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24 May 2019
Friday fold: Riffelhorn (+xenolith)
Naomi Barshi shares this “deskcrop” Friday fold… Naomi says she found this fold: near the Riffelhorn, Gornergrat, above Zermatt, Switzerland. The sample has befriended my other show-off sample of a mantle xenolith from San Quintin, Baja California. Thanks for sharing, Naomi! The xenolith is a nice bonus!
12 January 2018
Friday folds: Hayden Butte (“A Mountain”), Tempe
In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has …
15 July 2017
Flow-banded rhyolite from Vulcano, Italy
I collected only a single rock on my summer travels in France and Italy. (Those of you who know me will realize how extraordinary this low number is!) It’s a flow-banded rhyolite from Vulcano, in the Aeolian Islands north of Sicily a few weeks ago. It contains porphyritic vesicular basalt xenoliths. I featured a similar sample on Twitter yesterday on the occasion (supposedly) of “International Rock Day”: For #InternationalRockDay , …
15 April 2017
Basement xenoliths in Catoctin Formation, Compton Pass
My son and I hiked Compton Peak in Shenandoah National Park this morning, and saw these two lovely examples of xenoliths. The example above is small, but it shows clearly the difference between the coarse, felsic basement rock (Mesoproterozoic granitoid, comprising the xenolith) and the surrounding fine-grained dark green metabasalt of the Catoctin Formation (Neoproterozoic). Here’s another, bigger example: These two Blue Ridge examples both illustrate the principle of relative …
10 October 2016
Oddball Icelandic rocks, part II: granite!?!?
Silly Iceland! Don’t you know you’re not a continent?
7 January 2015
A mafic sill in Antarctica
My friend and colleague Lauren Michel, the King Family Fellow at the Perot Museum of Nature and Science in Dallas, Texas, sent me this image from her recent trip to Antarctica: (click to enlarge) This is a beautiful example of a mafic igneous sill, probably of the rock known as “dolerite” (or diabase, to us Yanks). Lauren and I think it must be part of the Ferrar Large Igneous Province. …
2 April 2014
The wonder of cracking open a xenobomb
On “Border to Beltway”‘s visit to Kilbourne Hole, after we whet our appetite with Hunt’s Hole, Michael finds a xenobomb. Ernie and Boris look on with envy: A “xenobomb” is a xenolith (in this case, of mantle peridotite), slathered in a coating of lava and tossed out of a volcano in the middle of a liquid droplet (a bomb). Here’s what they look like in cross-section: You can experience some …
16 May 2013
GigaPanning Kilbourne Hole
Kilbourne Hole is the crater of a maar volcano in southern New Mexico, just across the state line from El Paso, Texas. I went there the weekend before last with a team from El Paso Community College, led by Joshua Villalobos. This is the place where xenobombs come from! If you go to the right area, you can find dozens of these mantle xenoliths sheathed in fine-grained basalt, like chocolate-coated …
9 May 2013
Boudinage in northern Minnesota
This is my final post on the pre-GSA-Minneapolis structural geology field trip to the Superior Province. The photo above shows a roadcut exposure of boudinage in xenolith-bearing Vermillion Granitic Complex. Here’s another, smaller, more brittle example of boudinage, from another site, the following morning: Gee, it only took me 1.5 years to blog that trip in its entirety… Sheesh!
14 December 2012
Friday fold: the folded xenoliths of Duck Creek
On Duck Creek (Ellendale, NC quadrangle), you can see folded xenoliths within the Toluca Granite (383 Ma, 378 Ma, or 368 Ma, depending on which mineral you ask). The granite there contains xenoliths that contain pre-exisiting fabrics and structures, and we stopped at Duck Creek on the pre-GSA-Charlotte field trip I took to the Neoacadian Inner Piedmont to check out these xenoliths. Here are some less impressive xenoliths, strung out …
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.