andesite porphyry - hand specimen of a spectacular andesite porphyry
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This spectacular andesite porphyry is from a dike that was mostly removed during construction of Interstate 80 in Placer County, California. Andesite is associated with subduction zones, where an oceanic plate of the earth's crust is diving below a continental plate. It is intermediate in composition between basalt and granite and is derived from the subducting basaltic oceanic plate, from granitic sediments from the continent carried down with the plate, and from partial melting of the overlying mantle. Andesite forms the large volcanic cones that ring the Pacific, such as Mount St. Helens, the volcanos of the Aleutian chain and the Andes, Mt. Fuji in Japan and Mt. Mayon in the Philippines.
Porphyries show two stages of cooling. Slow cooling deep below the surface allows the growth of the large white orthoclase phenocrysts. Rapid cooling near the surface after injection into a fracture to form a dike, resulted in the aphanitic groundmass, where crystals are not visible to the naked eye. Aphanitic is derived from the Greek a for not, and phaneros for visible. Phenocrysts is a term derived from the same root, phaneros, for the large and very visible crystals of feldspar.
Students should recognize this as a porphyry and should be able to explain the different sizes of crystals, visible and too small to be seen. An identification as a basalt porphyry is an acceptable result. Exact identification, as an andesite, was done through examination of thin sections using a petrographic microscope.
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We don't have this rock type often, and the supply is extremely limited.
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