Rhyodacite is avolcanic rock intermediate in composition betweendacite andrhyolite. It is theextrusive equivalent of thoseplutonic rocks that are intermediate in composition betweenmonzogranite andgranodiorite. Rhyodacites form from rapid cooling oflava relatively rich insilica and low inalkali metal oxides.
UnderIUGS guidelines, rhyodacites are not formally defined in either theQAPF classification, used to classifyigneous rocks by their mineral content, or theTAS 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 whichplagioclase makes up about two-thirds of the totalfeldspar content.[1][2] Such a rock will contain between 69% and 72% silica by weight.[3]
The U.S. Geological Survey defines rhyodacite as volcanic rock containing 20% to 60% quartz and with plagioclase making up 35% to 65% of the total feldspar content. This makes rhyodacite the extrusive equivalent of monzogranite.[4]
In addition to its quartz and feldspar content, rhyodacite commonly containsphenocrysts ofandesine,biotite,hornblende, andpyroxene. Quartz andsanidine phenocrysts are less common than in rhyolite, and rhyolite has only scantmafic phenocrysts.[5]
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Like other silica-rich compositions, rhyodacites are more abundant aspyroclastic rock than aslava flows.[6] These include extensive ash flows and fallout sheets that are importantstratigraphic markers.[5]
Rhyodacite lava flows occur in northwesternFerry County (Washington),[4] and atAn Sgùrr on the island of Eigg in Scotland.[7]
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