Friday Rock Repost: Sand Boil

a sand boil after the 1970 Peru earthquake 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% 0.9% of a football field thanks LL!) across.

Normally, layers of sand and silt underground bear the weight of whatever’s on top of them through a network of contacts between individual soil grains. During an earthquake, this network is disrupted. But the stuff on top is still there, bein’ all heavy. Wacky hijinks ensue!

If the jiggling sand happens to be wet, all that 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!

This picture is totally going in my thesis.


  1. BrianR wrote:

    Yay! Sand “boil” … I like that … I’ve always called them sand volcano, but I think I like boil better.

    These conglomerate injection complexes we documented in Patagonia always made me wonder if there was a conglomerate boil on the sea floor … how awesome would that be.

  2. Maria Brumm wrote:

    There are pooploads of submarine mud volcanoes, of course, but I’m not sure what the max observed grain size is.

  3. Andrew Alden wrote:

    The seafloor mud volcanoes cough up boulders, of serpentinite.

    At the GSA section meeting yesterday, Hannah Wolf (a high-school student who won in the Westinghouse Science Fair last year) showed her poster about a fossilized sand volcano in southern Utah. She has a budding specialty in paleoseismites, and you’d all be proud of her.

  4. Bee wrote:

    Thanks – I never knew what to call those things.

    I giggled at your “metre = 0.33 of a football field”. It’s been a longtime annoyance of mine, the convention of comparing anything of size with football fields. I know how big a metre is, or a yard, or any part of either measurement. But like a whole huge number of people, I never watch football, have never attended a game, and have no clue what size a football field may be… might as well say “big as a hayfield” – I’ve seen plenty of those.

  5. Lab Lemming wrote:

    Are you sure that shouldn’t be 0.009 football fields long? Because although the word ‘football’ can refer to more than half a dozen different codes of ball sports, I don’t think any of them have 300 meter fields…

  6. Kadi wrote:

    Just in case you hadn’t heard yet, I live in Indiana and there have been reports of sand boils popping up along many of our rivers and creeks since the earthquake in Southern Illinois a couple of weeks ago. Students and scientists around here are now studying them to see what kind of meaning they have and are asking farmers to report any sightings of them. You might be interested in finding out more about what’s going on around here since it’s so current.

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