The entire surface of this remarkable flow is glass, a liquid that cooled without crystallizing. The striking differences you see from rock to rock are due to the number and size of bubbles.
Why is everything glass?
Whether natural or synthetic, the primary ingredient in glass is silica (silicon dioxide). The obsidian and pumice of this lava flow contain about 73% silica, like most window glass does. In a hot, molten state, silica's atoms tend to stick together and create webs of molecules that slow down the movement of all atoms. The surface of this lava flow cooled off before its atoms had time to organize into crystals, so we are left with glass instead.
Flow Facts
Age???1300 years, Central Oregon's most recent eruption
Length???1.0 miles (1.6 km)
Area???1.1 square miles (2.9 sq km)??640 football fields or 400 soccer fields
Thickness???Average of 150 feet (45 m). Height of a 17-story building
Flow Surface???About 10% obsidian and 90% pumice (frothy obsidian)
Rocky Type???A glassy variety of rhyolite (rhyolite is a volcanic rock with at least 72% silica)
Why is obsidian black? Like a drop of black ink in a glass of clear water, tiny magnetite crystals (iron oxide) give obsidian a black tint.
(Graphic Captions)
Crystal???Organized atom in crystals
versus
Glass???Disorganized atoms in glass and obsidian
Obsidian???Solid glass with no bubbles
Pumice???Frothy glass with small bubbles (nearly white pumice)
Pumice???Frothy glass with big bubbles (medium to dark gray pumice)
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