We had read about Yellowstone's Obsidian Cliff a few times when learning about rocks and geology, so we were all anxious to see it. But it seems like it's not really an "encouraged" place to go anymore! It's by the side of a road where you can't walk easily, and the informational sign has a very scolding tone about how many people have taken obsidian from the cliff and ruined it for everyone. (For the record: I think it's sad too. But I'm weary of didacticism and lecturing in public spaces. I suppose maybe it works and makes people want to forsake their sins and joyfully obey…but I doubt it.) You aren't supposed to climb on the cliff itself, so we didn't.
There was such a beautiful meadow across the road from the cliff. You could see big chunks of obsidian that I guess had been blown out of the volcano. As you probably know, obsidian is formed from lava that cools so quickly, it doesn't have time to form crystals, so it's smooth and glassy. We've been to places in Utah where you can find small pieces of obsidian, but it's amazing to see whole boulders of this, let alone a WHOLE CLIFF made of it!
Up close you can really see the glassy texture!
And even from far away, you can see the shiny black veins of obsidian in the cliff. It shone in the sunlight. Beautiful!
Conchoidal fracturing! Don't worry, we didn't take any pieces of obsidian (which we found on the ground—they are lying around all over even across the road). Just took pictures of them.
I especially liked this one, with stripes of obsidian across the surface.