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White Island is an active composite stratovolcano in the Bay of Plenty, New Zealand, that comprises many small volume (<0·1 km3) andesite–dacite lava flows and pyroclastic deposits with phenocryst contents of ∼15–44%. Minor high-Mg basaltic andesite explosive eruptions, such as those of 1976–1992, may have occurred at intervals throughout the history of White Island, but are rarely preserved. These alternate with major episodes of andesite–dacite lava extrusion. The high-Mg magmas form by hydrous melting of mantle, metasomatized by fluids from the dehydrating slab at the slab–mantle wedge interface, that rise rapidly to shallow magma chambers (2–7 km?) where limited mixing and contamination occurs before eruption. Some of this magma remains in the magma chamber where it interacts with the crystal mush, from which it inherits phenocrysts, to form so-called ‘dirty’ lavas. Total phenocryst content of these lavas is correspondingly higher. As more magma is intruded into the chamber, the heat flux will increase and melt fraction will eventually rise to the surface to form high-silica andesite–dacite magma (‘clean’ lavas) with fewer inherited phenocrysts. Similar multi-magma chamber plumbing systems, with complex evolution involving fractionation and contamination, probably occur in most andesite–dacite arc volcanoes.

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