stalactite and stalagmite

Image showing the six most common speleothems with labels. Enlarge to view labels.

A stalactite (UK /ˈstæləktaɪt/, US /stəˈlæktaɪt/; from the Greek stalasso, (σταλάσσω), "to drip", and meaning "that which drips") is a type of speleothem (secondary mineral) that hangs from the ceiling of limestone caves. It is a type of dripstone. The corresponding formation on the floor of the cave is known as a stalagmite.

Formation and type

Demonstration of drip stone formation in a lab.

Stalactites are formed by the 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 underground caverns.^[1] The chemical formula for this reaction is:^[2]

CaCO₃(s) + H₂O(l) + CO₂(aq) → Ca(HCO₃)₂(aq)

This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down. When the solution comes into contact with air the chemical reaction that created it is reversed and particles of calcium carbonate are deposited. The reversed reaction is:^[2]

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 fast-flowing water rich in calcium carbonate and carbon dioxide, these can grow at 3 mm (0.12 inches) per year.^[3]

Every stalactite begins 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 (0.5 mm), 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. 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 columns of calcium carbonate.

Stalactites can also form in lava tubes, although the mechanism of formation is very different.

Concrete

Concrete stalactites.

Stalactites can also form on concrete, and on plumbing where there is a slow leak and limestone (or other minerals) in the water supply, although they form much more rapidly there than in the natural cave environment (description and experiments see literature).

The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide in concrete. This calcium oxide reacts with any rainwater that penetrates the concrete and forms a solution of calcium hydroxide. The chemical formula for this is:^[2]

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Over time this calcium hydroxide solution reaches the edge of the concrete and, if the concrete is suspended in the air, for example, in a ceiling or a beam, then this will drip down from the edge. When this happens the solution comes into contact with air and another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate.^[2]

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(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 a diameter of approximately 5 mm (0.20 inches).^[2]

Records

The White Chamber in the Jeita Grotto's upper cavern in Lebanon contains an 8.2 m (27 ft) stalactite which is accessible to visitors and is claimed to be the longest stalactite in the world. Another such claim is made for a 20 m (66 ft) stalactite that hangs in the Chamber of Rarities in the Gruta Rei do Mato (Sete Lagoas, Minas Gerais, Brazil). However, vertical cavers have often encountered longer stalactites while exploring. One of the longest stalactites viewable by the general public is in 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 0.3 m² (3.2 sq ft).^[4]

Origin of the term

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 creation by the dripping of water. The term "stalactite" was coined in the 17th century by the Danish Physician Ole Worm^{[citation needed]} who created the word from the Greek root stalasso, (σταλάσσω), "to drip".

Photo gallery

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  Water droplet coming out of the central canal of a stalactite.

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  Stalactite in Doolin Cave, Ireland measuring 20ft (6.54m) long.

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  Mineralized water drop forming at bottom of stalactites.

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  Stalactites of the type called "soda straws" from the Choranche caves in the Vercors, France

See also

• Stalagmite
• Lavacicle
• Rusticle
• Karst
• Icicle

References

• Dripstone in time-lapse ("Tropfsteine im Zeitraffer") - Schmidkonz, B.; Wittke, G.; Chemie Unserer Zeit, 2006, 40, 246. doi:10.1002/ciuz.200600370

External links

Wikimedia Commons has media related to: Stalactite

• The Virtual Cave's page on stalactites
• "Stalactites" by Enrique Zeleny, Wolfram Demonstrations Project.