Abstract:
Domes of the Sumisu volcanic complex (western Pacific), having summits at ocean depths of 1100, 600, 245, and 95 m, are mantled with compositionally identical rhyolitic pumice that has similar vesicularity, but that varies systematically in distribution, size, and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth. At depths >500 m, the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant (meters to tens of meters) polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin. Vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace. At <500 m depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts enclosed by quenched margins and pockmarked by coarse (cm) vesicles. No carapace pumice occurs, and the summit is composed of craggy dense dome rock. These shallower water pumice clasts resemble those spalled from historic submarine dome-forming eruptions that buoyed to the sea surface. We interpret spalling to result from vent-derived, weak volatile-driven explosions that take place at water depths <500 m. Our study shows that an increase in hydrostatic pressures over a range of 12 MPa reduces volatile-driven explosivity for subaqueous, rhyolitic, dome-forming eruptions, but does not affect vesicularity. We conclude that meter-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m, and its morphology can be used to distinguish between explosive and effusive origins.
