Existing comment:
Obsidian:
Commonly called volcanic glass, obsidian forms when the rhyolitic lava cools rapidly. Obsidian is chemically the same as pumice and is often mixed with it in the same rock. Unlike pumice, obsidian contained little dissolved gas when it cooled and did not form "bubbles." In the Scenic Area, obsidian is found in the Mono Craters. |