A
speleothem (from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
for "cave deposit"), commonly known as a
cave formation, is a secondary mineral deposit formed in a
caveA cave or cavern is a natural underground void large enough for a human to enter. Some people suggest that the term cave should only apply to cavities that have some part that does not receive daylight; however, in popular usage, the term includes smaller spaces like sea caves, rock shelters, and...
. Speleothems are typically formed in
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
or
dolostoneDolostone 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
calciteCalcite 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 470°C, and vaterite is even less stable....
and
aragoniteAragonite is a carbonate mineral, one of the two common, naturally occurring polymorphs of calcium carbonate, CaCO3. The other polymorph is the mineral calcite. Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with...
(both
calcium carbonateCalcium carbonate is a chemical compound with the chemical formula CaCO
3. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural...
), or
gypsumGypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO
4·2H
2O.-Crystal varieties:...
(
calcium sulfateCalcium 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
precipitatePrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
. 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
A stalactite is a type of speleothem that hangs from the ceiling or wall of limestone caves. It is a type of dripstone.- Formation and type:...
s are pointed pendants hanging from the cave ceiling, from which they grow;
- 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
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
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. The corresponding formation on the ceiling of a cave is known as a stalactite...
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
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 sheetlike 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, 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 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 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 is a white, cheese-like 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
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 aragoniteAragonite is a carbonate mineral, one of the two common, naturally occurring polymorphs of calcium carbonate, CaCO3. The other polymorph is the mineral calcite. Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system with...
crystals
- Speleogens (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 is an uncommon type of speleogen occasionally found in caves.Boxwork is commonly composed of thin blades of the mineral calcite that project from cave walls or ceilings that intersect one another at various angles, forming a box-like or honeycomb pattern...
- Others:
- 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
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 15 centimeters in diameter. Most are smaller than one centimeter 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
Snottites are colonies of single-celled extremophilic bacteria. They hang from the walls and ceilings of caves and are similar to small stalactites, but have the consistency of "snot", or mucus....
s are colonies of predominantly sulfur oxidizing bacteria and have the consistency of "snot", or mucous;
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
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
,
copperCopper 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 rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
or
manganeseManganese 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
calciteCalcite 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 470°C, and vaterite is even less stable....
; CaCO
3, the primary mineral in
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
. It is a slightly soluble mineral whose
solubilitySolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution. The solubility of a substance strongly depends on the used solvent as well as on temperature and pressure...
increases with the introduction of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, CO
2. 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 coreAn ice core is a core sample from the accumulation of snow and ice over many years that have re-crystallized and have trapped air bubbles from previous time periods...
s as a proxy 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
QuaternaryThe Quaternary period is the youngest of three periods of the Cenozoic era in the geologic time scale of the ICS. It follows after the Neogene period, spanning 2.588 +/- 0.005 million years ago to the present...
period using the
uranium-thorium datingUranium-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 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 resonanceElectron 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 CaCO
3 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 Mn
2+, Fe
2+, or Fe
3+, 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.
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