Concrete Spelunking
One of the joys of noticing (and delighting in) the weird and obscure in nature is that sense of validation and camaraderie when someone else notices (and delights in) the same thing.
I love exploring the old forts that bracket Puget Sound, particularly Forts Ebey, Worden, and Flagler. They date from the late 1800's, so are hardly ancient, but exploring them helps sate my longing for distant European castles.
Several years ago at Fort Flagler on Marrowstone Island, I noticed stalactites growing from the concrete ceiling in one of the batteries. I was delighted, and recorded them in my nature journal.
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| Calthemites at Fort Flagler, Marrowstone Island |
Recently, I read David B. Williams new book, Wild in Seattle. I felt that spark of validation and camaraderie when I saw that he had a chapter about concrete "cave" formations. "I've seen those!" I thought to myself, "I love those!"
Now I know that they have a name. The concrete "cave" formations are properly called calthemites, which means lime mineral deposit. Concrete contains lime, which is made partly of calcium. As the concrete slowly breaks down the calcium can leach out and later be deposited as calcium carbonate. Calcium carbonate is one of the minerals that can form natural cave formations as well.
In search of more calthemites, we revisited Fort Flagler and once again explored the batteries. Now that I was specifically looking for these formations, I saw them everywhere; hanging from the ceiling, dripping down the walls, even building up on the floors.
Calthemites commonly form as hollow, cylindrical stalactites called soda straws. The longest ones we observed were perhaps eight inches long, and roughly the diameter of an actual soda straw. They seemed to often (but not always) form at the join between concrete and iron beams. As water seeps through cracks and pores in the concrete, it picks up calcium, which is deposited as it drips out. You can see drops of water on the ends of some of these soda straw; each drop is adding a little more to the end of the straw.
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| Soda straws. Notice the drops of water dangling off the ends. |
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| Broken soda straws, showing their hollow center |
All stalactites begin life as soda straws. In natural cave systems most stalactites gradually fill in and form a solid, conical shape. Calthemite soda straws grow far more quickly than cave soda straws (the reactions leading to the deposition of calcium carbonate are different), and as a result they remain hollow. Some do start to form cone shapes, though, as minerals build up at their bases.
We also found a few places were stalagmites were forming beneath soda straws, building upward with each drip from above. The stalagmites we saw were very low and broad; I wonder if they can form taller columns if left undisturbed?
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| Calthemite stalagmite, top view |
The most common calthemite formation at Fort Flagler was flowstone. Wherever cracks ran along the walls, sheets of flowstone dripped down in beautiful rippling patterns.
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| Calthemite flowstone |
Calthemites are generally white, but can be colored by copper or iron embedded in the concrete. You can see the rusty color of iron oxide in this flowstone.
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| Flowstone with iron oxide staining |
The flowing water that deposits these concrete formations is sometimes very caustic (it's a strong base in the same way that bleach is), so use caution when examining calthemites closely. The water flowing through the concrete at Fort Flagler luckily did not burn us, but now I want to go back yet again with some pH paper and test it.
Resources and further reading:
An excellent but more technical article on calthemites from Garry Smith. Scroll to page 23.
Different cave formations explained by the National Park Service.
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