Breaking crust: sonar finds new kind of deep-sea volcano.

Explorations east of Japan have revealed a previously unknown type of volcano.

Volcanoes typically emerge in one of three geological settings, explains Stephanie P. Ingle, a geochemist at the University of Hawaii in Honolulu. Some crop up along mid-ocean ridges, where Earth's tectonic plates spread apart. Others, such as those on land along the western coast of North America, form where one plate is being sub-ducted, or forced beneath, another plate. And volcanoes far from plate boundaries, such as those in Hawaii, appear where hotter-than-normal plumes of Earth's mantle well up (SN: 7/9/05, p. 24).

Ingle and her colleagues, however, have found inactive volcanic features in a completely new setting under the sea about 600 kilometers from Japan. "Finding these peaks was serendipitous luck" Ingle notes.

Analyses of ocean floor nearer Japan had hinted at the presence of undersea volcanoes. So, Ingle's team used sonar to scan the ocean bottom farther offshore. In a 6-km-deep location in the Pacific, the team spotted peaks about 1 km across and 50 meters high. Lava samples were then dredged from the exposed portions of those mostly buried volcanoes.

The thickness of mineral layers that had formed on the surfaces of lava chunks suggests that the undersea volcanoes were last active 50,000 to 1 million years ago, the researchers report in an upcoming Science. The scientists also describe the peaks they had found nearer Japan. Those erupted between 8 million and 4 million years ago.

Chemical analyses of crystals embedded in the lava hint that the material originated about 14 km below the ocean floor. However, current models of Earth's structure hold that any of the minerals expected at those depths should be solid, so they wouldn't provide lava to fuel volcanoes, says Ingle.

Unexpectedly high concentrations of water or other volatile substances in the rocks could lower their melting points, she notes. Then, the first substances to melt would yield lava containing high concentrations of sodium and potassium--just as the lava dredged from the newfound volcanoes does.

The researchers speculate that the unusual volcanoes formed where the ocean floor flexed and cracked as it headed toward the Japan subduction zone.

"This is a surprising place to find volcanism;' says Donald W. Forsyth, a marine geophysicist at Brown University in Providence, R.I. However, he notes, it's not clear that the volcanic activity is related to flexure of the ocean floor. Most plate-tectonic models suggest that ocean-floor stresses in that region wouldn't be high enough to fracture Earth's crust.

The team's findings call for a re-examination of whether mantle plumes caused other volcanoes that are far from tectonic-plate boundaries, says Marcia K. McNutt, a geophysicist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. The newfound undersea peaks are clear-cut examples of volcanoes that have been formed by an alternative method, and such a mechanism may be at work elsewhere as well, she notes.