Friday Rock Blogging: Sand Boils
So sand is just little weensy rocks, anyway. And this is a weensy volcano made of sand, in Peru. It’s about a meter (0.33% of a football field) across.
Normally, layers of sand and silt and crap underground bear the weight of whatever’s on top of them through a network of contacts between individual sand grains. During an earthquake, if the sand grains are loosely packed, this network is disrupted. But the stuff on top is still there, being all heavy and shit. Wacky hijinks ensue!
If the jiggling sand happens to be wet, all the overburden pressure is transferred to the water. Water, being incompressible, is not happy about being squeezed like this; if it finds a suitable weak spot, it will squirt out. And voilà, a pretty sand boil!