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Another Specimen

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Typical Minerals
Minerals too small to identify (except phenocrysts, which here are amphibole). Composition will be from middle of Bowen's Reaction series, and include such minerals as Ca/Na plagioclase, amphibole, and biotite.
     A fine grained (aphanitic) igneous rock, medium to dark in color.
      Color is such a poor indicator of rock identification in most cases, that it can only be a first approximation. Here, however, the amphibile phenocrysts put this specimen in the intermediate range.
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Tectonic Association
     Andesite, as with all intermediate rocks, is found principally in volcanic arc systems, either intraoceanic volcanoes (e.g. Japan, Aleutian Islands), or cordilleran systems (Andes mountains). It characteristically forms composite explosive volcanoes.
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