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The types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it erupted. Considering that the most abundant elements found in silicate magma are silicon and oxygen, the various types of magma (and therefore ash) produced during volcanic eruptions are most commonly explained in terms of their silica content.
Rhyolite (/ ˈraɪ.əlaɪt / RY-ə-lyte) [1][2][3][4] is the most silica -rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase.
Dacite from the Lassen Volcano National Park. Dacite (/ ˈdeɪsaɪt /) is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite.
Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro.
The Taupō Volcano erupts rhyolite, a viscous magma, with a high silica content, a feature associated with the middle portion of the Taupō Volcanic Zone within the Taupō Rift. This is an intra-arc rift in the eastern part of the continental Australian Plate , resulting from an oblique convergence with the Pacific Plate in the Hikurangi ...
Felsic or silicic magmas have a silica content greater than 63%. 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]
Basalt (UK: / ˈbæsɔːlt, - əlt /; [1] US: / bəˈsɔːlt, ˈbeɪsɔːlt /) [2] is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt.
2; the higher the silica content, the greater the ability of the magma to trap and hold on to gas and water vapor. When high-silica magma rises to the Earth's surface, the trapped gases and vapors can erupt explosively to produce ash clouds and pyroclastic flows that consist of superheated gas, ash and volcanic fragments.