R

◊ recrystallization ◙ rhyolites ◊

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rhyolite

Rhyolite is an aphanitic or porphyritic, felsic, extrusive igneous rock with a composition similar to that of granite. Rhyolites are typically composed of quartz, alkali feldspar, and plagioclase feldspar; with biotite and pyroxene as accessory minerals. Muscovite, which is common in granites, occurs very rarely in rhyolites, and then only as an alteration product. The potassium feldspar is microcline or microcline-perthite in granites, whereas the K-spar in most rhyolites is sanidine. A great excess of K over Na is not uncommon in rhyolites, yet it is uncommon in granites, except as a consequence of hydrothermal alteration. Rhyolites are ususally light gray or creamy through pink to brick red in color, although some rhyolites have a greenish tint. Rock textures are related to the mode of formation. Many rhyolites contain macroscopic phenocrysts, and some rhyolites also have natural glass as a constituent. The high silica content (>69%) of rhyolites, coupled with relatively low Fe and Mg, confers high viscosity on extruded rhyolitic lavas. Rhyolitic magmas erupt at temperatures of 700 to 850° C. Rhyolite often erupts explosively because its extremely high viscosity hinders degassing. When bubbles of volatiles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny hot particles of volcanic ash. The cloud of hot debris from a rhyolitic volcano is termed ignimbrite, and the very dangerous, pyroclastic phenomenon causes rhyolite volcanoes to be a major hazard. Rhyolites are also found in rhyolite breccias, in volcanic necks, and in dikes and sills. Rapidly cooling rhyolites do not grow crystals, and effusive eruptions form a natural glass or vitrophyre: the glassy rhyolites include obsidian, pitchstone, perlite, and pumice. The color of obsidian ranges from black to brown, to red, to green to blue and yellow. Explosive eruptions of rhyolite create rhyolite tuffs or pumice. Scorias have less gas vesicles, so are not as lightweight as pumice and do not float in water. Scoria is generally darker than pumice and may have a few small crystals of olivine or plagioclase visible. More slowly cooling forms of rhyolite have microscopic crystals with bubble-rich and crystal-rich layers that form as the lava flows onto the surface and advances, and so exhibit textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Rhyolites, like granites, are typically confined to continents or continental margins where they are produced by low-temperature partial melting at subduction zones; although small quantities of rhyolite do occur oceanic islands remote from any continent. The Mojave Desert has an unusual occurrence of rhyolite – occurring in bimodal volcanic fields comprised of basalt and rhyolite. The absence of the intermediate volcanic rock, andesite, in the Mojave suggests that the basalt and rhyolite had separate origins and that their parent magmas did not mix.

link: images: hand-specimens: rhyolite NZ (compare to andesite); Rhyolite; rhyolite with phenocrysts, 2; green tinted banded Rhyolite; rhyolite tuff and rhyolite breccia; black porphyritic rhyolite with zone plagioclase; red porphyritic rhyolite with large feldspathic and quartz crystals; red rhyolite cobble broken open, Sonoran, Mexico; leopardskin rhyolite, 2, 3, 4, orbicular rhyolite; Rhyolite Porphyry; Rhyolite Tuff; obsidian NZ; Obsidian; mahogany obsidian; Pumice; pumice NZ; Scoria; close-ups: rhyolite, 2, 3; banded rhyolite, 2, 3; sphere; banded rhyolite, Mamainse Point; garnet in rhyolite; topaz in rhyolite; Scoria; daisystone, Mamainse Point; thin-section: rhyolite ppl; rhyolite xp; rhyolite tuff; crystal-rich rhyolite containing phenocrysts of quartz, K-feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass; Blue Hills Rhyolite, 50x ppol, 50x xpol, 50x ppol, 50x xpol, wp; formations: Rhyolite Mountains - Fissure System looking toward the Namaskard Pass from Hverfell; Rhyolite domes, Bottle Creek Property, Nevada; rhyolite peak in Iceland; contact between a rhyolite intrusion and a gabbro, and rhyolites and conglomerates, Mamainse Point; contorted flow banded rhyolite, southern Langadalsfjall; edge of a large rhyolite flow in the Three Sisters Wilderness, Oregon; Palisade Rhyolite, Palisade and Shovel Point; rhyolite flow; rhyolite tuff/breccia; columnar rhyolite, sheeted dykes, rhyolite sills; pumice field close-up; webpages: Drexel U rocks.