New Volcano Type Found in Pacific?

Brian Handwerk
for National Geographic News
July 27, 2006
A new type of volcano may be heating up the floor of the western Pacific Ocean.

Scientists suspect the new volcanoes occur at a crack in tectonic plates caused by stress as the plates slide past each other.

The group of small volcanoes, called petit spot volcanoes, was discovered far from the tectonic-plate boundaries (such as mid-oceanic ridges) that often spawn volcanoes, earthquakes, and other geologic activity.

Until now, scientists explained volcanoes located outside tectonic boundaries using mantle-plume theory. According to the theory, molten rock from deep within the Earth rises to the surface to produce these volcanoes.

But geoscientist Naoto Hirano's team suggests that the petit spot volcanoes are instead fueled by partially melted rock from much closer to the Earth's surface.

The team will report its findings in tomorrow's edition of the journal Science.

Plume Theory

Mantle-plume theory suggests that "plumes" of molten rock originate deep within the planet and rise to the surface to produce "hot spot" volcanoes like those in the Hawaiian Islands.

It has been the dominant theory for nearly 30 years. (Related story: "Deep Sea Volcano Erupts on Film—A First" [May 24, 2006].)

But Hirano's team reports that petit spot volcanoes yield no evidence of a liquid rock source from deep within the Earth.

They believe the source of these volcanoes is melted rock from the upper mantle, much closer to the surface, which has been squeezed through cracks in the tectonic plate above.

When one tectonic plate slides beneath another in a process known as subduction, the forces flex and bend the plates. This may be responsible for creating such cracks.

(Learn more about volcanoes in an interactive feature.)

"This type of [activity produces] tiny volcanoes, possibly now active, on the old, cold subducting Pacific plate," said Hirano from his office at the Scripps Institution of Oceanography in La Jolla, California.

"This petit spot volcano theory suggests that this type of eruption can occur wherever the oceanic plate is flexed."

Second Opinion

Geophysicist Marcia McNutt, president and CEO of the Monterey Bay Aquarium Research Institute in Moss Landing, California, wrote a commentary on the new research.

"These small volcanoes—because of their location and the fact that there are these clear cracks that have formed due to the bending of the [tectonic] plate—almost assuredly did not form by a plume," she said.

"The coincidence that it would require to have a plume just happen to be right below those cracks in the plate—it's too big of a coincidence to be credible."

McNutt suggests that Hirano and colleagues have a far more likely explanation.

"Scientists used to assume that any time there was volcanic activity that was not at a plate boundary, there had to be a plume that created it," she said.

"Maybe [the petit spot theory] could be an explanation for a lot more than this chain of volcanoes," she added.

"Maybe many features on the ocean floor, which we have tried with sometimes limited success to make fit the plume theory, could actually be [caused] by this same mechanism."

Versions of a tectonic-plate "crack" theory have been around since the mid-19th century, but the new research may provide the first real-world example. Study author Hirano hopes to uncover more volcanoes that support the theory.

"We need more surveys of the ocean floor in order to answer [questions] about frequency of petit spot volcanoes," he said. "We know of only two sites."

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