rhyodacite
Sign in to saveright|thumb|A sample of rhyodacite from Slovakia Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.
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3 sectionsContents
- Description
- Occurrence
- References
right|thumb|A sample of rhyodacite from Slovakia Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.
==Description== thumb|QAPF diagram with the approximate location of rhyodacite highlighted thumb|TAS diagram with the approximate position of rhyodacite highlighted in yellow (T=trachyte or trachydacite; R=rhyolite; O3=dacite) Under IUGS guidelines, rhyodacites are not formally defined in either the QAPF classification, used to classify igneous rocks by their mineral content, or the TAS classification, used to classify volcanic rocks chemically. However, the IUGS allows the use of the term to describe rocks close to the boundary between the rhyolite and dacite fields in each classification scheme. Rhyodacite then describes a fine-grained igneous rock containing between 20% and 60% quartz and in which plagioclase makes up about two-thirds of the total feldspar content. Such a rock will contain between 69% and 72% silica by weight.