13 July 2010
Folded Archean gneiss
Posted by Callan Bentley
Here’s a nice sample of folded gneissic banding in basement complex of the Gallatin Range, Montana. According to Dave Mogk of Montana State University (who I met in person for the first time on Saturday), these high grade metamorphic rocks are as old as 3.2 Ga (billion years old). They experienced temperatures as high as 750 degrees C, and pressures equivalent to 25 kilometers of depth in the crust. The lovely folds were what caught my eye, however:
I was mapping this stuff today in the Pioneer Mountains. Pretty old stuff.