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How Stalactites and Stalagmites Are Formed

written by: Bruno Kos • edited by: Kellie Hayden • updated: 1/31/2013

Stalactites & Stalagmites: Which one points up and which one grows down? Even beyond elementary school, students remain unsure about the answer. Find out once and for all in this article--and also, find out how are stalactites and stalagmites are formed.

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    To Drip

    Students encounter the terms stalactites and stalagmites somewhere during elementary school science. But, when asked which one is up and which one is down, there is always a certain amount of insecurity before a final answer is given. Ultimately, the words are rather similar, with only two letters of difference.

    Words stalactite and stalagmite have their roots in the Greek language. More specifically, these terms are derived from the Greek word stalasso, which means "to drip." It is hard to find a more suitable word, since both stalactites and stalagmites are formed simply by water dripping from fractures in the ceiling of a cave.

    Further, both stalactites and stalagmites are a type of speleothem, which is actually a secondary mineral deposit formed in a cave. Within the caves, they grow very slowly, approximately 0.007–0.929 mm per year.

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    Formation of Stalactites

    A stalactite, as mentioned in the introduction, is a type of speleothem which hangs from the ceiling of caves that are made of limestone (calcium carbonate rock, dissolved by water and contains carbon dioxide). They are formed in the process of deposition of calcium carbonate (but some other minerals as well) Both calcium and limestone are precipitated from mineralized water solutions.

    This process can be described with the chemical formula, which is as follows:

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

    The above solution travels through the rock, until it finally reaches the roof of the cave. Once it does, it drips down. Once the solution reaches air, the following chemical reaction occurs (which is a reversed reaction of the previous solution):

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

    The formation of stalactites begins with nothing more but a single mineral-laden drop of water. Once this drop falls, it eventually leaves the thinnest ring of calcite. When the next drop falls, another calcite ring is deposited. After a certain amount of time, a "soda straw" stalactite will form (they are very fragile, though). If water begins to flow on the outside, more calcite will be deposited, and a cone-shaped stalactite will form.

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    Formation of Stalagmites

    Stalagmites are also a type of speleothem but as opposed to stalactites, they rise from the floor. Further, they are also formed in the process of deposition of calcium carbonate. The stalagmite and corresponding stalactite (meaning that they grow together) are called a "column".

    Stalagmites shouldn't be touched with the hands, since the oils from the skin may disturb the path where the mineral water will cling. Eventually, the formation of stalagmites would suffer.

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    Remembering Which Is Which

    If confused about which points up and which one hangs down, several memory tricks exist. Some of the simplest are:

    • Stalactite has a letter "t" in it ("t" for "top“).
    • Stalactite has a "c" in it ("c" for "ceiling“).
    • Stalagmite has a "g" in it ("g" for "ground“).
    • Stalactites cling "tightly" to the roof of the cave.
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    Besides the information on how are stalactites and stalagmites formed, a reader should also know that both stalactites and stalagmites are considered as natural heritage objects and, as such, they are usually protected by the laws. Accordingly, it is prohibited to collect them, destroy them, or to sell them.

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    References

    Chris Pellant, Helen Pellant: Sandstone and Other Sedimentary Rocks, Gareth Stevens Pub., 2007.

    Jackie Gaff: Stalactites hang down (and other questions about caves), Kingfisher, 2004.