Elongated mineral formation hanging down from a cave ceiling
For the elongate mineral formation found on a cave floor, seeStalagmite.
Image showing the six most common speleothems with labels. Enlarge to view labels.
Astalactite (UK:/ˈstæləkˌtaɪt/,US:/stəˈlæktaɪt/; from Ancient Greekσταλακτός (stalaktós)'dripping', from σταλάσσειν (stalássein)'to drip')[1] is a mineral formation that hangs from the ceiling ofcaves,hot springs, or man-made structures such asbridges and mines. Any material that is soluble and that can be deposited as acolloid, or is insuspension, or is capable of beingmelted, may form a stalactite. Stalactites may be composed oflava,minerals,mud,peat,pitch,sand,sinter, and amberat (crystallized urine ofpack rats).[2][3] A stalactite is not necessarily aspeleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.[2][4]
The corresponding formation on the floor of the cave is known as astalagmite.
This solution travels through the rock until it reaches an edge and if this is on the roof of acave it will drip down. When the solution comes into contact withair thechemical reaction that created it is reversed and particles of calcium carbonate are deposited. The reversed reaction is:[6]
Ca(HCO3)2(aq) →CaCO3(s) +H2O(l) +CO2(aq)
An average growth rate is 0.13 mm (0.0051 inches) a year. The quickest growing stalactites are those formed by a constant supply of slow drippingwater rich in calcium carbonate (CaCO3) and carbon dioxide (CO2), which can grow at 3 mm (0.12 inches) per year.[7][8] The drip rate must be slow enough to allow the CO2 to degas from the solution into the cave atmosphere, resulting in deposition of CaCO3 on the stalactite. Too fast a drip rate and the solution, still carrying most of the CaCO3, falls to the cave floor where degassing occurs and CaCO3 is deposited as a stalagmite.
All limestone stalactites begin with a single mineral-laden drop of water. When the drop falls, it deposits the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (≈4 to 5 mm diameter), hollow tube commonly known as a "soda straw" stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
Stalactite formation generally begins over a large area, with multiple paths for the mineral rich water to flow. As minerals are dissolved in one channel slightly more than other competing channels, the dominant channel begins to draw more and more of the available water, which speeds its growth, ultimately resulting in all other channels being choked off. This is one reason why formations tend to have minimum distances from one another. The larger the formation, the greater the interformation distance.
The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shapedstalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws". Given enough time, these formations can meet and fuse to create aspeleothem of calcium carbonate known as a pillar, column, or stalagnate.[9]
Another type of stalactite is formed inlava tubes while molten and fluidlava is still active inside.[10] The mechanism of formation is the deposition of molten dripping material on the ceilings of caves, however with lava stalactites formation happens very quickly in only a matter of hours, days, or weeks, whereas limestone stalactites may take up to thousands of years. A key difference with lava stalactites is that once the lava has ceased flowing, so too will the stalactites cease to grow. This means that if the stalactite were to be broken it would never grow back.[2]
The generic termlavacicle has been applied to lava stalactites and stalagmites indiscriminately and evolved from the word icicle.[2]
Like limestone stalactites, they can leave lava drips onto the floor that turn into lava stalagmites and may eventually fuse with the corresponding stalactite to form a column.
The shark tooth stalactite is broad and tapering in appearance. It may begin as a small driblet of lava from a semi-solid ceiling, but then grows by accreting layers as successive flows of lava rise and fall in the lava tube, coating and recoating the stalactite with more material. They can vary from a few millimeters to over a meter in length.[11]
As lava flows through a tube, material will be splashed up on the ceiling and ooze back down, hardening into a stalactite. This type of formation results in an irregularly-shaped stalactite, looking somewhat like stretched taffy[clarification needed]. Often they may be of a different color than the original lava that formed the cave.[11]
When the roof of a lava tube is cooling, a skin forms that traps semi-molten material inside. Trapped gases expansion forces lava to extrude out through small openings that result in hollow, tubular stalactites analogous to the soda straws formed as depositionalspeleothems in solution caves. The longest known is almost 2 meters in length. These are common in Hawaiian lava tubes and are often associated with a drip stalagmite that forms below as material is carried through the tubular stalactite and piles up on the floor beneath. Sometimes the tubular form collapses near the distal end, most likely when the pressure of escaping gases decreased and still-molten portions of the stalactites deflated and cooled.Often these tubular stalactites acquire a twisted, vermiform appearance as bits of lava crystallize and force the flow in different directions. These tubular lava helictites may also be influenced by air currents through a tube and point downwind.[11]
Ice stalactites on the gutter of a houseIce stalactites on a frozen beach inBete Grise, Michigan
A common stalactite foundseasonally or year round in many caves is the ice stalactite, commonly referred to asicicles, especially on the surface.[12]Waterseepage from the surface will penetrate into a cave and iftemperatures are belowfreezing, the water will form stalactites. They can also be formed by the freezing of watervapor.[13] Similar to lava stalactites, ice stalactites form very quickly within hours or days. Unlike lava stalactites however, they may grow back as long as water and temperatures are suitable.
