A speleothem commonly known as a cave formation, is a secondary mineral deposit formed in a cave

Cave

A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...

. Speleothems are typically formed in limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 or dolostone

Dolostone

Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is...

 solutional caves.

Origin and composition

Water seeping through cracks in a cave's surrounding bedrock may dissolve certain compounds, usually calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 and aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

 (both calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

), or gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

 (calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

). The rate depends on the amount of carbon dioxide held in solution, on temperature, and on other factors. When the solution reaches an air-filled cave, a discharge of carbon dioxide may alter the water's ability to hold these minerals in solution, causing its solutes to precipitate

Precipitation (chemistry)

Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...

. Over time, which may span tens of thousands of years, the accumulation of these precipitates may form speleothems.

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:

• Dripstone is calcium carbonate in the form of stalactites or stalagmites:
  • Stalactite
    Stalactite
    A stalactite , "to drip", and meaning "that which drips") is a type of speleothem that hangs from the ceiling of limestone caves. It is a type of dripstone...
    
    s are pointed pendants hanging from the cave ceiling, from which they grow;
    • Soda straw
      Soda straw
      A soda straw is a speleothem in the form of a hollow mineral tube. They grow in places where water leaches slowly through cracks in rock, such as on the roofs of caves...
      
      s are very thin but long stalactites having an elongated cylindrical shape rather than the usual more conical shape of stalactites;
    • Helictite
      Helictite
      A helictite is a speleothem found in limestone caves that changes its axis from the vertical at one or more stages during its growth. They have a curving or angular form that looks as if they were grown in zero gravity...
      
      s 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;
  • Stalagmite
    Stalagmite
    A stalagmite is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate. This stalagmite formation occurs only under certain pH conditions within the underground cavern. The corresponding formation on...
    
    s 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
  • Columns result when stalactites and stalagmites meet or when stalactites reach the floor of the cave;

• Flowstone
  Flowstone
  Flowstones are composed of sheetlike deposits of calcite formed where water flows down the walls or along the floors of a cave. They are typically found in "solution", or limestone caves, where they are the most common speleothem. However, they may form in any type of cave where water enters that...
  
  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
    Rimstone
    Rimstone, also called gours, is a type of speleothem in the form of a stone dam. Rimstone is made up of calcite and other minerals that build up in cave pools. The formation created, which looks like stairs, often extends into flowstone above or below the original rimstone. Often, rimstone is...
    
    , or gours, occur at stream ripples and form barriers that may contain water;
  • Stone waterfall formations simulate frozen cascades

• Cave crystals:
  • Dogtooth spar
    Dogtooth spar
    Dogtooth spar is a speleothem found in limestone caves that consists of very large calcite crystals resembling dogs' teeth . They are usually found near standing water, where crystals can grow for a very long time...
    
     are large calcite crystals often found near seasonal pools;
  • Frostwork
    Frostwork
    Frostwork is a type of speleothem composed of acicular growths almost always composed of aragonite or calcite replaced by aragonite . It is a variety of anthodite. In some caves frostwork may grow on top of cave popcorn or boxwork.-Formation:The origin of frostwork is somewhat controversial...
    
     is needle-like growths of calcite or aragonite;
  • Moonmilk
    Moonmilk
    Moonmilk is a white, creamy substance found inside caves. It is similar to other deposits, but its unique quality is that it does not harden or turn to stone...
    
     is white and cheese-like;
  • Anthodite
    Anthodite
    Anthodites are speleothems composed of long needle-like crystals situated in clusters which radiate outward from a common base. The "needles" may be quill-like or feathery...
    
    s are flower-like clusters of aragonite
    Aragonite
    Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
    
     crystals

• Speleogen
  Speleogen
  A Speleogen is a geological formation within a cave that has been created by the removal of bedrock, rather than as a secondary deposit. They are typically formed in limestone or dolostone solutional caves...
  
  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
    Boxwork
    Boxwork is an uncommon type of mineral structure, or speleogen , occasionally found in caves and erosive environments....
    
    

• Others:
  • Cave popcorn
    Cave popcorn
    Cave popcorn, or cave coral, is a very common cave formation. It actually comes in many sizes, shapes, and colors, but is most often found in small, knobby clusters resembling popcorn...
    
