Pacific volcanoes share split personality

Dual chemistry of island chains reflects variations in their deep source Web edition : Monday, September 19th, 2011 Kilauea powFireworks can ensue when lava meets ocean, seen here at Hawaii's Kilauea Volcano. Geochemical studies suggest that the molten rock forming Hawaii and other Pacific island chains may pull from two distinct sources.Michael Poland/USGS

Hawaii’s scenic volcanoes come in two chemical flavors, and now scientists think the igneous peaks on several other Pacific island chains do, too.

Two parallel lines of volcanoes stretch from the Big Island of Hawaii in the southeast to Molokai in the northwest.  Volcanoes on the Samoan and Marquesas islands are similarly paired. A new study finds that, as in Hawaii, one row is richer than the other in versions of elements such as lead and neodymium.

“This might be a common feature for all the Pacific hotspots,” says Shichun Huang, a geochemist at Harvard University and lead author of a paper appearing online September 18 in Nature Geoscience.

If so, these island chains may tap the same source deep in Earth’s mantle. Molten rock rises toward the surface in two chemically distinct streams, one stream feeding each row of volcanoes.

Geologists think Hawaii, Samoa, and the Marquesas each formed as a plate of Earth’s crust moved across a “hotspot,” the top of a plume carrying molten material from the planet’s deep interior. Like a welding torch passing across a piece of metal, the hotspot punched out island after island as the plate moved over it.

Recent studies have shown that the hotspots are more complex than once thought, says isotope geochemist Dominique Weis of the University of British Columbia. The mantle plume rising below Hawaii, for instance, feeds individual streams of chemically distinct magmas into Mauna Loa and Mauna Kea, both on the Big Island. Mauna Loa has a higher ratio of the most abundant form of lead on Earth, lead-208, compared with lead-206, which has two fewer neutrons in its nucleus.

By analyzing published data on lava samples, Huang and his colleagues have now shown that this chemical difference also exists in Samoa and the Marquesas.

The plumes feeding these island chains (as well as Hawaii’s) apparently tap a single massive reservoir that underlies much of the central and southern Pacific. This reservoir contains chemical signatures of ancient surface rock that plowed into the interior eons ago through plate tectonics. As a plume rises, it carries part of this material with it.

The new work shows how surface volcanoes can be linked to deep sources of magma, says geochemist Albrecht Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany. A few other scientists have questioned the existence of mantle plumes, but the new work “strongly suggests that at least these particular hotspots are actually mantle plumes that ascend from the lowermost mantle,” Hofmann says.

Huang’s group is now checking other Pacific island chains to see if they too show this same two-faced nature.

Found in: Earth and Earth Science

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Gassy volcanoes tied to mass extinction

SAN FRANCISCO — The greatest extinction in the history of life may have been caused, in part, by ozone-depleting gases spewed in a massive volcanic eruption, a new study suggests. Geologists have found surprisingly high amounts of the elements fluorine and chlorine in Siberian lavas dating back 250 million years — when about 90 percent of marine species and 70 percent of terrestrial species went extinct.

Benjamin Black, a graduate student at MIT, and his colleagues described their theory December 13 in a poster presentation at a meeting of the American Geophysical Union.

Researchers have long struggled to explain the “Great Dying” that occurred at the end of the Permian period. Some think that the extinction was a long, drawn-out affair caused by multiple factors — perhaps gradual changes in oceanic or atmospheric chemistry (SN: 5/28/05, p. 339). Others have blamed a single catastrophic event such as a belch of methane from the seafloor or an asteroid impact (SN: 2/24/01, p. 116) like the one thought to have wiped out the dinosaurs and other species 65 million years ago.

Volcanoes might be one of those calamities. In Siberia, around 250 million years ago, a series of massive volcanic eruptions spewed out lava over more than 2 million square kilometers. Some scientists have blamed these eruptions, known as the Siberian Traps, for climatic changes that contributed to the extinction.

Black and his MIT adviser, Lindy Elkins-Tanton, have been traveling to Russia for the past few summers to test such theories in the Siberian Traps.

The rocks contain tiny blobs of once-molten material, preserved like chemical time capsules from the earliest days of the eruption. Measuring the amounts of sulfur, chlorine and fluorine in those blobs, Black found surprisingly high levels of those elements — up to 0.75 percent chlorine and 1.95 percent fluorine, by weight, in one sample. That’s significantly more than the amounts found in similar deposits like the Deccan Traps in India and the Columbia River flood basalts in Washington and Oregon.

The chemicals probably weren’t in the magma as it began traveling up from deep within the Earth, the team proposed, but melted into the magma as it passed through salt-rich rock before erupting on the surface.

In all, the amount of chemicals in the Siberian rocks could translate to 9 trillion tons of sulfur, 8.5 trillion tons of fluorine and 5 trillion tons of chlorine spewing into the atmosphere during the eruptions. Such elements, when pumped out by power plants, can cause acid rain locally. If the eruptions were violent enough to lift the chemicals high into the atmosphere, the team proposed, the chemicals could have damaged the ozone layer just as chlorofluorocarbons do today — helping cause or at least exacerbate the mass extinction.

Stephen Self, a volcanologist at the U.S. Nuclear Regulatory Commission in Washington, D.C., said the main question is how long the chemicals would have stayed in the atmosphere. In a 2008 Science paper, Self and his colleagues reported finding high levels of sulfur and chlorine in the Deccan Trap lavas, though not as high as Black’s team reports.

“What happens to the chlorine is highly questionable,” Self said. It might, for instance, react with water droplets and rain out quickly.

Black’s team is now starting detailed calculations to see how high the chemicals would have gotten into the atmosphere. Regardless, the researchers reported, Siberia at least would have had one very bad time of it.

Found in: Earth and Earth Science

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