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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 ?
This group is dominated by andesitic to dacitic rocks with lesser quantities of basaltic to rhyolitic tuffs and flows and epiclastic sedimentary rocks.
Donoghue SL, Neall VE, Palmer AS (1995) Stratigraphy and chronology of late Quaternary andesitic tephra deposits, Tongariro Volcanic Centre, New Zealand.
A similar but coarser, rough, blocky texture is seen on the surfaces of more andesitic flows.
These include an interior andesiterhyolite assemblage of ash flow tuffs (31M-20M yr), a western assemblage of andesitic lavas (20M-12M yr), and a bimodal basalt-rhyolite assemblage with associated sedimentary rocks.
The deposit is composed of vein systems located within a tertiary andesitic volcanic core.
Below the fault, the lithology changed to barren magnetite and hematite rich andesitic intrusive.
The Cocos Plate is subducting beneath the Caribbean Plate along the Middle American Margin, building the Central Volcanic Range through magma ascent, and creating andesitic massifs such as the one on which Turrialba volcano is located.
It continues: "The stone quarried at Harden was formed approximately 390 million years ago, when magma from deep underground was forced up into the surrounding andesitic rocks, forming a thick blister-like intrusion of mica-porphyrite, which is now commonly called Harden red.
The lithology of the area consists of greenschists, mica schists, quartzites, hornblende schists, amphibolites, garnet-amphibolites, serpentinites and porphyritic hypabyssal rocks with andesitic to dacitic composition (Figure 1).