Quartz & fractures on the Billy Goat Trail
This weekend, my MSSE advisor John Graves was in town, and I took him out to a couple of field locations that I bring geology students to. We started off on Saturday afternoon on the Billy Goat Trail, where I went through the usual rigamarole, what with the Iapetus Ocean, Taconian Orogeny, migmatites, and what-not.
We also saw some cool fractures involving quartz, in two different situations, each instructive in its own way.
First, here at the base of the legendary "Traverse," is some metagraywacke that has fractured. Quartz-rich fluids flowed along these fractures, and the quartz they precipitated (presumably in interstitial spaces between grains?) made that particular zone on either side of the fracture more resistant to weathering than the non-quartz-infused metagraywacke. This "fortifying" effect falls off with increased distance from the fracture. Note that you can actually see the crack in each of these high-relief ridges; it's not a quartz vein per se, but a separate, related phenomenon. Penny for scale in both photos below -- one zoomed out, one zoomed in...
Second, check out these photos, of a spot near the downstream end of the Billy Goat Trail, where usually I don't have time to take students. The bedrock here is a migmatitic schist/gneiss. Here, you'll see ~vertical foliation cut by a ~horizontal quartz vein. Once again, a penny is for scale (this time held in place with some chewing gum, as the outcrop surface is vertical, striking at a right angle to foliation). These two structures are both representative of the same stress regime. With a dominant (tectonically-induced) stress directed ~horizontally, the various minerals in the original rock rotated (or grew) into new positions perpendicular to that stress (e.g., ~vertical). But that wasn't quite enough to accomodate the ~horizontal shortening. Some additional strain was accomodated by ~vertical extension through fracturing. That fracture was infilled with hydrothermal fluids that precipitated "milky" quartz, at almost a perfect right angle to the foliation:
John was suitably impressed, and we both appreciated the afternoon hike in EXCELLENT weather (55 degrees F; gorgeous!).
We also saw some cool fractures involving quartz, in two different situations, each instructive in its own way.
First, here at the base of the legendary "Traverse," is some metagraywacke that has fractured. Quartz-rich fluids flowed along these fractures, and the quartz they precipitated (presumably in interstitial spaces between grains?) made that particular zone on either side of the fracture more resistant to weathering than the non-quartz-infused metagraywacke. This "fortifying" effect falls off with increased distance from the fracture. Note that you can actually see the crack in each of these high-relief ridges; it's not a quartz vein per se, but a separate, related phenomenon. Penny for scale in both photos below -- one zoomed out, one zoomed in...
Second, check out these photos, of a spot near the downstream end of the Billy Goat Trail, where usually I don't have time to take students. The bedrock here is a migmatitic schist/gneiss. Here, you'll see ~vertical foliation cut by a ~horizontal quartz vein. Once again, a penny is for scale (this time held in place with some chewing gum, as the outcrop surface is vertical, striking at a right angle to foliation). These two structures are both representative of the same stress regime. With a dominant (tectonically-induced) stress directed ~horizontally, the various minerals in the original rock rotated (or grew) into new positions perpendicular to that stress (e.g., ~vertical). But that wasn't quite enough to accomodate the ~horizontal shortening. Some additional strain was accomodated by ~vertical extension through fracturing. That fracture was infilled with hydrothermal fluids that precipitated "milky" quartz, at almost a perfect right angle to the foliation:
John was suitably impressed, and we both appreciated the afternoon hike in EXCELLENT weather (55 degrees F; gorgeous!).
Labels: metamorphism, msse, piedmont, quartz, structure
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