A study of the shape of pumice from three adjacent submarine lava dome volcanoes in the western Pacific reveal that explosive volatility driven by the movement of molten magma is lower in deeper water. The shape of pumice stones, which are formed by expansion of magmatic volatiles as the magma rises to the sea surface, is different depending on the water depth and so can be a useful indicator of the evolution and eruption of underwater volcanoes.
Sharon Allen of the ARC Centre of Excellence in Ore Deposits and the School of Earth Sciences, at the University of Tasmania, Hobart, Australia and colleagues Richard Fiske of the Smithsonian Institution, Washington, DC, and Yoshihiko Tamura of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in Yokosuka Japan, used sampling and observations collected by a remotely operated vehicle of the three adjacent submarine lava dome volcanoes of the Sumisu, Izu-Bonin arc in the Western Pacific.
Domes of the volcanic complex have summits at ocean depths of 1100, 600, 245, and 95 metres and are mantled with pumice that is chemically identical but size, distribution, and surface texture varies enormously across the volcanic range.
According to a report in the May issue of the journal Geology, pumice generated from lava domes at water depths of more than 500 metres formed as a thick carapace on dense rock whereas at water depths less than 500 m pumice is blasted out. At shallower than 500 metre depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts (rock fragments) enclosed by quenched margins and pockmarked by coarse [centimetre-sized] vesicles, a rock fragment within which is trapped a bubble of gas, the team explains.
The study shows that an increase in hydrostatic pressures over a range of 12 megapascals [120 times atmospheric pressure] reduces volatile-driven explosivity of the dome-forming eruptions the team says, it does not affect the formation of rocky “bubbles, the vesicles. “We conclude that metre-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 metres, and its morphology can be used to distinguish between explosive and effusive origins,” they conclude.