A few days ago, on a field trip to Frenchman Coulee, I was with a group of Ice Age Floods enthusiasts when we ran across a curious geological phenomenon – groups of bowl-shaped polygons, some as wide as six feet across and almost a foot deep. Here’s a little of the information I’ve been able to dig up on these formations; more to follow if/when I get a chance to return to the subject.
The earliest mention of the phenomenon I’ve found so far was by R. Mallet, an Englishman, in 1874. I haven’t yet found an online copy of the paper (Volcanic Energy: An attempt to develop its True Origin and Cosmical Relations), but whatever his theory might have been, it (and he) were severely bashed by one of Lyell’s buddies, George Poulett Scrope – Note on Mr. R. Mallet’s “Prismatic Structures of Basalt,” Monthly Journal of Geology V.12, Jan-Dec, 1875.
The next reference to pop up was to a paper by R. B. Sosman titled Types of prismatic structures in igneous rocks, Journal of Geology Vol. 24, No. 3, (April-May 1916). On page 229 of this paper, Sosman discusses cross-columnar fracturing, which sometimes produces a ball and socket situation in which the bottom of the upper portion of the column is convex, the lower portion concave. Paragraph 6.2 is of particular significance in this regard. (This fits in with the notion that the formations above were created when the Missoula Floods ripped off the upper portions of the columnar basalts that make up so much of the Frenchman Coulee area. See photo of Echo Basin below.)
Re: https://www.jstor.org/stable/pdf/30079357.pdf
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A 2004 GSA article by C. Schafer & S. Kattenhorn (Character and evolution of fractures in low-volume Pahoehoe lava flows in eastern Snake River Plain, Idaho) provides information on how flow characteristic and differences in cooling rates (external vs internal / top-down to bottom-up)impact the development of cross columnar fractures. According to this paper, the concave/convex relationship (ball and socket relationship) tends to be favored in flow materials produced by the CRBG*. Another aspect of this fracturing noted by the authors is that it takes place in areas immediately outside the viscus materials, rather than inside them.
Re: http://gsabulletin.gsapubs.org/content/116/3-4/322.full.pdf+html
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I have to leave this post unfinished for the time being. Yard chores are calling me. But one more note, a personal observation: while studying my photos of these curiosities, it occurred to me that the slope angles involved here remind me a great deal of the kind of cleavages you get when you break certain types of basalt – one side is invariably concave, the other convex. I think that might be a significant observation, but am not geologically versed enough to know for sure. More on all this later; meanwhile, comments, observations, and additional references would all be greatly appreciated.
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*The earliest mention of this ball-in-socket phenomenon I’ve found was in the 1875 reference mentioned above. That discussion focused on the basalts in the Giant’s Causeway in Ireland. Does this mean the Grand Ronde Flood Basalt Group in Washington/Oregon/Idaho is somehow related to those from which the Giant’s Causeway’s spectacular columnar basalts originated?
Note: Echo Basin. If you double click on this photo, it should open to a larger size. Then look for the rock climbers in the upper right quadrant. Before the floods tore through this area, an entire assemblage of colonnades extended above the high ground shown here.