3 December 2010
Friday fold: gneiss one!
Posted by Callan Bentley
One other thing that I noticed during my visit last weekend to the Virginia Museum of Natural History in Martinsville, Virginia, was this fold. As soon as I saw it, I squealed “Oooh! That’s going to be the Friday fold!”
It says something about the crowd I was traveling with that everyone (a) knew what I was talking about and (b) knew before I did that I would photograph this fold and feature it as the Friday fold here on Mountain Beltway.
With no further ado, I give you a fold in gneissic banding of the Piedmont rocks of Henry County, Virginia (i.e., where Martinsville is):
Here’s a closer look, with my index finger tip for scale:
Now your life is complete. Happy Friday.
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.
I’m always interested in the foliation of gneiss. You can often see foliation in marble as well. I’ve never heard or read a good explanation of how and why foliation occurs. Not where or under what conditions, but why. Why is it not homogeneous? Why does it separate into bands of minerals?
It appears to be a migration of minerals to other like minerals. I find myself wondering if the compression energy that exists during formation manifests as a sort of standing pressure wave. Possibly one group of minerals forms or crystallizes out in the peak of the wave and the other in the trough. If you’ve seen a standing wave in a tank of liquid, you can see the surface separate into bands. Just a thought I’ve had.
Can anyone offer an explanation?