Stephanie and Allan explain how obsidian and pumice are formed. Obsidian is the same chemically as granite and rhyolite. Remember that granite is rock that cools within the crust and crystallizes very slowly. This is why you can see distinct "chunks" (or crystals) in your countertops. Obsidian is like rhyolite lava that is very thick and contains lots of silica. Its "flow" is so sticky that it hardly moves and crystals don't have a chance to form. This leaves the rocks with a glassy interior. Pumice is a foam of molten rhyolite material that cools quickly but has more gas bubbles in it than rock material, making the rock light enough to float on water!
Tuesday, July 14, 2009
Obsidian and Pumice Lesson
Stephanie and Allan explain how obsidian and pumice are formed. Obsidian is the same chemically as granite and rhyolite. Remember that granite is rock that cools within the crust and crystallizes very slowly. This is why you can see distinct "chunks" (or crystals) in your countertops. Obsidian is like rhyolite lava that is very thick and contains lots of silica. Its "flow" is so sticky that it hardly moves and crystals don't have a chance to form. This leaves the rocks with a glassy interior. Pumice is a foam of molten rhyolite material that cools quickly but has more gas bubbles in it than rock material, making the rock light enough to float on water!
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