Shenandoah geology class
On Saturday, I took a group of NOVA summer school students to Shenandoah National Park to look at some rocks. We had great weather, and saw evidence of Grenvillian mountain building, the breakup of Rodinia, and the transgression of the Sauk Sea. A real crowd-pleaser was an outcrop of what was once columnar basalt (the Catoctin Formation). I say "was once" because the basalt has been metamorphosed to greenstone, and the columns have been squashed into more lathe-like shapes.
Here's a few photos of the columns:
Here's a few photos of the columns:
Columns like these form as the cooling mafic lava contracts a bit in volume as it loses heat. This causes a series of fractures to form. The intersection of the fractures defines the hexagonal columns. We've seen the same phenomenon at the Giant's Causeway, Northern Ireland. When the lava of the Catoctin Formation was extruded, Rodinia was breaking up and the Iapetus Ocean was being 'born.' One of the things I really like about the Shenandoah columns is that, even though they're metamorphosed, they show clear 'ribs' on the side -- arrest lines as the propoagating fracture worked its way down into the flow. An inch at a time, the columns grew.
Finally, here's me doing some "arm waving" amongst rapt students (ha!):
All four photos by Nicole LaDue (NSF). Thanks Nicole!
Labels: blue ridge, field trips, primary structures, shenandoah, teaching
2 Comments:
What makes the fracture pattern survive metamorphism? Did the fractures become filled with veins, or is there some other mineralogical difference between the fractures and the centers of the columns?
I've never seen fractures that survived metamorphism before. (And I would never have thought that columnar jointing could be used as a strain marker in greenschist-facies (?) rocks.)
Yeah, it's unique in my experience too. Kind of weird. No, there's no vein-filling of the fractures. I did not note any mineralogical differences either. Pretty cool, eh? If you're ever out here, I'll tell you how to find them for yourself...
C
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