Thin sections of rock only 0.03 millimeter or 0.00118110236 of an inchthick on a glass slide offer us incomparable insight into the geological history of our Earth.
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| Shearing textures in a garnet schist gneiss – crossed polarized view |
As a geology student several years ago I spent hundreds of hours making and looking at hundreds of thin sections (for more details check out the third paragraph in this post). I was mesmerized by these slender slivers of rock that offer a window into a geologically microscopic world. There was always something new to see, something to learn, and so much to marvel at.
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| Shearing textures in a garnet schist gneiss – plane polarized view |
Metamorphic rocks are rocks that have been changed without melting from one type of rock to another under the influence of heat and/or pressure. In school I was totally consumed by the metamorphics in the Beaver Dam Mountains of extreme southwestern Utah. They still maintain a profound hold on my psyche, even though I am no longer a student.
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| Sheared gneiss with dark gray garnet being replaced by brown biotite – crossed polarized view |
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| Sheared gneiss with dark gray garnet being replaced by brown biotite – plane polarized view |
Petrographic microscopes allow us to study the optical properties of minerals. These mineral properties can offer intriguing clues to a rock’s tectonic history involving shearing, uplift, and decompression from pressures and temperatures deep within the Earth.
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| Mark C., captivated by his thin sections |
Can you discern a sense of shearing, a flowing movement in these ancient minerals?
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| Sheared gneiss mylonite, mainly quartz and biotite – crossed polarized view |
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| Sheared gneiss mylonite, mainly quartz and biotite – plane polarized view |
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| Shearing in black garnets and thready multicolored sillimanite – crossed polarized view |
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| Shearing in garnets and thready sillimanite – plane polarized view |
And so from this…
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| Sillimanite breaking apart within a schist – crossed polarized view |
…and this…
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| Sillimanite breaking apart within a schist – plane polarized view |
…and this…
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| Sheared plagioclase breaking apart in a schist-gneiss mylonite – crossed polarized view |
…and this…
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| Sheared plagioclase breaking apart in a schist-gneiss mylonite – plane polarized view |
…we can one day learn how this came to be.
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| Beaver Dam Mountains, southwestern Utah |
