The outflow is buoyant but not explosive. This could well make senseof a lot of geology that presumes explosive eruptions yet generateshuge deep flows of materials over a large area. When you considerit, we have no evidence of deep explosive sub sea events anywhereclose to what we experience on land. They must then behavedifferently and this shows us how.
It is quite right to attach a fresh name to this type of volcano.
It also suggests that during the sub sea stage, most of the materialwill accumulate close to hand and help build the cone that doeseventually break surface. It may even be a low faster than weactually expect. I the meantime, we think that they are rare, butthey may be merely invisable.
A new type ofvolcanic eruption
by Staff Writers
Southampton UK(SPX) Jan 24, 2013
http://www.terradaily.com/reports/A_new_type_of_volcanic_eruption_999.html
Scientists based inthe UK and New Zealand have described a "new" type ofvolcanic eruption. Volcanic eruptions are commonly categorised aseither explosive or effusive. But now, in research published thismonth in Nature Geoscience, researchers at Victoria University,Wellington and the National Oceanography Centre in Southampton haveuncovered a previously undocumented type of eruption in underwatervolcanoes - by looking at tiny original bubble spaces trapped involcanic rock.
Inside volcanoes,gases are dissolved in the molten magma as a function of the veryhigh pressures and chemistry of the magma. In the same way that gasesdissolved in carbonated drinks bubble up when you take the lid off,when magma is erupted as lava, the pressure is relieved and the gasesexsolve to form small gas bubbles or so-called "vesicles".
In explosive eruptionsthese vesicles expand so quickly they fragment the magma, violentlyejecting lava, which cools and degasses to form solidified pumicethat can be sufficiently light to float on water.In air pumice isobviously associated with violent, explosive eruptions. Consequentlyunderwater volcanoes flanked by highly vesicular pumice have, todate, also been interpreted as having erupted explosively.
But the results ofthis study indicate that there is a third eruptive style unique tounderwater volcanoes, which is neither effusive nor explosive.
"By documentingthe shape and density of bubbles in pumices generated by anunderwater caldera volcano in the southwest Pacific Ocean - theMacauley volcano - we found large differences in the number and shapeof "bubbles" in the same pebble-sized samples, different toanything previously documented," said Professor Ian Wright ofthe National Oceanography Centre, who co-authored the paper.
"This range ofbubble densities distinct in these pumice samples indicates that thelava erupting from the caldera was neither vigorous enough for anexplosive eruption, nor gentle enough for an effusive flow."
The study proposesthat rather than exploding in the neck of the volcano, theformation and expansion of bubbles in the magma created a buoyantfoam, which rose to the seafloor and then buoyantly detached from thevolcano as molten pumice balloons but with chilled margins.During its ascent to the sea surface, the vesicles within the molteninterior would have continued to expand as the pressure - this timefrom the weight of the seawater – reduced.
"These processesexplain the unique bubble structure seen in the samples analysed,which could have only occurred with an intermediate eruption styleand in an underwater setting," said Professor Wright.
"We conclude thatthe presence of widespread deposits of pumice on underwater volcanoesdoes not necessarily indicate large-scale explosive volcanism."
The authors proposedthat this style of eruption be named Tangaroan, the Maori god of thesea, and name of the research vessel used to collect the samples.
The study was led byMelissa Rotella, Professor Colin Wilson and Simon Barker from theSchool of Geography, Environment and Earth Sciences at the VictoriaUniversity of Wellington, New Zealand. The paper referred to is:Rotella M. D., Wilson C. J. N., Barker S. J. and Wright I. C. (2013)Highly vesicular pumice generated by buoyant detachment of magma insubaqueous volcanism. Nature Geoscience doi: 10.1038/NGEO1709.
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