    , or cave coral, are small, knobby clusters of calcite;
  • Cave pearl
    Cave pearl
    A cave pearl is a type of speleothem, a concretion of calcium salts which forms in limestone caves. They may be any shape but are formed of concentric layers, reaching up to in diameter. Most are smaller than wide. They can be found singly or grouped...
    
    s 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
    Snottites are colonies of single-celled extremophilic bacteria which hang from the walls and ceilings of caves and are similar to small stalactites, but have the consistency of "snot", a slang word for nasal mucus....
    
    s are colonies of predominantly sulfur oxidizing bacteria and have the consistency of "snot", or mucus;

Speleothems may also occur in lava tubes. Although sometimes similar in appearance to speleothems in caves formed by dissolution, these are formed by the cooling of residual lava within the lava tube.

Speleothems formed from salt, sulfur and other minerals are also known.

Speleothems made of pure calcium carbonate are a translucent white color, but often speleothems are colored by minerals such as iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 or manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

, or may be brown because of mud and silt particulate inclusions.

Chemistry

Many factors impact the shape and color of speleothem formations including the rate and direction of water seepage, the amount of acid in the water, the temperature and humidity content of a cave, air currents, the above ground climate, the amount of annual rainfall and the density of the plant cover. Most cave chemistry revolves around calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

; CaCO₃, the primary mineral in limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

. It is a slightly soluble mineral whose solubility

Solubility

Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

 increases with the introduction of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, CO₂. It is paradoxical in that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with the carbon dioxide, whose solubility is diminished by elevated temperatures; as the carbon dioxide is released, the calcium carbonate is precipitated.

Most other solution caves that are not composed of limestone or dolostone are composed of gypsum (calcium sulfate), the solubility of which is positively correlated with temperature.

As climate proxies

Samples can be taken from speleothems to be used like ice core

Ice core

An ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...

s as a proxy

Proxy (climate)

In the study of past climates is known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements , to enable scientists to reconstruct the climatic conditions that prevailed during much of the Earth's history...

 record of past climate changes.
A particular strength of speleothems in this regard is their unique ability to be accurately dated over much of the late Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

 period using the uranium-thorium dating

Uranium-thorium dating

Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral...

 technique. Stalagmites are particularly useful for palaeoclimate applications because of their relatively simple geometry and because they contain several different climate records, such as oxygen and carbon isotopes and trace cations. These can provide clues to past precipitation, temperature, and vegetation changes over the last ~ 500,000 years.

Absolute dating

Another dating method using electron spin resonance (ESR) — also known as electron paramagnetic resonance

Electron paramagnetic resonance

Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...

 (EPR) — is based on the measurement of electron-hole centers accumulated with time in the crystal lattice of CaCO₃ exposed to natural radiations. In principle, in the more favorable cases, and assuming some simplifying hypotheses, the age of a speleothem could be derived from the total radiation dose cumulated by the sample and the annual dose rate to which it was exposed. Unfortunately, not all the samples are suited for ESR dating: indeed, the presence of cationic impurities such as Mn²⁺, Fe²⁺, or Fe³⁺, humic acids (organic matter), can mask the signal of interest, or interfere with it. Moreover, the radiation centers must be stable on geologic time, i.e., to have a very large lifetime, to make dating possible. Many other artifacts, such as, e.g., surface defects induced by the grinding of the sample can also preclude a correct dating. Only a few percents of the samples tested are in fact suitable for dating. This makes the technique often disappointing for the experimentalists. One of the main challenge of the technique is the correct identification of the radiation-induced centers and their great variety related to the nature and the variable concentration of the impurities present in the crystal lattice of the sample. ESR dating can be tricky and must be applied with discernment. It can never be used alone: "One date only is No date", or in other words, "multiple lines of evidence and multiple lines of reasoning are necessary in absolute dating". However, "good samples" might be found if all the selection criteria are met.

External links

• The Virtual Cave: an online guide to speleothems
• Mineral aggregates from carst caves, formed in capillary film solutions
• Gallery of speleothems from NPS
  National Park Service
  The National Park Service is the U.S. federal agency that manages all national parks, many national monuments, and other conservation and historical properties with various title designations...
  
  Cave and Karst Program