Rock thin section: andesite

Photomicrograph of sample 09RDWES301 - an andesite collected during the Redoubt 2009 eruption. A rock thin section is created by gluing a small piece of rock onto a glass slide, then grinding it down to a thickness of 30 microns (the average human hair is about 100 microns in diameter) so that light shines through it when examined under the microscope. In this image, the brown crystal in the center is amphibole - and measures about 2.2 mm long. The white rectangles are plagioclase. The blue areas are void space (the epoxy holding the slide together is dyed blue for easy identification of empty areas). The fine-grained matrix holding the larger crystals together is mostly glass, plagioclase, and pyroxene microlites. Examining thin sections helps volcanologists better understand magma interactions, storage, and ascent.