There have been two types of volcanic eruptions for some time now – explosive or effusive. An explosive eruption is marked by a violent and explosive eruption, such as the Mount St. Helens eruption of 1980. An effusive eruption is marked by the outpouring of lava onto the ground.
However, new research has uncovered a previously undocumented type of eruption in underwater volcanoes.
The research was published in the journal Nature Geoscience and was authored by researchers at Victoria University, Wellington and the National Oceanography Centre in Southampton. In addition to hypothesising the nature of this third type of eruption, the authors also proposed that the new style of eruption be named Tangaroan, the Maori god of the sea, and name of the research vessel used to collect the samples.
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 you take the lid 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.
Above the oceans when pumice is located it generally represents an explosive volcanic eruption. Subsequently, when pumice is lining the underwater volcanoes an explosive eruption is assumed.
Not so, says the new research, which suggests that there is a third type of eruption utterly unique to underwater volcanoes which is neither effusive or explosive. Professor Ian Wright of the National Oceanography Centre explains;
“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.”
“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.”
Which leaves scientists needing to explain just how it got there.
The subsequent study proposed that the formation and expansion of the bubbles in the magma created a buoyant type of foam which rose to the seafloor and then detached from the volcano as molten pumice balloons. Its ascent to the sea surface would see a reduction in pressure as it climbed to higher depths, thus continuing the pumice balloons expansion.
“These processes explain the unique bubble structure seen in the samples analysed, which could have only occurred with an intermediate eruption style and in an underwater setting,” said Professor Wright.
“We conclude that the presence of widespread deposits of pumice on underwater volcanoes does not necessarily indicate large-scale explosive volcanism.”
Source: National Oceanography Centre