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.
All of the images below come from lavas erupted from Aucanquilcha in Chile (see above), a composite volcano that was mainly active from ~1.05 million years ago to the recent past (although likely it hasn't erupted for a few thousand years). I did my Ph.D. research at Aucanquilcha and beyond being an "extreme" location - the summit is ~6176 meters / 20,200 feet - it also has some amazing mineral textures. Let's take a look. One mineral that is ubiquitous to almost every lava erupted at Aucanquilcha is amphibole - a class of mineral that includes hornblende and pargasite. Aucanquilcha lavas have both and it differing states of reaction. The first image (see below) shows some relatively "happy" amphibole phenocrysts (crystallized in the magma) and microlites (tiny crystals in groundmass). The phenocryst in this image has a core of biotite mica, already betraying the complex history of magma at Aucanquilcha.
You might notice a scale bar below the crystal - that is 200 micrometers, so some of the microlites are pretty small while the phenocryst is pretty good sized - you would see it easily with the naked eye. We can zoom in on one of these large amphibole crystals and clearly growth bands showing the stages of the crystals growth (see below)
Not all phenocrysts in Aucanquilcha lavas are amphibole. There is a lot of biotite mica as well. Many of the biotite micas also have inclusions of zircon (a personal favorite of mine). Zircon is great for dating as it has abundant uranium and thorium. You can also see a thin reaction rim of pyroxene around the biotite, potentially formed by dehydration of the biotite (they contain a lot of water in the structure) during its ascent prior to eruption.
Finally comes my favorite (see below). This is a classic look of magma mixing, where we find a large plagioclase feldspar crystal apparently inter grown with a highly reacted amphibole crystal. The amphibole is clearly not happy (not in equilibrium with the magma around it) as it has reacted to form an armor of pyroxene and feldspar around it. This rim is likely the product of magma mixing. Now, whether these siamese crystals are truly joined or just a trick of how the thin section was cut is difficult to tell.
All of these minerals tell us a little something about how this magma was formed - and it is all at a scale where each of these reactions might be found in a few square centimeters of rock. (*Note: If you want to catch up on all the eruptions of the week, check out the Smithsonian/USGS Global Volcanism Program's Weekly Volcanic Activity Report.)
