone shape the cave morphology mapped by the British Geological Survey and described in works such as Foundations of the Mendips. Southeast Asia holds the planet's largest single passages, including Son Doong Cave and the Sarawak Chamber. This geographic spread reflects the simple rule that big caves need thick, soluble, fractured rock and abundant moving groundwater.

Cave Records and Superlatives

Cave records highlight the extremes that speleogenesis can reach over geological time. The notable superlatives include:

• Longest cave system — Mammoth Cave in Kentucky, with hundreds of miles of surveyed passage threaded through named routes such as Broadway, Cleveland Avenue, Gothic Avenue, Grand Avenue, Kentucky Avenue, and Violet City, and landmarks like Giant's Coffin and Frozen Niagara.
• Deepest cave — Veryovkina Cave in the Caucasus, descending more than two kilometres below its entrance.
• Largest cave passage and chamber — Son Doong Cave in Vietnam holds the largest passage, while the Miao Room and the Sarawak Chamber rank among the largest single chambers by volume.
• Longest lava tube — Kazumura Cave in Hawaii.
• Longest underwater cave — Sistema Ox Bel Ha in Mexico, among the longest explored flooded systems.

Cave names within a single system, such as the Snow Room, the Snowball Room, and the Snow Room's gypsum-coated chambers in Mammoth Cave, often describe the speleothems found there. Photographer Carsten Peter, working with National Geographic, has documented many of these record-setting caves and brought their scale to a wide audience.

Cave Exploration and Caving

Caving — also called spelunking or, in its scientific form, speleology — is the recreational and scientific exploration of caves. It ranges from guided walks through lit show caves to demanding expeditions into remote, unmapped systems, and it has a long human history, since caves have held cultural and archaeological significance as shelters, ritual sites, and sources of minerals for thousands of years. Prehistoric American Indians, for example, mined gypsum and other cave minerals deep inside Mammoth Cave, and the torch smoke and soot they left behind still stains some cave formations today.

Caving Activities and Techniques

Caving activities span a spectrum of difficulty and skill. Many caves are run as managed show caves with guided tours over built paths and lighting, while wild caving requires ropes, helmets, lamps, and specialized vertical and crawling techniques. Educational cave programs and student resources are widely offered at parks and managed caverns:

• Guided show-cave tours such as the Domes and Dripstones Tour, the Great Onyx Lantern Tour, and lantern-lit historic routes that take visitors past named formations.
• Self-guided walks along developed passages in caves operated by the National Park Service and members of the National Caverns Association.
• Wild or "adventure" caving trips that involve crawling, climbing, and squeezing through undeveloped passage.
• Cave diving, the most technical discipline, used to explore flooded phreatic systems and anchialine caves.

Cave Exploration Risks and Safety

Cave exploration carries real risks that demand preparation and respect for the environment. The main hazards include flooding from sudden rainfall, falls and rockfall, hypothermia in cold wet passages, getting lost or stuck, and exhaustion of light or air. Cave divers face the added dangers of restricted exits and silt-out. Beyond personal safety, cave formations are extremely fragile: a speleothem that took 50,000 years to grow can be snapped in a moment, and skin oils and lint permanently mar surfaces, which is why the caving principle is to take nothing but photographs and leave nothing but footprints. Access restrictions, gating, and permit systems protect both visitors and the cave, and managing bodies enforce them to preserve the formations and their wildlife.

For more on the human side of caves and the people who explore them, see this related article: People and caves. You can also browse more articles in our Speleology section.