A speleothem (; ) is a geological formation by

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, current accounts or any of several other types of accounts explained below. ...

that accumulate over time in natural

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

s. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on their depositional history and environment. Their chemical composition, gradual growth, and preservation in caves make them useful paleoclimatic proxies.

Chemical and physical characteristics

More than 300 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate (CaCO₃) minerals ( calcite or

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

). Less commonly, speleothems are made of

calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...

( gypsum or

mirabilite Mirabilite, also known as Glauber's salt, is a hydrous sodium sulfate mineral with the chemical formula Na2SO4·10H2O. It is a vitreous, colorless to white monoclinic mineral that forms as an evaporite from sodium sulfate-bearing brines. It is fo ...

) or

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...

. Speleothems of pure calcium carbonate or calcium sulfate are translucent and colorless. The presence of iron oxide or

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

provides a reddish brown color. The presence of manganese oxide can create darker colors such as black or dark brown. Speleothems can also be brown due to the presence of mud and

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...

. Many factors impact the shape and color of speleothems, including the chemical composition of the rock and water, water seepage rate, water flow direction, cave temperature, cave humidity, air currents, aboveground climate, and aboveground plant cover. Weaker flows and short travel distances form narrower stalagmites, while heavier flow and a greater fall distance tend to form broader ones.

Formation processes

Most cave chemistry involves calcium carbonate (CaCO₃) containing rocks such as

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

or dolomite, composed of calcite or

minerals. Carbonate minerals are more

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

in the presence of higher

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

(CO₂) and lower temperatures. Calcareous speleothems form via carbonate dissolution reactions whereby rainwater reacts with soil CO₂ to create weakly acidic water via the reaction: :H₂O + CO₂ → H₂CO₃ As the acidic water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction: :CaCO₃ + H₂CO₃ → Ca²⁺ + 2 HCO₃⁻ When the solution reaches a cave, the lower pCO₂ in the cave drives the precipitation of CaCO₃ via the reaction: :Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + H₂O + CO₂ Over time, the accumulation of these precipitates form dripstones ( stalagmites, stalactites), and flowstones, two of the major types of speleothems.

Climate proxies

Speleothem transects can provide paleoclimate records similar to those from ice cores or tree rings. Slow geometrical growth and incorporation of radioactive elements enables speleothems to be accurately and precisely dated over much of the late Quaternary by radiocarbon dating and uranium-thorium dating, as long as the cave is a closed system and the speleothem has not undergone recrystallization. Oxygen ( δ¹⁸O) and carbon ( δ¹³C) stable isotopes are used to track variation in rainfall temperature, precipitation, and vegetation changes over the past ~500,000 years. Variations in precipitation alter the width of speleothem rings: closed rings indicates little rainfall, wider spacing indicates heavier rainfall, and denser rings indicate higher moisture. Drip rate counting and trace element analysis of the water drops record short-term climate variations, such as

El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea te ...

(ENSO) climate events. Exceptionally, climate proxy data from the early Permian period have been retrieved from speleothems dated to 289 million years ago sourced from infilled caves exposed by quarrying at the Richards Spur locality in Oklahoma.

Types and categories

Speleothems take various forms, depending on whether the water drips, seeps, condenses, flows, or ponds. Many speleothems are named for their resemblance to man-made or natural objects. Types of speleothems include:Hill, C A, and Forti, P, (1997). Cave Minerals of the World, (2nd edition). untsville, Alabama: National Speleological Society Inc.pp. 217, 225 * Dripstone is calcium carbonate in the form of stalactites or stalagmites ** Stalactites are pointed pendants hanging from the cave ceiling, from which they grow *** Soda straws are very thin but long stalactites with an elongated cylindrical shape rather than the usual more conical shape of stalactites *** Helictites are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity **** Include forms known as ribbon helictites, saws, rods, butterflies, hands, curly-fries, and "clumps of worms" *** Chandeliers are complex clusters of ceiling decorations *** Ribbon stalactites, or simply "ribbons", are shaped accordingly ** Stalagmites are the "ground-up" counterparts of stalactites, often blunt mounds *** Broomstick stalagmites are very tall and spindly *** Totem pole stalagmites are also tall and shaped like their namesakes *** Fried egg stalagmites are small, typically wider than they are tall **

Stalagnate A stalactite (, ; from the Greek 'stalaktos' ('dripping') via ''stalassein'' ('to drip') is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble an ...

results when stalactites and stalagmites meet or when stalactites reach the floor of the cave * Flowstone is sheet like and found on cave floors and walls ** Draperies or curtains are thin, wavy sheets of calcite hanging downward *** Bacon is a drapery with variously colored bands within the sheet ** Rimstone dams, or gours, occur at stream ripples and form barriers that may contain water ** Stone waterfall formations simulate frozen cascades * Cave crystals ** Dogtooth spar are large calcite crystals often found near seasonal pools **

Frostwork In geology, frostwork is a type of speleothem (cave formation) with acicular ("needle-like") growths almost always composed of aragonite (a polymorph of calcite) or calcite replaced aragonite. It is a variety of anthodite. Frostwork can also be ...

is needle-like growths of calcite or aragonite **

Moonmilk Moonmilk (sometimes called mondmilch, also known as montmilch or as cave milk) is a white, creamy substance found inside limestone, dolomite, and possibly other types of caves. It is a precipitate from limestone comprising aggregates of fine cry ...

