Broad blankets of pumice that surround submarine volcanoes are generated by an intermediate eruptive style that is neither explosive nor flowing, reports a study published online this week in Nature Geoscience. Previously, such widely dispersed bubbly deposits were taken to indicate vigorous and explosive volcanism.

Melissa Rotella and colleagues analysed the textural characteristics of pumice rocks that had erupted from an underwater volcano in the southwest Pacific Ocean. The rocks had bubbly interiors, but less bubbly margins. The distribution of bubbles suggests that the pumice formed from the eruption of parcels of buoyant magma foam. They suggest that the parcel edges cooled quickly due to contact with the ocean water, whereas gas bubbles continued to form in the molten interior. These buoyant foam parcels seem to have been widely distributed by ocean currents, before disintegration and sinking.