MARK A. SCHNEIDER / SCIENCE PHOTO LIBRARY MARK A. SCHNEIDER / SCIENCE PHOTO LIBRARY
Andesite is an igneous rock consisting of plagioclase feldspar and hornblende, biotite or augite. It is extrusive, formed when lava reaches the surface of the earth through fissures, vents or volcanoes. It's texture is aphanitic (microcrystalline); mineral grains are too small to be seen by the naked eye.
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