This remarkable lava flow has an entire surface of glass, a liquid that cooled without crystallizing. The striking differences from rock to rock are due to the number and size of bubbles. The obsidian and pumice of this flow contain about 73% silica, like most window glass. In a hot, molten state, silica’s atoms tend to stick together and create webs of molecules that slow the movement of all atoms. The surface cooled before its atoms had time to organize into crystals, leaving glass instead. The flow is 1300 years old, Central Oregon’s most recent eruption, and measures 1.0 miles long and 1.1 square miles in area, with an average thickness of 150 feet. Its surface is about 10% obsidian and 90% pumice, a frothy obsidian, and it is a glassy variety of rhyolite, a volcanic rock with at least 72% silica. Tiny magnetite crystals give obsidian its black tint. Obsidian is solid glass with no bubbles, while pumice is frothy glass with small or large bubbles.