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Rhyolite is an extrusive igneous rock, formed from magma rich in silica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained ( aphanitic ) or glassy .
Rhyolitic magma is felsic and the most abundant in silica, potassium, and sodium but the lowest in iron, magnesium, and calcium. [1] [3] The silica composition of rhyolitic magma ranges from 65-75 wt.%. [1] It forms in the lowest temperature range, from about 1200 °F to 1470 °F. [1], [3] Rhyolitic magma has the highest viscosity and gas ...
They include rhyolite and dacite magmas. With such a high silica content, these magmas are extremely viscous, ranging from 10 8 cP (10 5 Pa⋅s) for hot rhyolite magma at 1,200 °C (2,190 °F) to 10 11 cP (10 8 Pa⋅s) for cool rhyolite magma at 800 °C (1,470 °F). [21] For comparison, water has a viscosity of about 1 cP (0.001 Pa⋅s).
Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into the system. When this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase. [27]
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in ...
Magma rich in silica and poor in dissolved water is most easily cooled rapidly enough to form volcanic glass. As a result, rhyolite magmas, which are high in silica, can produce tephra composed entirely of volcanic glass and may also form glassy lava flows. [2] Ash-flow tuffs typically consist of countless microscopic shards of volcanic glass. [3]
Dacitic magma was encountered in a drillhole during geothermal exploration on Kīlauea in 2005. At a depth of 2488 m, the magma flowed up the wellbore. This produced several kilograms of clear, colorless vitric (glassy, non-crystalline) cuttings at the surface. The dacite magma is a residual melt of the typical basalt magma of Kīlauea. [16]
Since felsic magma is viscous, volcanic eruptions that form dacite or rhyolite are explosive and violent. The Archean felsic eruption may be assigned to Vesuvius eruption type in the present day. [36] Submarine rhyolitic flows were widespread in the Archean but are uncommon in the modern volcanic environment. [39]