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Excerpts from Jim Conrad's
Naturalist Newsletter
from the September 25, 2011 Newsletter issued from Mayan Beach Garden Inn 20 kms north of Mahahual; Caribbean coastal beach and mangroves, ~N18.89°, ~W87.64°; Quintana Roo state, MÉXICO
FLOATING ROCKS
The other day a rock came floating ashore, as shown above. Of course it was pumice, and there's a fair bit of it strewn along the sandy beach these days.
Pumice is volcanic rock. When highly pressurized lava is violently erupted from a volcano, it simultaneously cools and depressurizes. As it depressurizes, many tiny gas bubbles form in it the way gas bubbles do in soda-pops when the tops are removed, lowering the pressure inside the bottles. If the lava cools very fast, as by falling into water, the lava hardens into bubble-filled rock. And the resulting pumice rock is so lightweight that it floats! The rock is also soft enough to cut with a pocketknife, or scrape with a fingernail. Pumice with a notch cut into it is shown below:
I haven't seen pumice float in like this during the five months I've been here, though I'm told it happens from time to time. Along with pumice there's an unusual number of large tree trunks suddenly cast onto the beach. Often the trunks are charred or bear machete cuts. I'm guessing that someplace downcurrent a hillside has given way during the current rainy season. Probably the hillside had been devastated and destabilized by slash-and-burn agriculture -- killing trees by girdling them, then setting fire to "clean things up." When plant roots no longer hold the soil in place and leaf littler no longer caps it, sometimes whole mountainsides slop away. And if it's in a volcanic area, there's a good chance there'll be a layer of ash and pumice below the topsoil. The tree trunks and pumice float.