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Dacite

Updated:2008-11-28 09:17:32

A light gray volcanic rock containing a mixture of plagioclase and other crystalline minerals in glassy silica, similar in appearance to rhyolite.

Typically light-colored, fairly silica-rich volcanic rock with a high viscosity when in a molten state; eruptions are commonly explosive (e.g., Mount St. Helens' eruption of May 18, 1980) and may produce voluminous tephra, pyroclastic flows, and lava domes.

A fine-grained extrusive rock with the same general composition as andesite, but having a less calcic plagioclase and more quartz; according to many, it is the extrusive equivalent of granodiorite. Syn:quartz andesiteancient Roman province of Dacia (now part of Romania).

Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000°C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome.

It is chemically very similar to the granitic rock, but because it was extruded by a volcano and cooled relatively quickly, it is much more fine grained. Rocks that are intrusive and cool slowly (over millions of years) can grow large crystals.

Dacite is intermediate in compostion between andesite and rhyolite, and, like andesite, it consists mostly of plagioclase feldspar with biotite, hornblende, augite or enstatite. It has an aphanitic to porphyritic texture, but they contain also quartz as rounded, corroded phenocrysts, or as an element of the ground-mass.

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