Dacite

Dacite usually forms as an intrusive rock such as a dike or sill. Examples of this type of dacite outcrop are found in northwestern Montana and northeastern Bulgaria. Nevertheless, because of the moderately high silica content, dacitic magma is quite viscous and therefore prone to explosive eruption. A notorious example of this is Mount St. Helens in which dacite domes formed from previous eruptions. Pyroclastic flows may also be of dacitic composition as is the case with the Fish Canyon Tuff of La Garita Caldera.

Dacite consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has quartz as some rounded, corroded phenocrysts, or as an element of the ground-mass.

Dacite can be used as a filler rock when paving and constructing roads, though it is not a popular choice due to its high silica content that may easily damage or weather. It may also be used for decorative purposes or stone carvings.

Dacite is relatively common and occurs in various tectonic and magmatic contexts: In oceanic volcanic series. Examples: Iceland (Heiðarsporður ridge), Juan de Fuca Ridge In limestone-alkaline and tholeiitic volcanic series of the subduction zones of island arcs and active continental margins. Examples of dacitic magmatism in island arcs are Japan, the Philippines, the Aleutians, the Antilles, the Sunda Arc (Mount Batur), Tonga and the South Sandwich Islands. Examples of dacitic magmatism in active continental margins are the Cascade Range, Guatemala and the Andes (Ecuador and Chile). In continental volcanic series, often in association with tholeiitic basalts and intermediary rocks. Sites of dacite in Europe are Germany (Weiselberg in the Saarland), Greece (Nisyros and Thera), Italy (in Bozen quartz porphyry, and Sardinia), Austria (Styrian Volcano Arc), Romania (Transylvania), Scotland (Argyll), Slovakia, Spain (El Hoyazo near Almería), France (Massif de l'Esterel) and Hungary (Csódi Hill). Sites outside Europe include Iran, Morocco, New Zealand (volcanic region of Taupo), Turkey, USA and Zambia. Dacite is found extraterrestrially at Nili Patera caldera of Syrtis Major Planum on Mars.