Tricia
TriciaHi Tricia,
I hope I don't tell you the same 5 things. Obsidian is natural glass that is erupted from volcanoes. It forms from magma that is high in silica. Glass is actually a very very very viscous liquid rather than a solid. This is because the definition of a solid is something that has a regular molecular structure (like the tinker-toy diagrams of molecules and minerals that you may have seen). A liquid, on the other hand, has no regular structure--it is all disorganized. If you saw a tinker-toy model of glass (such as obsidian), you might think that the person who made the model was very drunk when they made it. It would have no pattern to it, with the branches going every which way. This fits the definition of a liquid rather than a solid. So, even though glass isn't flowing at a rate that you can see, it is a liquid rather than a solid.
Certainly glass will flow faster if it is hot, and when obsidian is erupted from a volcano it is indeed hot. It still isn't able to flow very easily but it can at least flow a little. Because it is so viscous even when it is hot, it doesn't flow very fast or very far, nor can it spread out very much. Obsidian flows are usually only a few km long at most, and are often tens of meters high.
If you want to read still more about obsidians, check out page 161 of "Volcanoes" by Peter Francis.
Sincerely,
Scott Rowland
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