Speleothem
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A speleothem (from the Greek for "cave deposit") is a formal term for what is also known as a cave formation, or amongst cavers, collectively known as pretties. They are the result of the interactions among water, rock, and air within caves.
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As water seeps through cracks in rock, it dissolves certain compounds; for caves, these compounds are usually calcite and aragonite (both calcium carbonate), or gypsum (calcium sulfate). At first, this water creates passages which grow larger over time, forming a cave. Eventually, these voids within the rock grow large enough that the seeping water contacts air, causing its solutes to precipitate. This precipitation may be a function of concentration through water removal (calcium sulfate, calcium carbonate) or through the loss of carbon dioxide (calcium carbonate). Over tens of thousands of years, these drops cause speleothems to form. Formations may be produced on the ceiling, creating pendulous structures (stalactites) or structures that "grow" from the floor of the cave upwards (stalagmites). Given enough time, stalactites and stalagmites may grow together into a column.
Various types of formations develop, depending on whether the water drips, seeps, condenses, flows, or settles into pools. Many are named for their resemblance to the man-made or natural objects they resemble. Types of formations include:
- cave pearls, which are the result of water dripping from high above, then causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate;
- dogtooth spar, very large calcite crystals often found near pools that fill seasonally;
- flowstone, which can form a variety of structures, including cave bacon and drapery;
- soda straws, chandeliers, and popcorn, which form from the splattering of drops;
- helictites, which are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity;
- rimstone pools, which are small areas that build up enough deposits around their edges to contain water;
- and many more.
Occasionally (as is the case with cave bacon) they are colored due to the presence of minerals such as iron, copper, or, more rarely, manganese. Most speleothems are brown or mud-colored because of particulate inclusions from mud or silt.
Chemistry
Most cave chemistry revolves around calcite; CaCO3, the primary mineral in limestone. It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide, 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 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 cores as a proxy record of past climate changes [1] (http://www.ngdc.noaa.gov/paleo/speleothem.html). A particular strength of speleothems in this regard is their unique ability to be accurately dated over much of the late Quaternary period using the uranium-thorium dating technique.