From Sci-News.com via Nature -
Volcanic eruptions are commonly categorized as either explosive or effusive... In explosive eruptions these vesicles expand so quickly they fragment the magma, violently ejecting lava, which cools and degasses to form solidified pumice that can be sufficiently light to float on water.
In air pumice is obviously associated with violent, explosive eruptions. Consequently underwater volcanoes flanked by highly vesicular pumice have, to date, also been interpreted as having erupted explosively.
But the results of the study, published in the journal Nature Geoscience, indicate that there is a third eruptive style unique to underwater volcanoes, which is neither effusive nor explosive....