AsianScientist (Jan. 28, 2013) – Researchers from New Zealand and the U.K. have discovered a new type of underwater volcanic eruption that spews a foam-like mass of lava that rises slowly to the water surface.
Scientists had believed that underwater volcanoes erupt in a manner similar to those on land; Land volcanoes emit lava in two ways: in explosive eruptions where lava is spewed in violent explosions or in effusive eruptions where there is a seeping flow of hot magma. As a result, many underwater volcanoes had been presumed to be explosive during eruptions.
In their paper, published in Nature Geoscience, the researchers refute these presumptions and describe an intermediate kind of underwater volcanic eruption that is neither explosive nor effusive. The team made their discovery while studying pumices, highly porous volcanic rocks, on New Zealand’s Macauley Island.
“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, a co-author of the paper.
“This indicates that the lava erupting from the caldera was neither vigorous enough for an explosive eruption, nor gentle enough for an effusive flow.”
The researchers believe that underwater volcanoes emit a buoyant form of lava that is formed from the slow expansion of the vesicles. As the vesicles expand, molten lava is lifted from the seafloor and is detached to form a foamy mass.
These foam-like masses are hot on the inside and cold on the outside. As they rise to the sea surface, the vesicles within these lava masses continues to expand as seawater pressure is reduced.
The cooling of the lava then leads to the formation of pumice that are characteristically different from those hat result from explosive or effusive eruptions.
“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.
The authors have proposed that this new type of eruption be called Tangaroan eruptions, after Tangaroa, the Maori god of the sea, and the name of the research vessel used to collect the samples.
The article can be found at: Rotella et al. (2013) Highly Vesicular Pumice Generated By Buoyant Detachment Of Magma In Subaqueous Volcanism.
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Source: National Oceanography Center.
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