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Mazama ignimbrite, Oregon -- This pyroclastic flow was generated by the caldera-forming eruption of Mt. Mazama about 6,845 years ago. The ignimbrite shows magnificent compositional zonation. The pale (felsic) lower part has a rhyodacitic composition and the darker (mafic) upper part is andesitic. This vertical zonation is inverse of the zonation in the magma chamber before eruption. The upper part of the chamber (which erupted first) was rhyodacitic and the lower part of the chamber (which erupted last) was andesitic. The splendid pinnacles have been described as fossil fumarole pipes that are more resistant to erosion than the rest of the ignimbrite. Courtesy of Peter Francis. |