structure, tectonics

How do you make a green rock?

Typically when we think of rocks, we don’t often picture them as having vibrant hues. Minerals themselves often come in a spectacular array of colors, but rocks overall tend to have more muted tones and earthy colors.

These rocks (pictured above) from the Buckskin Mountains in western Arizona stand out in stark contrast to the brownish and beige surrounding rocks due to their bright green hues. They owe their green coloring to the minerals chlorite and epidote, which are abundant in greenschist facies metamorphic rocks.

Greenschists are metamorphic rocks formed under low temperature and pressure conditions. These particular rocks from the Buckskins are called chloritic breccias formed by metamorphism during detachment faulting that resulted in ~50 km of displacement. Detachment faults are associated with large-scale extensional tectonics and form metamorphic core complexes, where plutonic/metamorphic lower-plate rocks are brought to upper crustal levels and lie beneath unmetamorphosed upper-plate rocks.

The zone of chloritic breccia directly beneath the fault underwent chlorite-epidote alteration to become greenschist facies during deformation, which is why they are now such a lovely green color!

All images by author // Chloritic breccia below detachment fault; Parker, AZ // Detachment Fault cross-section

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