Dr Rex Taylor, Senior Lecturer in Ocean and Earth Science at the University of Southampton, investigated nodules and their trapped magma to see what caused the eruptions. He found that the nodules provide a record of the changes occurring in the magma plumbing right through to the moment the volcano erupted.
Dr Taylor says: "These nodules are special because they were ripped from the magma chamber before becoming completely solid -- they were mushy, like balls of coarse wet sand. Rims of crystals in the nodules grew from a very different magma, indicating a major mixing event occurred immediately before eruption. Stirring young hot magma into older, cooler magma appears to be a common event before these explosive eruptions."

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http://www.sciencedaily.com/releases/2012/10/121012074747.htm