Rhyolite is the name applied to a subset of the igneous rocks, those rocks formed by cooling from molten magma. Earth scientists separate igneous rocks based on the minerals that are present and the size of the mineral crystals. Rhyolite (rhymes with "WHY-oh-light") is an extrusive igneous rock, which means that it crystallizes at or near the Earth’s surface from a volcanic lava flow. Like other extrusive rocks such as basalt, rhyolite has an aphanitic texture; meaning that the mineral crystals that make up the rock are too small to distinguish with the naked eye.
Under a magnifying glass or microscope however, the minerals that make up rhyolite can be identified. Quartz, potassium feldspar (orthoclase) and sodium feldspar (plagioclase) are the dominant minerals in rhyolite. Other minerals such as biotite, hornblende and muscovite are generally present in smaller quantities. The mineral assemblage gives the rock the same range of chemical compositions as its coarse-grained equivalent, granite. Rhyolite and granite are both rich in silicon, potassium and aluminum.
Samples of rhyolite range in color from yellowish and brown through gray to pink and red. Where a fresh surface of basalt is usually smooth and even-textured, rhyolite often displays a glassy texture and banding or layering caused by the flow of the lava from which it cooled. Another common textural element is the presence of phenocrysts, which are noticeably larger mineral crystals embedded in a fine-grained matrix. The phenocrysts in rhyolite are most often quartz or feldspar. This dual-size texture is termed "porphyritic," and it represents two-stage cooling: slow cooling that allows the larger crystals to form, followed by more rapid cooling that creates the matrix or groundmass.
Rhyolite and granite are the rock types that are highest in silica, SiO2, more than 2/3 by volume. In contrast, basalt contains approximately 50 percent silica. The high silica content of rhyolitic lava causes it to have greater viscosity than that of relatively silica-poor lavas. As a result, rhyolite flows tend to be thick and slow-moving, often forming large dome-like structures instead of the thin, widespread sheets that often characterize basalt flows. The eruptions of rhyolitic volcanoes are likely to be more explosive than basaltic eruptions because of the high viscosity of the silica-rich lava. Where basalt lavas seem thin and runny, such as in the Hawaiian volcanoes, rhyolitic lavas are thick and gooey.
Rhyolite's high viscosity tends to trap bubbles of dissolved gas and water vapor, creating small cavities known as vesicles when the rock hardens. In extreme cases the gaseous components come out of solution so profusely that they create a foamy texture, much like the head on a beer or a warm soda. The bubbles are so numerous and their walls so thin that the resulting rock, known as pumice, can float.
Other extrusive rocks
Rhyolite differs from other extrusive igneous rocks on the basis of mineralogy. Basalt contains no quartz (a mineral of pure silica) and is dominated by dark iron and magnesium-bearing minerals. Andesite, named for the Andes Mountains, is intermediate between rhyolite and basalt; this rock type contains small amounts of quartz. Dacite contains the same amount of silica as rhyolite and thus has plentiful free quartz, however the feldspars are mostly sodium-bearing plagioclase instead of potassium-bearing orthoclase and sanidine. Dacite lava is also viscous and prone to explosive eruptions, such as the 1980 dacitic eruptions of Mt. Saint Helens in Washington, USA.
Rhyolitic volcanism is generally associated with volcanism caused by melting of silica-rich continental crust. In North America, rhyolite is widespread throughout the Rocky Mountains and along the west coast, including volcanism in the Mono Lake area near the California-Nevada border.