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Published experimental, thermodynamic, and other geochemical data on H2O-bearing silicic melts are used to obtain relationships which show that: (1) the rate of exsolution of H2O (vesiculation) from silicic magmas that contain more than a few tenths of 1 wt percent H2O is sufficiently rapid to contribute to the explosivity of pyroclastic eruptions; (2) the exsplution of only a few tenths of 1 percent H2O from a typical rhyolitic magma by the second boiling reaction—H2O-saturated melt crystals + H2O vapor releases sufficient mechanical energy (ΡΔV work of expansion)—to cause tensional fracture failure of wall rocks at pressures corresponding to ocean depths of at least 10 km; (3) the ΡΔV energy released by the exsolution of additional H2O, as a result of decompression following Wall-rock failure, is fully adequate to produce pyroclastic eruptions, even at these great ocean depths; (4) the crystallinity and vesicularity of the juvenile pyroclasts of the tuff units that host and underlie the Kuroko ores in the Hokuroku district of Japan are consistent with their having been erupted onto the sea floor at an ocean depth of 3.5 ± 0.5 km, from a magma chamber situated 1.1 ± 0.3 km beneath the sea floor; and (5) the submarine caldera model of Ohmoto (1978) for the formation of volcanogenic massive sulfide deposits appears, therefore, to be viable, at least for the deposits in the Hokuroku district. Application of these same relationships to the 1980 eruption of Mount St. Helens suggests that the March through May 18, 1980, eruptive sequence, including the intrusion of magma into the northern bulge and the landslide-triggering earthquake, was initiated by the second boiling reaction at a snbstantial depth beneath the summit. Furthermore, the devastating blast conld have been, but probably was not, cansed entirely by the virtually instantaneons exsolution of H2O from a higher level magma upon sudden decompression that accompanied the landslide.

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