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Second Favorite Geology Word: Crenulation – UPDATED

“Boudinage” and “transtensional” are #1 in my geo-lexicon, but Callan Bentley took boudinage and I’ll leave “transtensional” for another day as it was the topic of my first graduate thesis. “Crenulation” it is for Accretionary Wedge Episode 35: What’s Your Favorite Geology Word? This comes from my fascination with polyphase deformation, or multiple episodes of deformation, leaving their imprints on rocks in the form of cross-cutting fabric.

I used to explain crenulation to my students with their own palms, as shown in the image below. Start out with your palm facing up. Then, cup your palm until you see creases (folds) in it. Following that, take your other palm and push your folded palm in a direction roughly perpendicular to the folded palm’s fingers. See the interference pattern as new folds (D2) overprint the old ones (D1).

1. Flat open palm 2. Folds (indicated by blue lines) form perpendicular to fingers when hand is curled 3. Palm is folded again in direction perpendicular to previous curling. New folds (red lines) form; note deformation of previous folds.

No demonstration of deformation is complete without Play-Doh, which is luckily available from VatulBlog HQ’s always freshly-stocked shelves. (Gotta love the warning: Fun to play with, but not to eat. NOTICE TO PARENTS: CONTAINS WHEAT.)

Left: Folds form in Play-Doh roughly perpendicular to direction of squishing (yes, that's a technical term, and indicated by the black arrow; black lines are fold axes). Right: Second episode of deformation (indicated by red arrows and red fold axes) overprint previous folds (note that black fold axes are now folded themselves).

Now, my hands smell like Mmmm Play-Doh. Is there anything it cannot do?

Earlier, I mentioned the term “fabric.” Rock is not a homogeneous, isotropic substance like Play-Doh, but instead a collection of minerals that have different physical and chemical characteristics and geometrically rearrange themselves differently under varying stress-time/pressure-temperature conditions. Therefore, crenulation is best viewed at the microscopic level. At this scale, we can see what really happens to the rock’s constituent minerals when they undergo successive episodes of deformation.

Metamorphic thin sections in the next exciting installment of CRENULATION!

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