An acquaintance of mine recently discovered these odd discs of ice.



The discs floated freely in an eddy behind some fallen logs on the Nooksack River’s south fork. See other photos and a short video at

http://picasaweb.google.com/karenhealy11/January12011IceFloes#5557968238304251890

What we think happened is this: Foam floating about on the water, which you can see in the video, started to freeze, probably at night. Bits of frozen foam got pushed around in the eddy, and in the ensuing collisions, became roughly circular. Perhaps each disc grew radially when smaller pieces of unfrozen foam struck the disc, adhered and spread out a bit along the perimeter, and then froze in place.

The raised rims are undoubtedly due to the collisions. One sees such rims on “pancake ice”, which is also said to be due to collisions, but what about the concentric raised regions inside the discs? None of the images of pancake ice on Google seem to show these inner lines.

Here’s an idea about those lines. The air temperature oscillated during the growth of these discs: colder at night due to the clear-sky conditions and resulting radiative cooling, but warmer in the day. The discs may have grown at night, collecting new foam, then during the day, when the discs softened in the sun, softening particularly around the edges, the collisions raised up the rims. The next night, further radial growth followed by a new rim the next day at a greater radius. The pictures and video show 4-5 such rims, indicating growth over 4-5 days, which is roughly consistent with the duration of our cold snap.

- JN