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13 July 2010
Folded Archean gneiss
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 …