Finding foliation

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Looking down the wadi, image courtesy of Pablo Yañez

The past few weeks have been primarily focused on understanding and beginning the preliminary processes of quantifying strain in the rocks that I collected while abroad in Oman. Strain is the change in the shape (either 2D or 3D) of an object due to deformation, and in my area of study the rocks have enjoyed quite a bit of deformation. You can see evidence of deformation in both the macro and micro scale in rocks, which (to me) makes them so fascinating to study. Deformation is mainly caused by tectonic processes; either the shortening, elongation, distortion, or rotation of rocks due to stress. The stress in the earth’s crust originates from tectonic movement, with the buildup of pressure within the crust causing rocks to either bend or break as a result. The “bending” part of rocks can be seen in folding and foliation, while the “breaking” can be seen in faulting and fractures.

On the macro scale, we see foliation. This is a planar feature in rocks that can most easily be visually described as a layered pattern, however it is created when rocks have a plastic texture and the particles in the fabric of the rock realign to form planes. Many of the rocks in my area display foliation planes and much of my work so far has been determining the orientation of the foliation and mapping it on a geologic map to create a general overview of the current structures in the study area.
Foliation

On the micro scale, geologists make thin sections, a slice of rock around 0.03mm thick attached to a glass slide that can be observed under a petrographic microscope. From that thin section we can determine the microtectonic processes a rock has experienced. We can see evidence of flow patterns, twinning (gliding), diffusion, and creep that can give us clues to the type and extent of deformation that occurred.

I have cut the rocks down into slabs, sent a few samples off to be made into thin sections, and am currently awaiting their return. In the meantime, I have been researching the extensive mineralogy of the rocks in my study area so that I can better understand what I will see in thin section when they return.

Comments

  1. Hey Cece! It sounds like stress and strain are very much related in a causal relationship. I can imagine that the temperature and pressure that the stress occurs at has a heavy impact on what type of strain occurs (bending versus breaking)and to what degree it occurs. Similarly, deformation sounds a lot like what I’ve learned about metamorphism. Perhaps they’re easily confused amongst novices?
    I’ve always wondered how geologists make thin sections of such hard rocks as granites and gneisses. While I’ve seen the saws used, it’s hard to imagine that something could really break rocks in a precise manner. I’d love to hear more about how you deal with these tools!
    It sounds like you made quite the Odyssey to map out the geology of this region!