thumb|300px|right|Image showing the six most common speleothems with labels. Enlarge to view labels.

A stalactite (, ; , ) is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble and that can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, minerals, mud, peat, pitch, sand, sinter, and amberat (crystallized urine of pack rats). A stalactite is not necessarily a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.

The corresponding formation on the floor of the cave is known as a stalagmite.

Formation and type

right|thumb|upright|Demonstration of drip stone formation in a lab. The blue color is due to the addition of cupric ions (Cu<sup>2+</sup>) to the mother solution.

Limestone stalactites

The most common stalactites are speleothems, which occur in limestone caves. They form through deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. Limestone is the chief form of calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution in caverns. The chemical formula for this reaction is: The drip rate must be slow enough to allow the CO<sub>2</sub> to degas from the solution into the cave atmosphere, resulting in deposition of CaCO<sub>3</sub> on the stalactite. Too fast a drip rate and the solution, still carrying most of the CaCO<sub>3</sub>, falls to the cave floor where degassing occurs and CaCO<sub>3</sub> 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&nbsp;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.

Pillars

thumb|right|upright|Pillars in the [[Caves of Nerja, Spain]]

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-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws". Given enough time, these formations can meet and fuse to create a speleothem of calcium carbonate known as a pillar, column, or stalagnate.

Lava stalactites

Another type of stalactite is formed in lava tubes while molten and fluid lava is still active inside. 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.

Splash stalactites

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. Often they may be of a different color than the original lava that formed the cave. Water seepage from the surface will penetrate into a cave and if temperatures are below freezing, the water will form stalactites. They can also be formed by the freezing of water vapor. 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 under sea ice when saline water is introduced to ocean water. These specific stalactites are referred to as brinicles.

Ice stalactites may also form corresponding stalagmites below them and given time may grow together to form an ice column.

Concrete stalactites

right|thumb|Concrete stalactites

thumb|left|upright|[[Calthemite soda straw stalactites under a concrete slab]]

Stalactites can also form on concrete, and on plumbing where there is a slow leak and where there are calcium, 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 in concrete, outside of the "cave" environment, can not be classified as "speleothems" due to the definition of the term.

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 of calcium 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 form calcium hydroxide (Ca(OH)<sub>2</sub>). The chemical formula for this is:

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Over time, any rainwater that penetrates cracks in set (hard) concrete will carry any free calcium hydroxide in solution 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 with air on the underside of the concrete structure, another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate. Another such claim is made for a limestone stalactite that hangs in the Chamber of Rarities in the Gruta Rei do Mato (Sete Lagoas, Minas Gerais, Brazil). However, cavers have often encountered longer stalactites during their explorations. One of the longest stalactites viewable by the general public is in Pol an Ionain (Doolin Cave), County Clare, Ireland, in a karst region known as The Burren; what makes it more impressive is the fact that the stalactite is held on by a section of calcite less than .

Etymology

Stalactites are first mentioned (though not by name) by the Roman natural historian Pliny 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 Physician Ole Worm, who coined the word from the Greek word σταλακτός (stalaktos, "dripping") and the Greek suffix -ίτης (-ites, connected with or belonging to).

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Image:Upclose caves.jpg|Stalactites at the Puerto Princesa Underground River, Palawan, Philippines

Image:Stalactite-drop.jpg|Mineralized water drop forming at bottom of stalactites

Image:Choranche_caves_stalactites.jpg|Stalactites of the type called "soda straws" from the Choranche caves in the Vercors, France

Image:Tubular lava stalactites.jpg|Tubular lava stalactites

Image:Tubular lava helictite.jpg|A tubular lava helictite

</gallery>

See also

  • Stalagmite
  • Lavacicle
  • Rusticle
  • Karst
  • Icicle
  • Bottlebrush - Stalactite coated with pool spar.
  • Brinicle

References

  • Dripstone in time-lapse ("Tropfsteine im Zeitraffer") - Schmidkonz, B.; Wittke, G.; Chemie Unserer Zeit, 2006, 40, 246.
  • The Virtual Cave's page on stalactites
  • Stalactites by Enrique Zeleny, Wolfram Demonstrations Project.