Friday Rock Blogging: Gabbro
I have a confession to make: I have absolutely no idea what this picture means. And most of you probably don’t either, which is okay, because you’re not running around the Internets pretending to be a geologist.
This is what’s colloquially known as a thin section – a piece of rock sliced so thin that you can shine light through it, and then stare at it under a microscope until you get a headache. Minerals that look similar in a hand sample will refract light very differently in thin section, which makes it useful for obsessive mineral-identifiers. You can also use thin sections to see anything interesting that might have happened to the crystals themselves – changing conditions as they grew, deformation afterwards, all sorts of things. Thin sections are part of the standard geology toolset.
As a geophysics major, though, I never had to spend umpteen hours in petrology lab learning how to interpret the things. And as it turns out, this is not the sort of skill you can just pick up by osmosis at colloquia… especially not at seismology colloquia.
So I’m putting this up as an example of gabbro, but for all I know it is Portland cement.
- The mineral grains are big enough to see with the naked eye
- It has some plagioclase and some dark stuff
- It doesn’t have any quartz
- It has a label
If you take a bit of the Earth’s mantle, heat it until it just starts to melt, and then store the resulting liquid someplace where it can cool very slowly, you get gabbro.