Virginia water well shows seismic waves

This site, from the USGS, shows depth to the water table for a well in Virginia:


You’ll notice the tidal influence on the water table (broad sine-curve-like up and down crests and troughs at ~12 hour intervals), and then a sudden perturbation which caused some wiggles almost two and a half feet of magnitude! This, presumably, is the seismic waves from the Chilean earthquake arriving — surface waves, I would guess, but I’ll happily be corrected if that’s not the case.

Major hat tip to Cian Dawson, who tweeted a link to the site around 6am D.C. time (which would be, what, 3am in California, Cian? Sheesh!). In the same tweet, a link to this hydrograph in Christiansburg, Virginia, showing its response to various historical earthquakes.


Frozen soil lifts off

When I was out poking around in the woods, confirming for local geophile Barbara that indeed her local geologic map wasn’t 100% accurate, I noticed this on the frozen ground:

We have seen this before, in a post back on NOVA Geoblog, almost exactly a year before I took this photo. Here’s another shot from the more recent excursion, taken a foot or so over from the first one:


What’s happening here is not that I am showing you particularly high-contrast photos of pebbles and cobbles in the mud. Instead, the reason for the dark line around the sedimentary clasts is that the mud is frozen. When water freezes into ice, it expands in volume by about 9%. This extra volume means that the ice can’t occupy the same space that the liquid water did. So it pooches upwards, as “up” is the direction in which it is least “hemmed in.” Down? No — the expanding ice is not capable of pushing the entire Earth out of its way. North/south? or East/west? Well, there’s already soil there, and it’s pushing back, so there’s no expanding out in those (horizontal) directions. So, “up” it is. That’s all we’re left with: “up” is σ3.

If I were to draw this as a cartoon, here’s the “before” picture:


As the sheet of frozen mud expands upwards, it detaches from the non-expanding (in fact, shrinking, but not by anywhere near 9%) cobbles and pebbles. As the mud ice lifts up higher and higher, the gap between it and the clasts gets more and more pronounced.


Shadows in those gaps make them appear dark to the camera lens.

Ice pulls all kinds of neat tricks like this in the winter. What’s a cool ice phenomenon you’ve observed lately?