Geology Word of the “Week” Triple Feature- Word I: Speleothem

After seeing Rebecca and other skepchicks at the recent Amaz!ng Meeting 7, I realized that it’s time to start writing for Skepchick again. My geology word of the week slipped to a geology word of the month, so to make up for that I’m going to post a triple feature of geology words this week. I’ll post one word today, one on Monday, and one on Wednesday. After this kickstart, I’ll go back to my weekly word and try to write up some other skeptical tidbits now and again.

That said, the first geology word of the week is: Speleothem. I was working on some speleothem samples this morning in lab, so I thought this would be a fitting word for today.

If you’ve ever been to a cave, you’ve probably seen speleothems. Speleothem is an encompassing term which is used to describe all types of chemical precipitate minerals that form in caves. Speleothems generally precipitate from groundwater which has percolated through the bedrock surrounding the cave and leached various elements and compounds. When the enriched water reaches the cave, changing conditions (a large open space has very different pressure and temperature than pore spaces in bedrock) allow gases, such as carbon dioxide, to escape from the water. Evaporation can also occur. The changing composition of the water encourages the (usually very, very slow) precipitation of speleothem minerals from cave waters.

Chemically, the speleothems which form in a particular cave are similar in composition. Most caves are formed in limestone, and so the speleothems will generally be formed of calcite, the dominant mineral in limestone. However, depending on how and where the speleothem is precipitated, it can take on a variety of shapes. Scientists and other cave explorers have given different names to these various morphologies. Examples of speleothems are stalactites, stalagmites, flowstones, cave coral, cave drapery, cave curtains, and cave crystals. There are dozens of names for various cave formations, so speleothem is a nice catch-all phrase for geologists to use.

Most speleothems form over thousands upon thousands of years. Thus, you shouldn’t touch or remove speleothems unless you’re doing so for legitimate science. Even when collecting speleothems for science, one should be conservative. Take small samples and obtain the necessary permissions. Fortunately, for my own research in Oman I am often able to collect speleothems which have fallen on the ground and are no longer growing.

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Here’s a picture of me with a stalagmite in Oman. I didn’t sample this one, but I did sample some of its neighbors. This particular stalagmite isn’t forming in a true cave but rather in an open hallow underneath a layer of rock. Water percolated through the layer of rock and formed speleothems in the hallow underneath. I’ll try to write more about my research in Oman in coming weeks.

Now, for those of you who still confuse stalactite and stalagmite, here’s a reminder of something you probably learned in grammar school but may have forgotten by now.

StalaCtites hang tite to the Ceiling while stalaGmites grow up from the Ground.


Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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  1. I was on a cave tour once. The tour guide mentioned that some stalagmites are “alive” or growing. The ones that are “growing” will light up when you stick a flashlight behind them. The “dead” ones block the light. Is this because the “alive” ones are wet. I asked the tour guide, but he didn’t know. (college kid, summer job)

  2. @MathMike: I went there with my old boy scout troop! We camped out the night before, and the cup of water I left on the patrol box overnight froze solid.

    The place I saw them first was at Beaumont scout ranch, though. They have a cave on site called the “Mud Cave.”

  3. @ZachTP: I spent a week one summer giving tours of that cave. 1983 I think. Not that I had intended it, but I don’t think I’ve been back to that camp since then.
    I camped at Onondoga one night when the fog was so thick you could follow the beam from your flashlight just a few feet before the fog swallowed it.

  4. I am working with ammonia oxidizing bacteria, and they are commonly found on rocks where they oxidize ammonia into nitrite and nitrate. By generating that acid, they cause dissolution of calcium carbonate. I suspect that some of the growth of caves is due to oxidation of ammonia by them. Particularly in caves where there are bats where bat guano can generate levels of ammonia that are toxic to most everything else.

    Have you looked for nitrate and nitrite? The equilibrium of CO2 and water is pretty slow which is why virtually all organisms have carbonic anhydrase. I suspect that carbonic anhydrase from organisms may be important in modifying CO2 release kinetics.

  5. @ZachTP: I used to live in Springfield MO. I’ve been through Fantastic Caverns and the caves under Silver Dollar City (Branson) several times. Very interesting stuff. I was so interested that I forgot that I don’t like closed in spaces much. I’ve seen every one of the examples that Evelyn mentioned above. I’m not interested enough to go spelunking, though…

    Oh, and there were bats, too! :-D

  6. Is there any commercial cave anywhere in America that does NOT contain a speleothem that looks like Abraham Lincoln?

    (You can tell I’ve been on way too many tours.)

  7. Bats sure are cool…but there’s a major outbreak of white nose disease in the WV bat population. It could mean 90%+ casualties…In essence, a complete wipeout of our local bat populations.

    I’ve tried to get bats to use a bat house here, but I really don’t have a place that is “good enough” to attract them. :-(

  8. @QuestionAuthority: The suburbs for a few miles around my house have tiny little parks scattered through them, and one friend of mine made some bat houses for them as an Eagle project.

    The last time I went to see them, there were some bats in ’em, but I think they were torn down.

    I would hate to see an outbreak like that around here; we have enough mosquito problems already.

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