My sister posted the above photo on Facebook. She discovered this curiosity on a hike and called it a “beach mushroom.” At first glance, it certainly looks like a mushroom. There are many species of mushrooms that thrive in beach sand, but this isn’t one of them. It’s a weathered pebble of what appears to be basalt balanced on a narrow column of sand. How did that happen?
It starts with the pebble lying flat with its entire lower surface supported by sand. The weight of the pebble applies pressure on the sand. Not much for a pebble this size — I’d estimate around 0.04 psi. But that’s sufficient to lock the irregular sand grains together more tightly so they resist erosion by wind or water.
As the surrounding sand is blown or washed away, erosion starts to undercut the pebble. When this happens, the pressure begins to increase because the same weight is being supported by less area of sand. By the time it gets to the point shown in the photo, where the supporting column of sand appears to have a diameter of around 1 cm, the pressure on the sand has increased by a factor of 16.
Here’s a graphic showing an idealized version of the geometry:
Erosion of the supporting column will continue, albeit at a reduced pace, but this unstable situation can’t last long. My sister was lucky to catch it when she did. Pretty sure it was toppled by the next day.
I did a lot of hiking along Lake Michigan as a kid and never saw one of these “beach mushrooms.” And I was a rock hound, always looking down for interesting finds. So it seems these things must be fairly rare, but I did some research to see if geologists had an official name for “beach mushrooms.” Turns out there are several names for structures like these, but they usually apply to larger scale features. Geologists often use the term mushroom rock (aka “rock pedestal”).
For a diminutive structure like a “beach mushroom” that label just doesn’t seem right. But I did find one that seems to apply — hoodoo. The word is suspected to come from the Hoodoo spirituality that originated in the communities of enslaved Africans in the early American south. Indigenous peoples believed certain naturally occurring geological features had mystical powers, not the least of which was to inspire awe or wonder. And somehow the term hoodoo got repurposed for these features.
Now don’t get me wrong … I’m not suggesting there’s any “magic” associated with “beach mushrooms,” and maybe we should just leave it as “beach mushroom.” But considering the transient nature of these structures, maybe there should be a little “awe and wonder” if you’re lucky enough to spot one.
Next Week in Sky Lights ⇒ Golden Sunsets