Scrubbing up knowledge of submarine volcanoes

A study of the shape of pumice from three adjacent submarine lava dome volcanoes in the western Pacific reveal that explosive volatility driven by the movement of molten magma is lower in deeper water. The shape of pumice stones, which are formed by expansion of magmatic volatiles as the magma rises to the sea surface, is different depending on the water depth and so can be a useful indicator of the evolution and eruption of underwater volcanoes.

Sharon Allen of the ARC Centre of Excellence in Ore Deposits and the School of Earth Sciences, at the University of Tasmania, Hobart, Australia and colleagues Richard Fiske of the Smithsonian Institution, Washington, DC, and Yoshihiko Tamura of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in Yokosuka Japan, used sampling and observations collected by a remotely operated vehicle of the three adjacent submarine lava dome volcanoes of the Sumisu, Izu-Bonin arc in the Western Pacific.

Domes of the volcanic complex have summits at ocean depths of 1100, 600, 245, and 95 metres and are mantled with pumice that is chemically identical but size, distribution, and surface texture varies enormously across the volcanic range.

Sharon Allen
Sharon Allen

According to a report in the May issue of the journal Geology, pumice generated from lava domes at water depths of more than 500 metres formed as a thick carapace on dense rock whereas at water depths less than 500 m pumice is blasted out. At shallower than 500 metre depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts (rock fragments) enclosed by quenched margins and pockmarked by coarse [centimetre-sized] vesicles, a rock fragment within which is trapped a bubble of gas, the team explains.

The study shows that an increase in hydrostatic pressures over a range of 12 megapascals [120 times atmospheric pressure] reduces volatile-driven explosivity of the dome-forming eruptions the team says, it does not affect the formation of rocky “bubbles, the vesicles. “We conclude that metre-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 metres, and its morphology can be used to distinguish between explosive and effusive origins,” they conclude.

Links

Geology, 2010, 38(5), 391-394.
Sharon Allen homepage

Volcanic greenhouse

Volcanoes, such as Mount Vesuvius, that sit on carbonate sediments could represent a previously underestimated source of atmospheric carbon dioxide and may therefore be a contributor to global warming, according to Italian geoscientists.

Giada Iacono-Marziano of the National Institute of Geophysics and Vulcanology in Palermo, and colleagues Fabrice Gaillard, Bruno Scaillet, and Michel Pichavant of the University of Orleans, France, and Giovanni Chiodini of the Vesuvius Observatory, point out that Mount Vesuvius, one of the most infamous of the world’s volcanoes, has been quiescent since 1944. However, this inactivity is currently associated with an insidious phenomenon, elevated carbon dioxide emissions from the volcano.

Vesuvius

Vesuvius

This carbon dioxide, which amounts to about 300 tonnes per day, is the equivalent of yearly anthropogenic carbon dioxide emissions by a region of the world such as Chad, Tonga, or the British Virgin Islands. During the last eruptive period, 1631 to 1944, the carbonate-sourced carbon dioxide comprised 4.7 to 5.3 percentage mass of the vented magma, the team adds.

Unfortunately, the precise origin of this carbon dioxide is a matter of debate. However, Iacano-Marziano and colleagues point out that Mount Vesuvius sits on a carbonate sedimentary sequence several kilometres thick, and to them the source of the carbon dioxide is fairly obvious.

Iacono-Marziano and colleagues have now demonstrated how the red-hot basaltic magma of Mount Vesuvius coming into contact with this carbonate bedrock subsumes it and liberate carbon dioxide gas in so doing. This, the team says, explains the carbon dioxide emissions measured at the surface around the volcano.

The team suggests that this kind of assimilation of carbonate rocks by magma probably contributes, to some degree, to the carbon dioxide degassing of several other volcanic centres that are either dormant or active but located over sedimentary rocks. Magma-carbonate interactions could therefore have a major role in global carbon dioxide emissions from volcanoes, which has been underestimated so far, the team concludes.

Further reading

Geology 2009, 37, 319-322
http://dx.doi.org/10.1130/G25446A.1

Istituto Nazionale di Geofisica e Vulcanologia volcano research
http://portale.ingv.it/research-areas/vulcanoes/volcanoes

Suggested searches

volcanoes
carbon dioxide
global warming

Volcanoes of the Moon

Even though astronauts have set foot on the Moon, analysed its surface and brought samples back to Earth, we do not yet fully understand the Moon’s origins nor how it has evolved during the last few billion years since its formation. New clues have now emerged from a study of the Moon’s past volcanic activity that suggest that volcanic activity began 4.35 billion years ago (+/- 0.15billion), a relatively short time after the formation of our planet’s biggest satellite.

Kentaro Terada of Hiroshima University, in Japan, and Mahesh Anand of The Open University, Milton Keynes, UK, and their colleagues have dated minerals from a fragment of a lunar meteorite dubbed Kalahari 009, which overturn the notion that volcanic activity on the Moon began a mere 3.8-3.9 billion years ago. The fragment is classified as a very-low-titanium breccia a piece of crystallised magma from a lunar mare (sea) basalt. The team used ion microprobe uranium-lead dating of phosphates in this chunk of moon rock to reset the volcanic clock.

Information locked in Moon rocks could provide clues about the early solar system (Credit: Institut fur Planetologie, Universitaet Muenster)

Information locked in Moon rocks could provide clues about the early solar system (Credit: Institut fur Planetologie, Universitaet Muenster)

The researchers suspected that the fragments of crystallized magma come from the earliest eruptions on the Moon’s vast plains. These ancient phosphate ages are thought to represent the crystallization ages of parental basalt magma, making Kalahari 009 one of the oldest known mare basalts, the researchers say. Lunar rock samples collected by Apollo astronauts resulted from more recent eruptions although these high-titanium collected samples are much older than other known low-titanium samples.

The authors suggest that the Moon’s volcanism must have started at the time its crust was forming and that the Kalahari 009 meteorite is the first example of a ‘cryptomare’ sample from the Moon as it conveys a hidden signature of the Moon’s earliest history.

Mahesh Anand

Mahesh Anand

According to Anand, rocks from the Moon are the only source of samples that allow us to probe back to the first 500 million years of the solar system. Such ancient rocks have long since been subsumed by plate tectonics on Earth, the Moon rocks represent a geological fossil record.

The Moon

The Moon

Further reading

Nature, 2007, 450, 849-852;
http://dx.doi.org/10.1038/nature06356

Mahesh Anand homepage
http://www.open.ac.uk/planetarygeology/

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Moon