Cordón Caulle, Chile
The folks at NASA's Earth Observatory posted one of the best images of a lava flow I've seen in a long time this week — it shows the dark obsidian flow from the 2011-2012 activity at Cordón Caulle in Chile. The flow is a dramatic feature against the light-colored background of volcanic ash and debris (tephra). The best part of this flow is that according to reports of volcanologists on the ground, the flow is still creeping along even though the eruption has been over for almost a year. I won't spoil the story, so be sure to read the great description of the flow on the NASA EO page linked above.
However, you don't need to travel to Chile to see great obsidian flows — flows made of glassy lavas, usually rhyolitic (high silica). The Cascades and Long Valley in the western United States have multiple domes and flows that are fully obsidian or at least partially the glassy, crystal-free lava that is obsidian. I've had a chance to visit a few of these, so I thought we could take a virtual tour of some of the obsidian flows in our national backyard. There are even more than pictured in this gallery, but these here are some of the most striking.
Image: NASA Earth Observatory.