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Volcanic Activity Discovered at Samoan "Hot Spot"

Underwater volcanic explosions could produce danger as a major navigational hazard

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December 12, 2000

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Scientists from Woods Hole Oceanographic Institution and Scripps Institution of Oceanography have discovered "strong evidence" for current volcanic activity at the Vailulu'u summit east of Samoa.

In the electronic journal Geochemistry, Geophysics and Geosystems, published by the American Geophysical Union, Stan Hart of Woods Hole and Hubert Staudigel of Scripps Institution report that Vailulu'u's crater is filled with smoggy waters, more turbid than any other underwater volcano studied.

They analyzed the distribution of particulate matter, or underwater "smog," that forms when hot volcanic rocks react with seawater. This smog spills over the crater rim and forms a halo around the Vailulu'u summit. These reactions produce particulates, including minerals formed from the reactions between seawater and volcanic rock, and from elevated levels of biological activity that is characteristic of most submarine eruptions and hydrothermal systems.

"The discovery of a 'smoking' active underwater volcano at the eastern end of the Samoan Island chain is significant because it supports the interpretation of the Samoan islands as a hot spot chain, similar to Hawaii," said Hart . "It also provides a unique natural laboratory where submarine volcanism and its physical, biological, and chemical effects may be studied in close proximity to an island."

Locating a new hot spot volcano such as Vailulu'u provides fundamental insights into how ocean island chains form and is important for determining the absolute motion of the oceanic plates.

But such underwater volcanoes also may be dangerous, the authors said. "Submarine volcanoes like Vailulu'u are potentially hazardous because a 600-meter-deep underwater volcano may become highly explosive during underwater eruptions," said Staudigel. "Such underwater explosions are dangerous because the volcanic explosive strength can be highly amplified by the force of the vaporization of seawater. This may cause tidal waves and could present a major navigational hazard."

The report is the result of two expeditions aboard the Scripps research vessel Melville and the U.S. Coast Guard ice breaker Polar Star.

Vailulu'u is located at the eastern leading edge of the Samoan ridge and rises from 5,000 meters to within about 600 meters of the sea surface, including a 400-meter-deep and 2-kilometer-wide summit crater. Three rift zones extend from this crater to the east, west, and southwest, very closely resembling the physical appearance of Ta'u, the neighboring island to the west. Vailulu'u is similar in size to many well known volcanoes, such as Mt. Fuji in Japan and Mt. Ranier in Washington State.

Rocks dredged by the scientists from Vailulu'u had shiny fresh glassy surfaces and a sulphurous odor that are characteristic of very recently erupted undersea lavas.

The timing of Vailulu'u's volcanic activity is close to events that have been recorded by global seismic networks. Major earthquake activity was recorded at or near Vailulu'u in July 1973 and January 1995. The authors say it is quite possible that these two seismic swarms were related to the eruptions that produced their dated rock samples.

Using isotopic tracing techniques, the scientists determined that the volcano was formed by partial melting of materials in the earth's mantle with a characteristically "Samoan" composition or pedigree. The examinations suggested volcanic activity within the past 5 to 50 years, further evidence that Vailulu'u is the current location of the Samoan hot spot.

As an interesting sidelight, the evidence for volcanic activity at Vailulu'u may have an historical antecedent in Samoan mythology. Kanaloa, the oldest Polynesian volcano god, is said to have been involved in a violent battle to the East of Ta'u after which he landed on its eastern slopes. The site of his landing was honored with temple to Kanaloa, built about 3,000 years ago. This temple faces the site of Kanaloa's battle, and current location of Vailulu'u. Perhaps the ancient Polynesians knew about Vailulu'u all along?

In March 2000, the scientists deployed ocean bottom seismographs on the summit of Vailulu'u to listen to microseismic activity that will produce data to provide further clues to the presence of volcanic activity and help determine when and where magma moves within the volcano.

The deployment of seismographs included one deployment by helicopter — a method of marine instrument deployment that has never been done before, which may prove very useful in the quick response to a submarine volcanic crisis.

The research cruises were funded by the National Science Foundation.

Originally published: December 12, 2000