1. Education

Obsidian Gallery

By

Obsidian is an extreme variety of igneous rock with a glassy texture. Most popular accounts say that obsidian forms when lava cools very quickly, but that is not quite accurate. Obsidian starts with a lava very high in silica (more than about 70 percent), such as a rhyolite. The many strong chemical bonds between silicon and oxygen make such a lava very viscous, but equally important is that the temperature range between fully liquid (the liquidus) and fully solid (the solidus) is very small. Thus obsidian does not need to cool especially fast, because it solidifies especially fast. Another factor is that a low water content may inhibit crystallization.

More Extrusive Igneous Rocks

About Igneous Rocks

Images 1-12 of 12
From a distanceObsidian FlowTypical of flowsObsidian BlocksComplex texturesObsidian Flow TextureLittle mineral clustersSpherulites in Obsidian
Black as anthraciteFresh ObsidianJust slightly weatheredObsidian CobbleFrosty coatingObsidian Hydration RindDirect from lava to clayWeathering of Obsidian
Delicate natural sculptureWeathered ObsidianGood enough to workObsidian ToolsPossibly work fragmentsObsidian of Glass Buttes, OregonGlorious rubbishObsidian Chips
  1. About.com
  2. Education
  3. Geology
  4. Rocks & Rock Collecting
  5. More on Igneous Rocks
  6. Obsidian Gallery - Pictures of Obsidian

©2014 About.com. All rights reserved.