I finally figured out why obsidian is the way it is, thanks to Hans-Ulrich Schmincke's fine Volcanism. Obsidian is supposed to be a fast-quenched lava, a volcanic glass, but most localities don't look any different from ordinary lava domes, and we've watched lava domes and they don't cool super fast. Now basalt glass you need to quench pretty hard, for instance in a lava pillow in the deep sea or in the hyaloclastite layer of a tuya. But the viscous high-silica lavas (usually rhyolite) that become obsidian turn glassy so easily because their solidus is so close to their liquidus. Most molten rock solidifies over a range of temperature, allowing high-temp minerals to crystallize before low-temp ones. But not rhyolite. It doesn't cool fast, it hardens fast. Obsidian is fascinating stuff — have a look at some.
_{Obsidian tools — Geology Guide photo}