4 May 2012
Friday fold: ball & pillow in volcanic ash
Posted by Callan Bentley
Another guest Friday fold – this one supplied by Ander Sundell of the College of Western Idaho, and his student Katie Ursenbach, who took the shot and gave permission for me to re-post it here.
You’re looking at cuspate-lobate folds due to primary sedimentary settling. the Snake River Canyon in southwestern Idaho. A pile of volcanic ash erupted into water in the Paleogene, and soon after covered by a series of basalt flows while it was still wet.
Dense, viscous layers sank downward into bulbous lobes, and in between them, less viscous, less dense material squirted upward into “flame” like structures. This stretches the Friday fold into a new realm: primary sedimentary features. This isn’t a tectonically-induced structure, but it’s cool anyhow!
Thanks to Ander and Katie for sharing!
just to add a detail:
the viscosity doesn’t actually determine whether flame structures will form, that’s totally density-driven. the relative viscosity will help determine the shape though.
good photo, thanks callan, ander and katie!