South Sister domes, Oregon

South Sister is a composite volcano that is part of the Three Sisters volcanic cluster -- it is also the youngest and most active, but not from the summit crater. The southern flanks of the volcano are covered in a series of rhyodacite (slightly less silica than rhyolite) domes that are almost obsidian. I say almost because you can actually find crystals in many of these lavas, but it can be challenging. The domes are separated into Rock Mesa and the Devil's Hills Chain. The former likely erupted ~2350 years ago producing a VEI 4 eruption, while the Devil's Hills were likely erupted about 300 years later and in total are slightly smaller than Rock Mesa (0.3 versus 0.5 cubic kilometers of lava). Research done on Rock Mesa and the Devil's Hills by colleagues at UC Davis found that many of the zircon crystals in these rhyodacite domes were scavenged from residual magma of rhyolites erupted at South Sister over 20,000 years ago.

Image: USGS