December 4, 2019
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Note: Yes, this really involves yogurt…specifically Greek yogurt. If that’s what you came to see, scroll down several photos.
En echelon tension gashes are a crowd favorite for geologists and non-geologists alike. Their orderly patterns look really interesting on a rock face, and common gash-filling minerals (often quartz or calcite) tend to be white or very light colored, making the gash sets stand out. The example below is found in lowermost Cambrian quartzite exposed in the French Broad River in western North Carolina.
The dramatic black-and-white examples here are from the Tavignanu River southeast of Corte, in Corsica. I somehow failed to take pictures of even better examples in this area; many were absolutely stunning.
Tension gashes are geologically interesting because their development and orientation reflect the stress field experienced by the rock mass while it was still buried. The tips of the gashes, which are Mode 1 fracture type, point towards the orientation of maximum compressive stress, and the gashes open towards the minimum compressive stress (see 2 photos down). The really cool en echelon sets are associated with shear zones which develop at an acute angle to the maximum compressive stress. Movement on the shear zone stretches the rock, promoting gash opening towards the minimum stress, and can also deform the gashes into “S” (see picture above) or “Z” shapes once they have formed. Subsequent gashes can grow across the “S”s or “Z”s (look towards the bottom of the first picture in the post) to indicate incremental deformation. Mineral material filling and preserving the gashes precipitates from high-pressure fluids present in the rock mass when it is still deeply buried; the fluids move into the space made available by gash development.
Tension gashes and their relationship to a stress field are ubiquitous in structural geology textbooks, and a Google search of the term will serve up an endless supply of very impressive real examples and annotated diagrams. I think the block diagram shown below does a good job of offering context; its source is linked in the caption. Note the intersecting conjugate shears/faults and the “X” shape…this pattern shows up frequently in the video. Being able to understand stress orientation and fracture tendencies (or history) has plenty of application, both in terms of modern-day stress field in rock still deeply buried (deep injection of fluids for various purposes, for example) and in exhumed rock at the surface (potential planes of weakness or failure, from an engineering standpoint).
As a student, I thought the gash patterns were great, but I always struggled to understand their relationship to shear zone orientation in the context of the stress field. I visualized someone grabbing either side of the zone and inducing the shear, such that the “maximum stress” would be parallel to the shear zone because its movement was being forced. Because I could not picture the shear zone developing at a material-specific angle to the maximum compressive stress, the angle between the gashes and shear didn’t make much sense to me. I needed a moving visual to watch where I could see shear (and fracture/gash) formation occurring due to compression from an obvious orientation. This is where the yogurt comes into play. Some very nice en echelon tension gash sets appeared in my Greek yogurt a few weeks ago when I squeezed the plastic container.
I thought these were really nice. When the yogurt container is squeezed, its rim is deformed from a circle into an ellipse. Maximum compression produces the short axis of the ellipse, which grows outward (long axis) towards the minimum compressive stress (general idea can be seen in the video). As the shape of the yogurt contents change, tension gashes and shear zones develop to accommodate the change. I messed around with squeezing various other brittle materials in flexible containers to try to produce a comparable and classroom-expedient visual. Yes, these experiments are unconfined. Yes, they lack pressurized, solute-rich fluids at high pressure, etc. etc. Even so, the development of geometric and oriented open fractures and shear/extensional shear zones is easy to appreciate, as is the stress field and mass deformation that produces them. I wish I had been able to do this as a student way back when…
So, in a video that is every bit as dry as the flour and cornmeal used to make it, here are some cool geometric fracture networks that form in various readily available (i.e., edible), cohesive brittle materials. Yogurt is at the end of the video.
As stated above, these are just basic visuals to try to give context for how certain outcrop-scale features reflect the stress field experienced by a rock mass. I tried actively squeezing a round flexible container, as well as loading material into a container that was already squeezed, which was then released. This second setup created some cool fracture networks as well as apparent en echelon features, which were sometimes destroyed by subsequent shear movement. These are shown in the photos below, and can be seen in a sequence starting at 1:46 in the video (as well as the first example shown). Be careful with the 1:46 example; it may cause motion sickness.
“X” patterns created by conjugate shear sets were very common. These offer a nice visual indicator of stress field orientation, as the maximum compressive stress bisects the narrow ends of the “X.”
Using these cohesive brittle materials, the deformation features remain after squeezing as long as the shape of the container does not re-deform them when it is released. It would thus be possible for students to determine paleo-stress orientation by looking at patterns on the material surface even if they didn’t watch deformation happen.
Cornmeal was mechanically the best dry material, but its patterns are more difficult to see due to its grainy appearance. The features produced in the yogurt, however, were by far the coolest and most “realistic” looking. That said, you can only deform yogurt once, and the uneven surface produces local variations in stress orientation and response to deformation. That being said, I will always have camera in hand upon opening a Greek yogurt container from this day forward.
Some more pictures from the French Broad River:
French Broad River outcrop locations: 35.881175N 82.771175W , 35.855342N 82.751756W
Approximate Tavignanu River location: 42.218195N 9.276694E
This post was first published on The Geo Models blog.