Methods: Processing Eclogite Samples and Structural Calculations

I have just returned from the Lofoten Islands, in northern Norway, and I could not be more excited about the rock samples and measurements I was able to collect for my thesis research. One of my advisers, Professor Chuck Bailey, and I braved the arctic snow, hail, wind, and rain in order to go on multiple traverses across two eclogite-facies shear zones near the town of Nusfjord, on the island of Flakstadøy. We collected twenty-four rock samples of eclogite and gabbronorite (approximately 50 pounds-worth) and many more structure measurements during our time there, as well as a plethora of pictures in order to fully capture the outcrops of the shear zones in all of their snowy glory.

Now that I have returned to the United States, the sample processing phase of my research will commence. The first steps will be to use the rock saw in the W&M Geology Department to cut the rocks into appropriate sizes for making thin sections, or thin slivers of the rock that can be set on a microscope slide and viewed with a petrographic microscope. Once the rocks have been made into thin sections, I can begin analyzing the samples for mineral assemblages and microstructure fabrics. Analyzing the thin sections for structural fabrics and mineral grain shape changes will help in determining the vorticity number of these samples through the Porphyroclast Hyperbolic Distribution method. The Porphyroclast Hyperbolic Distribution method uses the ratio of back-rotated grains to forward-rotated grains in a rock sample to determine the position and angle of the eigenvector boundary between the variably rotated grains. With the eigenvector determined, the angle of the opening of the eigenvector can be used to calculate the vorticity number. As a brief reminder, the vorticity numbers I calculate will tell me if these shear zones experienced simple shear, pure shear, or general shear.

Along with analyzing thin sections, I will also start making structural measurement calculations. I have thus far produced numerous stereonets displaying the foliations and boundary measurements taken at the shear zones, and with these diagrams I will be able to visually notice patterns in shear direction. Additionally, I will calculate the angle between the shear zone boundary and the foliations at different regions of the shear zones (margins, interior, and the spaces in between), and use that to, in part, determine the vorticity number as well. This approach is called the Rs/Θ method, with Θ being the angle aforementioned. Calculating Θ can also be used to determine the displacement the shear zone enjoyed, by using it to further evaluate the shear strain experienced across the zone. Integrating the shear strain values from each place across the zone and accounting for the width of the zone will lead to the calculation of the amount of displacement.

In the coming weeks, I will be able to start to get an idea of the type of shear these rocks underwent based on my samples, measurements, and estimated vorticity numbers. Once the thin sections are made I will be able to calculate vorticity numbers with even more accuracy, and hopefully some of the mysteries surrounding these eclogite-facies shear zones will be unveiled.


Katie Valery points to an eclogite-facies shear zone at her study site in Nusfjord, in the Lofoten Islands.

Katie Valery points to an eclogite-facies shear zone at her study site in Nusfjord, in the Lofoten Islands.


  1. The sense of shear that a rock experienced can tell us how the rocks were deforming deep beneath the surface. You can have pure shear (a flattening of the rock, like squishing a ball of playdoh into a pancake), simple shear (when the top and the bottom of the rock move in opposite direction, as if you were pushing just the top of a deck of cards causing a parallelogram shape as a result), or general shear (a combination of pure shear and simple shear).

    We care about shear because depending on the type of shear a rock experienced, you can estimate how much displacement occurred during the shearing event. Was it meters? Kilometers? It is valuable to know these things so that we can accurately depict the geologic history of an area, or even entire regions, depending on the size of the shear zone. Additionally, my shear zones in this part of the Arctic are particularly interesting because there is an extremely deformed and metamorphosed rock situated directly adjacent to undeformed igneous rock on either side of it. So if the rocks truly were deep enough to be sheared in the eclogite facies of metamorphic conditions (the deepest, hottest metamorphism you can have on this planet), then why do we see no such evidence in the surrounding pristine, undeformed, unmetamorphosed rocks? Did they not reach the same depths? And if not, then how did they get to be surrounding the eclogite shear zone? It is a very peculiar situation found in my particular shear zones that I am researching, and knowing something about the sense of shear and displacement can help us start to unravel these mysteries of how these rocks ended up in the positions that they are in today. I hope this answers your questions!

  2. jngranger says:

    Hello! Your research sounds very exciting! Forgive me because I have next to no geology experience, but what is ‘shear’ and why do we care what kind of shear these rocks are undergoing?

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