Dacite ( / ˈ d eɪ s aɪ t / ) 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 . It is composed predominantly of plagioclase feldspar and quartz .
15-542: Roca Consag (Consag Rock) is a granitic (or more precisely, dacitic ) outcrop located in the northern Gulf of California about 22 miles ENE of the town of San Felipe . The outcrop stands about 286 feet above the sea, and is named for the Croatian Jesuit missionary Ferdinand Konščak who was active in the early exploration and missionary work in Baja California . The ruins of an abandoned lighthouse can be seen on
30-401: A thick felsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium. As the young, hot oceanic plate is subducted under continental crust, the subducted slab partially melts and interacts with the upper mantle through convection and dehydration reactions. The process of subduction creates metamorphism in the subducting slab. When this slab reaches
45-606: Is found extraterrestrially at Nili Patera caldera of Syrtis Major Planum on Mars . Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Aphanitic Aphanites (adj. aphanitic ; from Ancient Greek αφανης (aphanḗs) 'invisible') are igneous rocks that are so fine-grained that their component mineral crystals are not visible to
60-863: Is impractical. The rock must then be classified chemically based on its content of silica and alkali metal oxides ( K 2 O plus Na 2 O ). The TAS classification puts dacite in the O3 sector. In hand specimen, many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially pyroxene-bearing dacites, are darker colored. In thin section, dacites may have an aphanitic to porphyritic texture. Porphyritic dacites contain blocky highly zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite (or enstatite ) are found in some samples. The groundmass of these rocks
75-425: Is often aphanitic microcrystalline , with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline. 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
90-489: The naked eye (in contrast to phanerites , in which the crystals are visible to the unaided eye). This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not the same as that of volcanic glass (e.g., obsidian ), with volcanic glass being non-crystalline ( amorphous ), and having a glass-like appearance. Aphanites are commonly porphyritic , having large crystals embedded in
105-604: The subalkaline tholeiitic and calc-alkaline magma series . The word dacite comes from Dacia , a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania and Moldova ) where the rock was first described. The term dacite was used for the first time in the scientific literature in the book Geologie Siebenbürgens ( The Geology of Transylvania ) by Austrian geologists Franz Ritter von Hauer and Guido Stache. Dacite
120-452: The history of the magma. The formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, buoyant, perennial rock from a mafic, dense, short-lived one. The process by which dacite forms has been used to explain the generation of continental crust during the Archean eon . At that time,
135-428: The mantle and initiates the dehydration reactions, minerals such as talc , serpentine , mica and amphiboles break down generating a more sodic melt. The magma then continues to migrate upwards causing differentiation and becomes even more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende. Accessory minerals like pyroxenes provide insight to
150-470: 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 . Dacitic magma is formed by the subduction of young oceanic crust under
165-507: The production of dacitic magma was more ubiquitous, due to the availability of young, hot oceanic crust. Today, the colder oceanic crust that subducts under most plates is not able to melt prior to the dehydration reactions, thus inhibiting the process. Dacitic magma was encountered in a drillhole during geothermal exploration on Kīlauea in 2005. At a depth of 2488 m, the magma flowed up the wellbore. This produced several kilograms of clear, colorless vitric (glassy, non-crystalline) cuttings at
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#1733114825885180-563: The rhyolites. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram . This defines dacite as having a content of 20% to 60% quartz, with plagioclase making up 65% or more of its feldspar content. However, while the IUGS recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, dacites are often so fine-grained that mineral identification
195-853: The surface. The dacite magma is a residual melt of the typical basalt magma of Kīlauea. Dacite is relatively common and occurs in various tectonic and magmatic contexts: The type locality of dacite is Gizella quarry near Poieni, Cluj in Romania . Other occurrences of dacite in Europe are Germany ( Weiselberg ), Greece ( Nisyros and Thera ), Italy (in Bozen quartz porphyry, and Sardinia ), Austria (Styrian Volcano Arc), 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
210-461: The west side of the outcrop. The waters in the vicinity of the Roca Consag are a popular fishing area and also favored for spotting whales and porpoises. This article about a regional geological feature is a stub . You can help Misplaced Pages by expanding it . Dacite Dacite is relatively common, occurring in many tectonic settings. It is associated with andesite and rhyolite as part of
225-609: Was originally defined as a new rock type to separate calc-alkaline rocks with oligoclase phenocrysts (dacites) from rocks with orthoclase phenocrysts (rhyolites). Dacite consists mostly of plagioclase feldspar and quartz with biotite , hornblende , and pyroxene ( augite or enstatite ). The quartz appears as rounded, corroded phenocrysts , or as an element of the ground-mass. The plagioclase in dacite ranges from oligoclase to andesine and labradorite . Sanidine occurs, although in small proportions, in some dacites, and when abundant gives rise to rocks that form transitions to
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