From Wikipedia: (http://en.wikipedia.org/wiki/Volcanic_rock)
The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks).
The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.
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