(redirected from Andesites)
Also found in: Dictionary, Thesaurus, Wikipedia.


Very finely crystalline extrusive rock of volcanic origin composed largely of plagioclase feldspar (oligoclase or andesine) with smaller amounts of dark-colored mineral (hornblende, biotite, or pyroxene), the extrusive equivalent of diorite.



an extrusive, dark-colored (such as dark gray, brown, or black) mountain rock. Its structure is porphyritic or hyalopilitic; in the latter case the microlites resemble felt saturated with glass. It is formed as a result of the freezing, at the top surface or close to it, of volcanic lava containing 56–60 percent silicic acid and much magnesium, calcium, and iron. Andesite is usually a fresh rock composed of crystals of medium plagioclases, andesine, and pyroxenes and less frequently of magnetite, hornblende, and other minerals immersed in glass. Andésite is widely spread throughout contemporary and ancient volcanic regions such as Kamchatka, the Caucasus, Middle Asia, and the Mediterranean. Together with basalt, it constitutes the principal mass of extrusive rocks. It is used as an acid-proof material and for special glazing.

References in periodicals archive ?
Andesite is an aphanitic and extrusive igneous rock composed of the Na-rich plagioclase, pyroxene and amphibole.
Lava flows are mainly andesites and porphiritic andesites, but some differences in phenocrysts abundances are observed between ancient and young lava flows (Forero and Zuluaga, 2010).
The Pliocene volcanics are generally dominated by andesites, whereas dacites occur as dominant phase in Pleistocene volcanics.
The andesite containing the attractive minerals is dark brown in color with a purple tint.
low-silica andesite in the lower part of the Porten Road Formation is highly enriched in LREE and LFSE.
We describe the petrography and geochemistry of these igneous rocks in order to evaluate possible regional correlations and to contribute to the understanding of the petrogenesis of orogenic andesites.
The principal rock between Pueblo Nuevo and Jocuistle is an andesite porphyry sill which crops out prominently in the arroyo between El Zapote and Agua Caliente.
OPW-2 and OPW-3 were drilled to 140m core lengths at 45 and 60 degrees respectively at the north end of the property intersecting similar spotted andesites, in Flow horizon 4.
The area's geology is fairly typical of the northern Sierra Madre Occidental with structurally disrupted andesite volcanics of the Lower Volcanic Complex (LVC) exposed in the more deeply eroded parts of the district and gently tilted tuffaceous rhyolite ash-flow tuffs of the Upper Volcanic Supergroup (UVS) occupying the higher areas.
The dominant rock types in the area are flows and tuffs of pyroxene-feldspar propyllitic andesite later intruded by andesite.
These trenches encountered altered and locally silicified andesites with broad zones of massive, low angle quartz veining, crosscut by high angle narrow veins.