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18 November 2011
Friday fold: mafic metavolcanics
Okay – in spite of numerous distractions (see every other post so far this week), it’s time to return to the pre-GSA Minneapolis structural geology field trip. Our final stop of the second day in the field was a series of folded up mafic metavolcanics. I’ve got some photographs of them. These mafic volcanics were strained in a relatively incompetent (weak) fashion between two more competent (stiff) gneiss domes in …
7 November 2011
Pseudoboudins!
You may recall that I kind of like boudinage. So it piqued my interest when our field trip leaders (on the pre-GSA Minneapolis trip to examine the structural geology of the sub-province boundaries in the Superior Craton) said our next stop was to visit “pseudoboudins,” segments of granitoid pegmatites that looked like boudins but probably represented a totally different process. Here’s the idea: With regular old boudinage, the solidification of …
2 November 2011
The Ottertail Pluton
After the awesome outcrops and pavements of strained metaconglomerates from the Quetico / Wabigoon subprovince boundaries of the Superior Craton, my pre-GSA field trip visited the most charmingly-named magma chamber I’ve ever seen, the cuddly-sounding Ottertail Pluton. This is an Algoman-type pluton which is discordant to tonalite-composition gneisses in the area. As with the Giants Range Batholith that we saw near Virginia, Minnesota, the Ottertail Pluton shows lots of cool …
19 October 2011
Bookshelfed mullions from Norway
Elizabeth Eide of the National Academies shared this image with me a year or two ago, when she gave a talk on Norwegian geology for the Geological Society of Washington. Those are some bookshelfed gneiss layers sandwiched between upper-left and lower-right carbonates. The “bookshelfing” refers to the numerous parallel brittle fractures along which the gneiss slabs have slid down the face of their neighbors, like a set of encyclopedias tilted …
14 October 2011
Friday fold: isoclinal dextral asymmetric granitoid vein
The “Friday fold” is an isoclinal dextrally-asymmetric granitoid vein exposed north of Fort Frances, Ontario. Taken on October 7, 2011, the photo features a centimeter-demarcated pencil for scale. It shows thickened hinges, boudinage of the lower right long limb, and incipient boudinage of the upper left long limb.
12 October 2011
The GSA meeting experience, 2011
I’m on the plane home from the annual meeting of the Geological Society of America, held this year in Minneapolis, Minnesota. This annual event features a robust smorgasbord of science, with talks and posters detailing the research efforts of thousands of geoscientists from the US and other countries. It’s an amazing experience on many, many levels, and as I fly home now after a week in Minnesota, my feelings are …
28 September 2011
Roadside wonders of Route 287
Northern Colorado’s route 287 connects Fort Collins, Colorado with Laramie, Wyoming. Along its length, it displays roadcuts into Archean-aged basement complex. Two of these outcrops are featured in this post: one metamorphic (mostly), and a second igneous (mostly), with some intriguing polka-dotted plutons.
1 August 2011
My wife with a boudin
Lily poses with amphibolite boudin (showing extensional veins) on the road in Kootenay Bay, British Columbia.
10 July 2011
Cascade Canyon
Following up on Friday’s fold, I wanted to share a few other images from our hike last week in Cascade Canyon of Grand Teton National Park. Lots of cool exposures of Archean basement rock there. Folds in gneissic banding: Big, angular mafic blocks in a felsic soup: Swarm of granite dikes on a mountainside: Asymmetric feldspathic porphyroclast in amphibolite, with apparent reaction rim: Boudinage and an isoclinal fold: Ptygmatic fold: …
20 June 2011
“Boudinage” is my favorite geology word
The current edition of the Accretionary Wedge geology blog carnival (hosted by Evelyn Mervine of Georneys) is built around the theme of favorite geology words. My favorite geology word is derived from the French boudin, for sausage. It’s “boudinage,” and it’s best said with a heavy French accent and a leering, dirty expression. “Boo-din-ahhdj” I love pronouncing it; it’s a delicious word, like a good boudin itself. So what is …
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.