How is obsidian formed?
Is it always deposited during volcanic activity? (Question sent by Brooke, 6th Grader, St. Paul’s School, Clearwater, Florida) Obsidian is a natural form of glass. Obsidian is formed during volcanic activity when lava, with the right chemical composition (high in acid), cools very quickly upon contact with the atmosphere during an eruption. The rapid cooling prevents the formation of the distinct crystals you often see in volcanic rocks. If it cooled slowly, allowing the crystals to form, it would be classed usually as dacite or rhyolite. Most obsidian is black, but there are red and green varieties as well. Obsidian is found in the Aegean Sea on the island of Melos and one or two other places. In the west Mediterranean it is found on Lipari island among other places. Obsidian is also found in Turkey, Hungary, Ethiopia, and many other countries. In the United States obsidian is found in Yellowstone National Park and in the Pacific mountain ranges of California and Oregon. About how lon
