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10 February 2023
Friday fold: Kelvin-Helmholtz waves in granite?
Reader Christian Gronau returns with another “guest Friday fold” submission. Christian writes, Greetings from a cold and wet west coast. A good time to root through old rock samples – and let the imagination run free … The little compilation below strikes me as visually compelling (both photographs are mine) – but how likely is it that the suggested analogy has any merit ? Would it have some …
11 June 2021
Friday fold: inadvertent kink fold analogue model
It’s Friday! Adam Forte, a geology professor at LSU, posted this image yesterday on Twitter: It’s a box of sheets of newsprint, stored vertically and ignored for a while, now rotated 90° so we’re looking at a cross-sectional view. To me, this is an excellent example of a physical analogue modelling experiment (albeit inadvertent) that demonstrates the process by which kink folds form. The key parameters to get this distinctive …
28 February 2020
Friday fold: sandbox
The Friday fold is a lovely little sandbox analogue model by Prof. Marco Martins-Ferreira, who posted it on Twitter this week: As deformation proceeds, you can see the layers develop folds that then morph into faults, shoving deeper layers atop more shallow strata. As a bonus, you can hear Marco’s baby cooing in the background! Here’s a stabilized, sped-up version, courtesy of Anna Williams: Happy Friday, all!
8 May 2014
Honey crystallization as an analogue for magma segregation and cumulate textures
Check this out: Maybe I’ve got low blood sugar, but I think I see a magma chamber in that jar of honey. There is clearly some crystal settling going on there, and it appears that the more crystals there are, the easier it is to trap bubbles. When the clots of crystals get too dense, they peel off (stope) and drop down to the floor of the jar. Similar sorting …
7 March 2013
Snow day aftermath
So we ended up getting 13 or 14 inches of snow yesterday. It was wet, and the temperature was right around freezing. As soon as it stopped falling, it started melting. One interesting aspect of this is that big wads of slushy snow were plopping off the trees every time a gust of wind came through. The structures these “plops” formed exhibit much of the same morphology of meteorite impact …
17 October 2012
Convection in a dirty dish
Saw this in a greasy / soapy baking pan in my kitchen sink the other day: Do you see those lobe-shaped light areas, separated by dark septae? I think that’s the semi-gelled signature of gravitational instability, perhaps thermally driven. I’m speaking of convection: upwelling in the round light areas, and sinking of denser material in the dark seams in between. A closer look at the structure of the suspension, and …
18 August 2012
Migmacatization
Evelyn put up a cat photo on Geokittehs earlier today, and it reminded me of anatexis, the process of partial melting. Anatexis is my favorite way to produce a migmatite. In this model, the light-colored (felsic) ginger cat is derived from the partial melting of another cat, partly dark (mafic) and partly felsic (ginger). Where the low-melting-temperature minerals have been extracted, the source cat is much darker. Fresh injections of …
23 May 2012
Fossil Falls fun
A few shots from Fossil Falls, in the southern Owens Valley, California… This is the now-dry river bed of the Owens River. There’s abundant evidence of water-induced erosion (potholes, polishing, etc.), but nary a drop of water to be seen – Though this particular portion of the Owens River drainage dried up in the Pleistocene, it parallels the more recent history of the valley’s water resources, which are now being …
12 February 2012
Fractures on a seismic sign
On the campus of the University of Texas at El Paso, there is a sign for the Kidd Seismic Observatory. The sign has seen some brittle deformation of its reflective letters. I infer this due be due to differential shrinkage of the letters (relative to the sign they are attached to) in the intense Texas heat. I like seeing deformation in materials other than rocks. This is a neat example …
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.