When it flows out of the volcano or crack, usually mixed with steam and gas, it is called lava. Enormous quantities of lava, enough to flood the whole countryside, may be produced by the eruption of a major volcano. Others move at the rate of only inches per day. The speed of the flow depends on the temperature and composition. All lavas contain a high percentage of silica, a compound made up of the chemical elements silicon and oxygen.
Rhyolites melt at lower temperatures and are lighter in weight and color than basalts.
Rhyolitic lavas are quite viscous, or thick, and contain large quantities of gas. The gas often boils off with explosive force, expelling large amounts of glowing ash and cinders. Sometimes, however, it is released more slowly or even trapped as bubbles when the lava hardens. When the bubbles are tiny and closely packed, a lightweight stone called pumice is formed. Any kind of lava may turn into pumice, but most of it develops in rhyolites.