A water-rich magmatic gas phase escaped explosively from Mount Pinatubo on June 15, 1991, taking with it a load of crystalline and molten material sufficient to form pumice and tephra deposits with an estimated total dense-rock-equivalent volume of 3-5 km3 (Scott et al., 1991), and carrying in it enough sulfur to form a 20 Mt SO2 cloud in the stratosphere. Application of the "petrologic method" for estimating sulfur degassing during the climactic event from the sulfur content of trapped glass inclusions and matrix glasses in the pumice deposits requires an unacceptably large volume of erupted magma to account for SO2 in the stratospheric cloud. The ubiquitous presence of primary vapor bubbles in glass inclusions and unaltered anhydrite phenocrysts in the pumice suggest that sulfur was present in a separate H2O-rich gas phase of the Pinatubo magma before eruption. Thus, for this eruption, and perhaps others, the petrologic method for estimating sulfur degassing is prone to substantial underestimation of sulfur release and the potential climatic impact of past explosive eruptions.

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