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3 July 2012
Granite dikes that have been folded and boudinaged
It seems like a good morning to return to Ontario, and to the Archean shear zones exposed at the Quetico/Wabigoon subprovince boundary of the Superior Craton. Readers will hopefully recall that I spent several days absorbing structural goodness from these rocks on a field trip before the Minneapolis GSA meeting last fall. The trip was led by Basil Tikoff, Bob Bauer, Dyanna Czeck, and Peter Hudleston. Our final Canadian stop …
25 June 2012
Leopard rock
Callan shares some photos of “leopard rock” (porphyritic mafic dikes) seen along the Beartooth Highway, northeast of Yellowstone National Park.
29 May 2012
Snowball students visit the University of Maryland
The final meeting of my spring semester Snowball Earth class was a field trip to the University of Maryland, hosted by Snowball guru Jay Kaufman, a specialist in chemostratigraphy using stable isotopes. Here, Jay welcomes the class to his wet lab: Doing chemostratigraphy takes lots of samples. Here’s a drawer full of samples from one of Jay’s many field areas: Then it was time for show and tell. Jay brought …
2 May 2012
Veins perpendicular to foliation
To recap the week so far here on Mountain Beltway: On Monday we looked at some sweet vertical boudinage along the plane of tectonic cleavage (not to mention those folds in a (formerly) horizontal granite dike, now bearing vertical axial planes), and then on Tuesday we looked at a horizontal cut through that same outcrop, and it showed moderately distorted lava pillows, And now today, let’s look at one more …
1 May 2012
Deformed pillow basalts from southern Ontario
Have you ever seen Archean pillow basalts? How about Archean pillow basalts that have grown much taller through vertical extrusion along a ductile shear zone? Come to the southern Superior Craton to see more!
30 April 2012
Exquisite boudinage in Ontario
Callan shares images of extraordinary boudinage outcrops in strained Archean meta-basalts on the Quetico-Wabigoon subprovince boundary within the southern Superior Craton. Ogle these gorgeous structures in awe and wonder.
26 April 2012
Slickensides on a fault cutting granite
Another shot from my pre-GSA structural geology field trip to the Superior Craton last fall, up in southern Ontario, Canada. The image shows slickenlines coated with brownish fault gouge and oxide staining, cutting through a high-potassium granite (see fresh surface at upper left). The slickenlines are little gouged grooves where asperities (bumps) on the block of rock opposite this one ground into this rock along the fault surface as the …
28 February 2012
Kopjes
Spheroidal weathering in Kruger National Park, South Africa. This outcrop is Archean granite of the Kaapvaal Craton. It’s producing a nice little inselberg in the low veld; good klipspringer habitat. “Kopje” is the word I learned to call these things in East Africa, but I guess the proper Afrikaans spelling is “koppie.”
23 February 2012
Contorted fractures in the Contorted Bed
Today, let’s zoom in on a little something I saw at the outcrop of the Contorted Bed in downtown Johannesburg, South Africa: What are these things? I only saw them at this one spot. These look to me like en echelon fractures at a high angle to bedding, that were then deformed due to dextral (top to the right) shearing. But they’re really closely spaced, and they show no apparent …
6 February 2012
Folded BIFs of Soudan, Minnesota
Why are these people smiling? photo by Yvette Kuiper Because they are structural geologists, and they are psyched to be at an extraordinary outcrop: This is a famous pavement outcrop of polyphase-folded banded iron formation (BIF) near Soudan, Minnesota. I went there last fall before the annual meeting of the Geological Society of America, on the Structural Geology of the Superior Craton’s sub-province boundaries field trip. “Oh YEAH!” What you’re …
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.