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15 June 2011
This past weekend I took a day to go revisit the Valles Caldera. While I was wandering the trails, I decided to do a little “rockhounding”. Geologists are often notorious for their rock collections, and I felt like mine was missing something that no volcanologist should be without: Obsidian.
Obsidian is basically volcanic glass – the result of lava cooling so quickly that it doesn’t have time to develop crystals. Pure glass is basically SiO2, and obsidian tends to form from rhyolitic magmas (which already have a high percentage of SiO2, typically greater than 68%). “Heavy” minerals in the obsidian (such as iron and magnesium) make the glass dark, and (depending on their oxidation state) can give it reddish or greenish hues.