is white and cheese-like **

Anthodite Anthodites (Greek ἄνθος ''ánthos'', "flower", ''-ode'', adjectival combining form, ''-ite'' adjectival suffix) are speleothems (cave formations) composed of long needle-like crystals situated in clusters which radiate outward from a common ...

s are flower-like clusters of aragonite crystals ** ''Cryogenic'' calcite crystals are loose grains of calcite found on the floors of caves formed by segregation of solutes during freezing of water. *

Speleogen A speleogen is a geological feature within a karst system that is created by the dissolution of bedrock. As rain water falls through the atmosphere it picks up carbon dioxide and more as it passes through organic material in the soil. As water mov ...

s (technically distinct from speleothems) are formations within caves that are created by the removal of bedrock, rather than as secondary deposits. These include: ** Pillars ** Scallops ** Boneyard ** Boxwork * Others **

Cave popcorn Cave popcorn, or coralloids, are small nodes of calcite, aragonite or gypsum that form on surfaces in caves, especially limestone caves. They are a common type of speleothem. Appearance The individual nodules of cave popcorn range in size from ...

, also known as "coralloids" or "cave coral", are small, knobby clusters of calcite ** Cave pearls are the result of water dripping from high above, causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate **

Snottite Snottite, also snoticle, is a microbial mat of cell (biology), single-celled extremophile, extremophilic bacteria which hang from the walls and ceilings of caves and are similar to small stalactites, but have the consistency of nasal mucus. In th ...

s are colonies of predominantly sulfur

oxidizing Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

bacteria and have the consistency of "snot", or mucus **

Calcite rafts Calcite crystals form on the surface of quiescent bodies of water, even when the bulk water is not supersaturated with respect to calcium carbonate. The crystals grow, attach to one other and appear to be floating rafts of a white, opaque materi ...

are thin accumulations of calcite that appear on the surface of cave pools ** Hells Bells, a particular speleothem found in the El Zapote cenote of Yucatan in the form of submerged, bell-like shapes **

Lava tubes A lava tube, or pyroduct, is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave. Formation A lava tube is a type of lava ca ...

contain speleothems composed of sulfates, mirabilite or opal. When the lava cools, precipitation occurs.

Calthemites

The usual definition of speleothem excludes secondary mineral deposits derived from

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

, mortar, or other calcareous material (e.g. limestone and dolomite) outside the cave environment or in artificial caves (e.g. mines, tunnels), which can have similar shapes and forms as speleothems. Such secondary deposits in man-made structures are termed

calthemite Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4 ...

s. Calthemites are often associated with concrete degradation, or due to leaching of lime, mortar, or other calcareous material.

Gallery

File:HallOfTheMountainKings.jpg, Various formations in the Hall of the Mountain Kings, Ogof Craig a Ffynnon, South Wales,

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...

File:NaturalBridgeCaverns6.jpg, Stalactites and columns in Natural Bridge Caverns,

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

File:NaturalBridgeCaverns7.jpg, More formations in Natural Bridge Caverns,

File:Kl 0005356.jpg, Cave curtain formation in the Marble Arch Caves, County Fermanagh,

Northern Ireland Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...

File:CaliforniaCaveFormation.jpg, California Caverns, Calaveras County, California; one of many caverns located in the Sierra Foothills of California File:Dripstone column in Biserujka Cave, Dobrinj, Island of Krk.jpg, Stalagnates (columns) in the cave Biserujka,

Dobrinj Dobrinj is a village and municipality in the Primorje-Gorski Kotar County in northwestern Croatia, on the island of Krk. There are 2,078 inhabitants in the municipality, with 91% Croats. Geography Dobrinj and the municipality of the same name ar ...

Island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...

Krk Krk (; it, Veglia; ruo, Krk; dlm, label= Vegliot Dalmatian, Vikla; la, Curicta; grc-gre, Κύρικον, Kyrikon) is a Croatian island in the northern Adriatic Sea, located near Rijeka in the Bay of Kvarner and part of Primorje-Gorski Kot ...

Croatia , image_flag = Flag of Croatia.svg , image_coat = Coat of arms of Croatia.svg , anthem = "Lijepa naša domovino"("Our Beautiful Homeland") , image_map = , map_caption = , capit ...

File:Druipsteen3.jpg, Various formations in the cave of Remouchamps, Aywaille,

Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to th ...

File:Zuil.jpg, Stalagnate (column) in the cave of Remouchamps, Aywaille,

File:Cave pearls.jpg, Image of Cave Pearl formation File:Flowstone in Mammoth Cave, KY.jpg, Image of flowstone in Mammoth Cave, KY

References

External links

The Virtual Cave: an online guide to speleothems

* ttps://web.archive.org/web/20130123003018/http://www.nature.nps.gov:80/geology/caves/photo_speleothems.htm Gallery of speleothemsfrom

NPS NPS may refer to: Organizations * National Park Service, U.S. * National Pension System, India * National Pension Service, Korea * National Phobics Society, UK charity * National Piers Society, UK charity * National Poetry Slam, competition * N ...

Cave and Karst Program (archived on 23 January 2013) {{Caves Calcium minerals Dinaric Alps Dinaric karst formations Incremental dating Karst formations Karst Limestone Paleoclimatology Speleothems