Ice stalactites can also form undersea ice whensaline water is introduced to ocean water. These specific stalactites are referred to asbrinicles.
Ice stalactites may also form corresponding stalagmites below them and given time may grow together to form an ice column.
Stalactites can also form onconcrete, and on plumbing where there is a slow leak and where there arecalcium, magnesium or other ions in the water supply, although they form much more rapidly there than in the natural cave environment. These secondary deposits, such as stalactites, stalagmites, flowstone and others, which are derived from the lime, mortar or other calcareous material inconcrete, outside of the "cave" environment, can not be classified as "speleothems" due to the definition of the term.[8] The term "calthemite" is used to encompass these secondary deposits which mimic the shapes and forms of speleothems outside the cave environment.[14]
The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is due to the presence ofcalcium oxide in cement. Concrete is made from aggregate, sand and cement. When water is added to the mix, the calcium oxide in the cement reacts with water to formcalcium hydroxide (Ca(OH)2). Thechemical formula for this is:[6]
CaO (s) +H 2O (l) →Ca(OH) 2 (aq)
Over time, any rainwater that penetrates cracks in set (hard) concrete will carry any freecalcium hydroxide insolution to the edge of the concrete. Stalactites can form when the solution emerges on the underside of the concrete structure where it is suspended in the air, for example, on a ceiling or a beam. When the solution comes into contact withair on the underside of the concrete structure, anotherchemical reaction takes place. The solution reacts withcarbon dioxide in the air andprecipitatescalcium carbonate.[6]
Ca(OH) 2 (aq) +CO 2 (g) →CaCO 3 (s) +H 2O (l)
When this solution drops down it leaves behind particles of calcium carbonate and over time these form into a stalactite. They are normally a few centimeters long and with adiameter of approximately 4 to 5 mm (0.16 to 0.20 inches).[6] The growth rate of stalactites is significantly influenced by supply continuity ofCa2+ saturated solution and the drip rate. A straw shaped stalactite which has formed under a concrete structure can grow as much as 2 mm per day in length, when the drip rate is approximately 11 minutes between drops.[14] Changes inleachate solutionpH can facilitate additional chemical reactions, which may also influencecalthemite stalactite growth rates.[14]
The White Chamber in theJeita Grotto's upper cavern in Lebanon contains an 8.2 m (27 ft) limestone stalactite which is accessible to visitors and is claimed to be the longest stalactite in the world.[citation needed] Another such claim is made for a 20 m (66 ft) limestone stalactite that hangs in the Chamber of Rarities in theGruta Rei do Mato (Sete Lagoas,Minas Gerais, Brazil).[citation needed] However,cavers have often encountered longer stalactites during their explorations. One of the longest stalactites viewable by the general public is inPol an Ionain (Doolin Cave),County Clare, Ireland, in akarst region known asThe Burren; what makes it more impressive is the fact that the stalactite is held on by a section of calcite less than 0.3 m2 (3.2 sq ft).[15]
Stalactites are first mentioned (though not by name) by theRoman natural historianPliny in a text which also mentions stalagmites and columns and refers to their formation by the dripping of water. The term "stalactite" was coined in the 17th century by the Danish PhysicianOle Worm,[16] who coined the word from theGreek word σταλακτός (stalaktos, "dripping") and the Greek suffix -ίτης (-ites, connected with or belonging to).[17]
^Baird, A.K. (1982). "Basaltic "stalactite" mineralogy and chemistry, Kilauea".4 (4). Geological Society of America Bulletin, abstracts with programs:146–147.{{cite journal}}:Cite journal requires|journal= (help)
^abcBunnell, Dave (2008).Caves of Fire: Inside America's Lava Tubes. p. 124.
