A speleothem (; ) is a geological formation made by mineral deposits that accumulate over time in natural .
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 (CaCO
3) minerals (
calcite or
aragonite). Less commonly, speleothems are made of
calcium sulfate (
gypsum or
mirabilite) or
opal.
Speleothems of pure calcium carbonate or calcium sulfate are translucent and colorless. The presence of
iron oxide or
copper 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.
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 (CaCO
3) containing rocks such as
limestone or dolomite, composed of
calcite or
aragonite minerals. Carbonate minerals are more
solubility in the presence of higher
carbon dioxide (CO
2) and lower temperatures. Calcareous speleothems form via carbonate dissolution reactions whereby rainwater reacts with soil CO
2 to create weakly acidic water via the reaction:
- H2O + CO2 → carbonic acid
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 (, ), and , two of the major types of speleothems.
Climate proxies
Speleothem transects can provide paleoclimate records similar to those from
or
Dendrochronology.
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 (δ
18O) and carbon (δ
13C) stable isotopes are used to track variation in rainfall temperature, precipitation, and vegetation changes over the past ~500,000 years.
The Mg/Ca proxy has likewise been used as a moisture indicator, although its reliability as a palaeohygrometer can be affected by cave ventilation during dry seasons.
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 (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
[[File:Tropfsteine.svg|thumb|upright=1.4|Types of speleothem:
(A) Stalactite
(B) Soda straws
(C) Stalagmites
(D) Coned stalagmite
(E) Stalagnate or column
(F) Drapery
(G) Drapery
(H) Helictites
(I) Moonmilk
(J) Sinter pool, rimstone
(K) Calcite crystals
(L) Sinter terrace
(M) Karst
(N) Body of water
(O) Shield
(P) Cave clouds
(Q) Cave pearls
(R) Tower cones
(S) Shelfstones
(T) Baldacchino canopy
(U) Bottlebrush stalactite
(V) Conulite
(W) Flowstone
(X) Trays
(Y) Calcite rafts
(Z) Cave popcorn or coralloids
(AA) Frostworks
(AB) Flowstone
(AC) Splattermite
(AD) Speleoseismites
(AE) Boxworks
(AF) Oriented stalactite
(AG) collapsed rubble]]
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). Huntsville, pp. 217, 225]
-
Dripstone is calcium carbonate in the form of stalactites or stalagmites
-
are pointed pendants hanging from the cave ceiling, from which they grow
-
are very thin but long stalactites with an elongated cylindrical shape rather than the usual more conical shape of stalactites
-
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
-
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 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, or gours, occur at stream ripples and form barriers that may contain water
-
Cave crystals
-
Dogtooth spar are large calcite crystals often found near seasonal pools
-
Frostwork is needle-like growths of calcite or aragonite
-
Moonmilk is white and cheese-like
-
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.
-
(technically distinct from speleothems) are formations within caves that are created by the removal of bedrock, rather than as secondary deposits. These include:]]
-
Others
-
Cave popcorn, also known as "coralloids" or "cave coral", are small, knobby clusters of calcite
-
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
-
are colonies of predominantly sulfur Redox bacteria and have the consistency of "snot", or mucus
-
Calcite rafts 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 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, lime, 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
. Calthemites are often associated with concrete degradation, or due to leaching of lime, mortar, or other calcareous material.
==Gallery==
See also
External links
