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26 March 2014
The Great Unconformity in the Franklin Mountains
Good morning! Let’s take a walk up the east side of the Franklin Mountains, north of El Paso, Texas, to walk across the Great Unconformity. The basement rock exposed here is the Red Bluff Granite, a 1.1 Ga felsic magma that intruded the columnar basalts of the Mundy “Breccia” and the Castner Marble. (It is unknown what substrate the Castner Marble was deposited upon.) This is what the Red Bluff …
24 March 2014
Monday migmatite
Here’s a sweet little sample of migmatite (~470 Ma late Ordovician Taconian Orogeny, U/Pb date from zircon), that my students and I spotted last week on the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province: Note the white translucent quartz, the orangey (partially kaolinitized and rusty stained) opaque potassium feldspar, and the shreds of biotite torn and tangled between them: I love the fact that I …
7 March 2014
Friday fold: Obsidian on display at ASU
Happy Friday! Here’s a beautiful folded obsidian sample, replete with conchoidal fractures, on display outside the geology department at Arizona State University in Tempe, where I was last week:
2 March 2014
A chip off the ol’ charnockite
We visited the Philip Carter Winery this weekend with family. Baxter and I were pleased to see outcrops of charnockite scattered over the property (located in the middle of the Blue Ridge geologic province). As any 18-month-old will tell you, charnockite is a pyroxene-bearing granitoid. It’s a distinctive and common rock type in Virginia’s Proterozoic basement complex. Here’s a close-up: The dark green is pyroxene. The white is plagioclase feldspar. …
14 February 2014
Friday fold: Valentine’s Day at Newberry Caldera’s big obsidian flow
Scott Johnson contributed this special “Valentine’s Day” edition of the Friday fold: a lovely primary igneous structure that evokes a heart: That’s a close-up view (lens cap for scale) that Scott took when he GigaPanned this feature: link … the big obsidian flow at Newberry Crater in Oregon. That’s a really dry lava flow that oozed out of the Earth sometime in the past thousand years or so. It’s even …
26 November 2013
Geology of RdV vineyard and winery
Last month, I led a fun new tour for the Smithsonian Associates: an all-day tour of the geology of Virginia wine country. Wine-making is a bigger-and-bigger business in Virginia these days, and I’ve been exploring it on and off over the past few years, ever since participating on a wine-themed geology field trip through the Geological Society of Washington. So we got a bus, and I waxed on about the …
1 November 2013
Friday fold: GSA display sample
At the GSA booth of Kansas State University, they had some nice specimens of rock on display. Here’s an example of a fold that caught my eye: rhyolite (flow banding, I presume) from the Owyhee Mountains: According to the gentleman manning the booth, the sample belongs to Dr. Matt Brueske. Happy Friday!
3 October 2013
Sand fom Þorlákshöfn, Iceland, under the GIGAmacro lens
I think this one of the most fascinating batches of sand we’ve yet had the pleasure of macro-GigaPanning: link So much igneous goodness hidden in those grains, collected from a beach on the south of Iceland… The image was made by Robin Rohrback-Schiavone (my student at NOVA) as part of the Mid-Atlantic Geo-Image Collection (M.A.G.I.C.). The sand sample was donated by my colleague Beth Doyle. Thanks to both of them!
16 April 2013
Squashed pillows
Join Callan for a look at submarine volcanism and later deformation near Ely, Minnesota. Archean tectonics fluffed these pillows up and squashed them down again.
24 November 2012
Rock Cycle III: Igneous → Sedimentary
The Boulder Batholith outside of Butte, Montana, is actively weathering, and shedding off grus. In the third installment of the Transitions of the Rock Cycle series, we watch an igneous rock turn to sediment.
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.