Unravelling a Volcano Using Crystals Under a Microscope

Normally at this time of the week, I look at volcanoes from far away - usually not even from the planet. However, this week, I'm going to go in the opposite direction* (so, if you want to skip ahead to the active volcanoes, go ahead). I'll be looking at a volcano from up close, so up close that you need a microscope to see these details. This isn't a normal microscope, but a petrographic microscope that utilizes the special optical properties of minerals cut thin, down to ~30 microns thick (we call them "thin sections"). Light will pass through many minerals at that thickness but the crystalline lattice of the mineral will refract, or bend, the light. The trick is you need polarized light that is vibrating in a single direction. So, if you stick a mineral in a light beam that passes through one polarizer before the mineral and one polarizer after the minerals, the refraction, specific to each mineral, will cause the minerals to have a variety of colors and other optical properties. When I want to know what happened to minerals in a volcanic rocks before the rock erupted, I look at the minerals in thin section. They can show me textures and reactions that betray events such as reheating, mixing of magmas, cooling and even the process of eruption itself.