These resemble aa in having tops consisting largely of loose rubble, but the fragments are more regular in shape, most of them polygons with fairly smooth sides. Andesite is very common product of subduction zone volcanism. Rhyolitic lava is a silica rich form of lava that is relatively "thick", as compared to basaltic and andesitic lava. is the resistance to flow (opposite of fluidity). - from the rock Rhyolite composition like granite It is … Basalt is a common extrusive igneous rock formed by the rapid cooling of basaltic lava exposed at or very near the surface of Earth. Andesite is composed of 52 to 63%silica (SiO2). Andesite is a dark, fine-grained, brown or greyish intermediate volcanic rock which is a commonly found in lava. Santorini, Greece. Basaltic magma - 1000 to 1200 o C ; Andesitic magma - 800 to 1000 o C ; Rhyolitic magma - 650 to 800 o C. Viscosity of Magmas. Basaltic andesite. A unique feature of basaltic lava is the formation of pillow lavas. Viscosity . Andesite can look very much like basalt to the unaided eye, but it is usually less dark and sometimes greenish in colour. Lavas of andesitic or intermediate composition commonly form a somewhat different type of flow, known as a block lava flow. Glossary Terms. For the extinct cephalopod genus, see Andesites. Composition Volcanic rock, commonly dark gray to black, with about 53-57 percent silica. composition is referred to as ‘‘andesitic surface’’ or ‘‘ande-site.’’ Spectra of terrestrial particulate samples of basaltic andesite and flood basalt also proved to match well with the martian surface spectra [Bandfield et al., 2000a; Christensen et al., 2000a]. ... changing its composition, or decreasing its pressure. Andesite is similar to basalt, but it contains more silica and is generally lighter in color. Viscosity depends on primarily on the composition of the magma, and temperature. Basaltic flows are the hottest. Andesite / ˈ æ n d / is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. [9] Using linear deconvolution methods, Bandfield et al. Width of sample 7 cm. Andesite is recorded as being an important rock type associated with 422 of the 721 active volcanoes of the Earth; most of the modern Andesite occur above the Benioff seismic zone and the mean chemical composition of andesite is broadly similar to the estimated mean chemical composition of the continental crust of the Earth. • Basaltic to andesitic composition • cone of ejected lava fragments (pyroclastics) cinder • pea to walnut sized • gentle eruption • steep cone - because pile of loose cinder fragments • small - 1 to 2 km's - because only erupts a few years • Example: Paricutin, Mexico, 1943 eruption. Geologic unit mapped in Oregon: Lava flows and flow breccia of hypersthene and olivine andesite, basaltic andesite containing plagioclase and pyroxene phenocrysts, and basalt; many flows contain phenocrysts of both hypersthene and augite. Includes interbedded volcaniclastic and epiclastic rocks mostly of andesitic composition, but partly of dacitic or rhyodacitic composition. Crystals found in andesite are mainly plagioclase feldspar, pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. In a general sense, it is the intermediate type between basalt and dacite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. White crystals are plagioclase phenocrysts, but they contain less Ca and more Na than plagioclase in basalt does. Basaltic lavas, the most abundant lava type on earth, are the focus of this lesson. Lava exposed at or very near the surface of Earth, but it is usually less dark sometimes! 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