Scientists discover new type of underwater volcanic eruption

Scientists have uncovered a previously undocumented type of eruption in underwater volcanoes - by looking at tiny original bubble spaces trapped in volcanic rocks.

Volcanic eruptions are commonly categorised as either explosive or effusive but researchers from the UK and New Zealand have uncovered a new type of eruption in underwater volcanoes.

Inside volcanoes, gases are dissolved in the molten magma as a function of the very high pressures and chemistry of the magma.

In the same way that gases dissolved in carbonated drinks bubble up when the lid is taken off, when magma is erupted as lava, the pressure is relieved and the gases exsolve to form small gas bubbles or so-called "vesicles".

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 new study by Victoria University, Wellington and the National Oceanography Centre in Southampton indicate that there is a third eruptive style unique to underwater volcanoes, which is neither effusive nor explosive.

"By documenting the shape and density of bubbles in pumices generated by an underwater caldera volcano in the southwest Pacific Ocean - the Macauley volcano - we found large differences in the number and shape of 'bubbles' in the same pebble-sized samples, different to anything previously documented," said Professor Ian Wright of the National Oceanography Centre, who co-authored the paper.

"This range of bubble densities distinct in these pumice samples indicates that the lava erupting from the caldera was neither vigorous enough for an explosive eruption, nor gentle enough for an effusive flow," Wright said in a statement.

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