Pulling together the themes of glowing ocean creatures from last week and coastal geology from the week prior, this week’s topic is glowing rocks. Obviously, the rocks didn’t eat glowing dinoflagellates, as was my hypothesized explanation for the creatures I saw on the sand one night. In fact, these rocks wouldn’t be visible at all if you were out for a walk in the dark. Instead, it takes smacking them together to create light. I tried this on a dark night and it was pretty amazing.
Many bioluminescent plankton glow in response to being disturbed. This is why, when you swirl your hand in water with these plankton, you see sparkles, but when you stop that motion, they go dark again. What you’re seeing is a chemical reaction — that of luciferase with oxygen. Most bioluminescent animals also contain luciferase, which is a catalyst that speeds up the reaction and makes a brighter glow. If you look closely at a glow stick, you can see a tiny glass tube inside the bigger plastic piece. When you crack that tube, you mix the two chemicals. This is similar to what happens in glowing marine organisms. Many of these organisms use bioluminescence as a defense mechanism — flash a glowing “go away” signal in response to a predator.
Not being living organisms, rocks aren’t sparking to defend themselves. So, what does make them glow? Given that you have to hit them together, it obviously has something to do with friction. Instead of bioluminescence, which names something living or “bio”logical, this type of glow is called triboluminescence. Unlike some types of friction, triboluminescence doesn’t create heat but instead creates light. Like much bioluminescence, it is a cold light. Apparently, the light can be different colors depending on the minerals being tested. For example, if you’ve done the Wint O Green Lifesavers experiment, you might have seen a blue light when you bite one in the dark. That comes from the crystalline sugars in the mint oil which are somewhat similar to the crystalline structure of quartz.
While quartz doesn’t glow because it is “excited” by predators, it is thought that the light produced is the result of electrons in the rocks getting “excited.” Another theory is that the flash of light is like a tiny lightning bolt generated by the force of the two pieces of quartz being smacked together. This was an interesting theory in light of last week’s thunderstorms following the intense heat. In an attempt to test this theory, it did appear that the harder we smacked, the bigger the spark.
I have yet to try this with other minerals commonly found on the shore in Maine, but I imagine there are others to test. If you want to learn more about triboluminescence, you can find more information at geology.com/minerals/triboluminescence/. Or just start with something simple and easy to find by collecting a couple of quartz pieces and making them spark in the dark. And don’t forget to test local waters on dark nights to see if you can see the biological version of this in flashing, tiny plankton.
Susan Olcott is the director of operations at Maine Coast Fishermen’s Association.
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