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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.
Some studies have been carried out recently to use andesite as a coating material and raw material in the ceramics sector.
Material and Method: Andesite tablets were gathered from the mine and cut in the factory to desired dimensions.
low-silica andesite in the lower part of the Porten Road Formation is highly enriched in LREE and LFSE.
Various volcanic facies including massive porphyritic rhyolite, porphyritic rhyolite breccia, dacite breccia, and minor andesite breccia and flows have been recognized in the newly defined Porten Road Formation.
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.
K/Th shows a positive correlation, which suggests that Ba and K behave coherently with respect to Rb and Th among the Ross Island andesites.
A third hole was completed to test this mineralized structure down dip (assays pending), but none of the holes reached the primary target, which is the mineralized structure where it intersects the underlying andesite rocks.
Opawica has intersected the previously discovered sulphide bearing andesites (Flow 1) at vertical depths from 250 to 300 meters beneath a small open pit mined and abandoned in the early 1970's.
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.
These sequences were subsequently intruded by 38 million year old intrusions and overlain by Miocene basalts and andesites.