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A Geologic Mystery: #ThinSectionThursday and #TBT… on Friday

Geologists like me like to cut and grind thin slices of a rock [1] – thin enough to see through – and look at them in a microscope using polarized light. The polarized light allows us to identify minerals more easily than we can by eye [2]. The thin slice of rock is called a thin section. When made with care, thin section photomicrographs can be beautiful (in this geologist’s opinion). Geologists have been posting them on Twitter under the hashtag #ThinSectionThursday for a while. For example:

Thursday was also #tbt (throwback Thursday), and this #thinsectionthursday photo fills both niches. This is a slide I found in my old desk when I was a grad student at the Scripps Institution of Oceanography. The desk used to belong to Gustaf Arrhenius, I think.

Slide of "Rock 17" thin section, back. Inscription: "Copy Transparency / Fritz Goro / Reproduction Prohibited Without Permission"

Slide of "Rock 17" thin section, front. Inscription: "gas bubble/ 40x / 62 / Rock 17"

“Rock 17” contains gray and white striped crystals, which are plagioclase feldspar (a calcium-sodium-aluminum silicate), as well as blobby blue-orange crystals of clinopyroxene (a calcium-iron-magnesium silicate). There are some black blocky particles of opaque minerals (probably the iron oxide magnetite) and at least one gas bubble or vesicle (the black round thing in the middle). The rock’s composition is therefore similar to that of a basalt, one of the most common rock types on Earth. The crystal sizes are difficult to determine without a some sort of a scale [3]. However, given the presence of the vesicle, I think this was probably erupted on Earth’s surface or on the seafloor. That would mean that the rock was cooled fairly quickly and the crystals are probably relatively small. I’d go with basalt as the name of this rock.

Where did it come from? One clue is the identity of the photographer: Fritz Goro. Goro was a well-known photographer around the middle of the 20th century whose work focused on science and scientists. His photos often illustrated articles in Life magazine. Goro took this photo of Arrhenius (along with Roger Revelle and others) on the Project Mohole cruise, an early attempt at scientific ocean drilling. I suspect this rock was one of those recovered as the 1961 drilling project attempted to drill through Earth’s crust into the mantle. Although Project Mohole failed in this respect, it became the foundation for the Deep Sea Drilling Project, which became the Ocean Drilling Program, and later the Integrated Ocean Drilling Program. The effort continues today as the International Ocean Discovery Program – the program in which I begin participating at the end of this month. [4]


[1] See Dave Hirsch’s page on making thin sections. We can do this, with the initial stages here at UW Tacoma and the finishing at University of Puget Sound or Pacific Lutheran University.

[2] We can do this, too: we have a Leica DM 750 petrographic microscope in the lab available for student use. Basic principles of petrography are presented in this document. To learn the details of how to use the scope, take TESC 347: Earth Materials.

[3] The “40x” inscription doesn’t help much – that’s just the microscope’s objective lens magnification. The microscope, camera and reproduction process also introduce some enlargement or reduction.

[4] I’ve been hesitating to post this because of the label on this slide, but I think my photo probably constitutes fair use in that it’s not for the purposes of reproducing the image. I took a photo of the slide itself, as an artifact.

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