Stalactite

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Image:Mosses around Stalactite with Waterdrop.jpg

A stalactite (Greek stalaktos, (Σταλακτίτης), "dripping" or "dropped"), or dripstone, is a type of speleothem that hangs from the ceiling or wall of limestone caves.

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Stalactites are formed from the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. The corresponding formation on the floor underneath a stalactite is known as a stalagmite. Given enough time, these formations can meet, resulting in a formation known as a column.

Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind 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.

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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."

While it has been claimed that the longest stalactite known hangs in the Chamber of Rarities in the Gruta Rei do Mato (Sete Lagoas, Minas Gerais, Brazil) and is 20 metres long, vertical cavers have often encountered longer stalactites while exploring.

Stalactites can also form on concrete ceilings, and on plumbing where there is a slow leak and limestone (or other minerals) is in the water supply, although they form much more rapidly there than in the natural cave environment. Stalactites can also form in lava tubes, although the mechanism of formation is much different.