15 February 2010
Check out these cool structures in one of the amphibolite bodies exposed along the Billy Goat Trail (C&O Canal NHP, near Potomac, Maryland):
Those are shear bands — basically small shear zones that are discretely localized within a larger body of less-deformed rock. Note the grain-size reduction visible in the shear bands, their dextral sense of offset, and their induration (making them more resistant to the forces of weathering and erosion: they stand up at least a centimeter higher than the rest of the amphibolite outcrop). We have seen a larger indurated shear zone before.
Note that the upper photo is truncated by the format of this blog template — click on it to go to the original image on Flickr, which allows you to see the sense of scale, and a wider view.
Here’s a cool YouTube video showing the process by which these things form (in a nice conjugate set given a homogenous material and plane strain):