Speleothem
Speleothems is the formal name for what is commonly known as «cavity formations». The word, coming from the Greek (σπήλαιον, spḗlaion, 'cavity' + θέμα, théma, 'deposit') and generally refers to secondary mineral deposits formed in caves after their genesis. There are not only secondary speleothems, but also primary ones, depending on the type of cave they are found in. The term speleothema was coined in 1952 by the American speleologist G.W. Moore.
Speleothems in karst caves (secondary speleothems)
General information
Water seeping through cracks in the ground near a cave can dissolve certain compounds, typically calcite, aragonite (calcium carbonate), and gypsum (calcium sulfate). The amount of dissolved mineral depends, among other factors, on the concentration of carbon dioxide and the temperature of the solution. When this solution reaches an air-filled cavern, the discharge of carbon dioxide alters the water's ability to hold these minerals in solution, causing them to precipitate. Over time, which can be tens of thousands of years, the accumulation of these precipitates can form secondary speleothems.
Secondary speleothems are not only formed in karstic caves, although it is there where they are best appreciated. They can also form in any other cavity where the dripping of mineral-laden water from the ground has the opportunity to precipitate. In this way, secondary speleothems can be found in volcanic tubes found in humid terrain and even in artificial cavities, such as mines, as long as the necessary time for their formation elapses.
Common Secondary Forms
- Stalactites;
- Stalagmites;
- Acreation columns, by union of stalactite and stalagmite;
- Exclusive or helictite;
- Coladas;
- Banderolas;
- Flatlets or pearls of caverns;
- Gours;
- Antiestalagmites or conulites;
- Mondmilch, Moonmilk Or moon milk.
Secondary speleothems formed by pure calcite are transparent white, but are generally colored by minerals such as iron, copper, or manganese, or may be brown due to the inclusion of mud or sediment particles.
Chemistry
Many factors influence the shape and color of secondary speleothems, including the amount and direction of water seepage, the amount of acid in the solution, the ambient temperature and humidity of the cave, air currents, weather of the surface, the amount of annual rainfall and the density of the external vegetation cover.
Most of the karst cave chemistry revolves around calcite, CaCO3, the primary mineral in limestone. It is a sparingly soluble mineral, whose solubility increases with the introduction of carbon dioxide, CO2. It is paradoxical that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with carbon dioxide, whose solubility decreases with elevated temperatures. When carbon dioxide is released, calcium carbonate precipitates.
Many other cavern solutions are not made of limestone or dolomite, but of gypsum (hydrated calcium sulphate), whose solubility increases proportionally with temperature.
Witnesses to the weather
Secondary speleothems can be sampled, as can an ice core as a record of past climatic changes. A characteristic of these speleothems is their unique ability to be dated with great precision well beyond the Quaternary period using the uranium series dating technique.
Stalagmites are particularly useful for creating paleoclimate records because of their relatively simple form as traces of oxygen, carbon, and cation isotopes. These can provide clues about precipitation, temperature, and vegetation changes over the past ~500,000 years.
Absolute dating
Another dating method is Electron Paramagnetic Resonance (EPR). It is a spectroscopic technique sensitive to unpaired electrons. It allows calculating the age of the speleothem from the total radiation dose accumulated in the sample and the annual dose to which it is exposed. Unfortunately, not all samples are valid for RPE dating: The presence of cationic impurities such as Mn2+, Fe2+, or Fe3+ , organic matter, can alter the sample. The radiation levels must be stable in geological time, that is, they must have a very long lifetime to make dating possible. In addition, surface defects induced by the grinding of the sample can cause incorrect dating. In fact, only a few percentages of the samples tested are suitable for dating. One of the main challenges of the technique is the correct identification of radiation-induced free radicals and their great variety related to the nature and variable concentration of the impurities present in the crystal lattice of the sample. The application of EPR/ESR spectroscopy can be complicated and must be applied with discernment. It is not a technique that by itself supports a dating: Multiple lines of evidence and multiple lines of reasoning are needed in absolute dating.
Speleothems in volcanic caves (primary speleothems)
Speleothems also form in volcanic caves like lava tubes. Although they are sometimes similar in appearance to those present in karst caves, the primary speleothems present in volcanic tubes are formed by the cooling of residual lava inside the cave. Depending on the age of the volcanic tube and the terrain in which it is located, other secondary speleothems can form inside, such as small stalactites and various concretions, thanks to contributions of water with dissolved minerals that end up precipitating.
The most common primary speleothems in lava tubes are:- Staphites or lava stalactites, of various types (conics, stridos, laminated...);
- Castles or lava stalagmites;
- Way forward.
The most common secondary speleothems in lava tubes of a certain age are:
- Concretions, usually calcareous, which are usually grouped into clusters, or plaster;
- Micro-stalactites. Beginning of formation of a stalactite, usually on a pre-existing stafilite;
- Microgours. Beginning of formation of gours on smooth lava areas.