Window Into A Microscopic World

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

Katherine Hanson

Hey there, I'm Katherine Hanson, the curator of watchingforrocks.com, a site dedicated to uncovering the hidden gems of the USA. With a passion for exploration and a love for discovering the beauty in every corner of this vast country, I'm on a mission to share the best cities, national parks, historic landmarks, and entertainment hotspots that the USA has to offer. From towering mountains to bustling cities, there's so much to see and experience. Join me as I embark on adventures and uncover the wonders that make America truly remarkable.
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