Rhyolite is formed when rhyolitic magma explodes, or if crystallization occurs when the rhyolitic magma is deeply buried. Rhyolite is high in silica, and it is typically found as ash deposits, in shiny forms, such as obsidian, or in lightweight forms, including pumice and perlite.
Rhyolite lava is thick and flows slowly but cools quickly. When rhyolite magma explodes, the overall temperature is generally low. Most rhyolites crystallize quickly on the surface of Earth, resulting in a groundmass that consists mainly of small crystals or small glassy crystals. An igneous rock that often crystallizes before surfacing, rhyolite forms complex chains of silicate minerals. Rhyolite magma that is rich in gas tends to form porous rocks, such as pumice. Porous rhyolitic magma that is able to flow can result in degassing and the formation of glassy rocks, such as obsidian. The form of the crystals in rhyolitic magma affects how quickly or slowly the magma moves.
Some rhyolites made up of larger crystalline material can be mistaken for granite. Rhyolite and granite have similar chemical compositions, but differences, including feldspar, potassium and sodium amounts, contribute to the separate categorization of the two. Depending on the amount of feldspar and quartz in the crystalline matrix of rhyolite, the physical appearance of the rock may be as rough as raw granite's appearance, smooth or glassy.