The rocky area at the center of this image is the Big Obsidian Lava Flow, the youngest lava flow in the state of Oregon. The lava flow slowly oozed out of a vent in the Newberry Caldera structure during an eruption 1300 years ago.
The lava flow covers a different unit from the same eruption. The eruption began with an explosive phase as volatiles like H2O and CO2 forced their way to the surface. Those gases formed vesicles that built pressure and blasted the molten rock out of its vent.
Some time after that initial explosive eruption, a portion of the remaining magma within the volcanic plumbing system reached the surface and began to ooze its way forward. The lava flow was a viscous rhyolite and probably only moved forward by at most a couple meters per day during the eruption. Because rhyolite is high viscosity, it can be difficult for crystals to begin growing in the lava before it cools off; a lava flow is able to cool off before any crystals begin to form is the basic requirement of forming obsidian and this lava flow is one of several sources of that glassy rock found in this state. Via EarthStory