SCIENTISTS DIVE TO ISLANDS' WOMB : JET OF MOLTEN ROCK NEAR HAWAII SPAWNS VOLCANOES, GUSHES LAVA.

Byline: William J. Broad The New York Times

Rising from deep within the Earth is a jet of molten rock that cuts gargantuan holes in the Pacific seabed, forming new volcanoes and eventually whole new islands as well, its brood including Maui and Oahu.

The jet, most intense on Earth, annually spews enough lava to build a road that could circle the planet twice. It has been thundering and exploding and erupting lava for millions of years.

Recently, scientists have descended in a submersible craft to probe an episode of explosive violence at the jet's leading edge. Their target, a half mile down, was the summit of Loihi, which has suddenly become one of the world's most active volcanoes.

``It's nerve-racking,'' Alexander Malahoff, the expedition's chief scientist, said of his dives into the dark, churning waters where fits of volcanic fury accompany the birth throes of a new Hawaiian Island. ``The top of the volcano is a physical wreck.''

In July and August, the deep site was rocked by thousands of seaquakes, including the strongest ever recorded around Hawaii. Since the volcanic raging is only 17 miles southeast of the big island of Hawaii, disaster officials feared the deep violence might spawn tidal waves at the surface that could devastate the big island and more distant shores of Oahu, including Honolulu and Waikiki Beach.

Land above water was spared, but Malahoff and the other scientists who dove down to the craggy recesses of the undersea volcano discovered a riot of landslides, toppled rock formations and bus-size volcanic boulders strewn over four or five miles.

This was not the result of a major eruption. The turmoil at the volcano's top had collapsed its summit, creating a murky crater more than a half-mile wide and 1,000 feet deep.

``This was a Mount St. Helens-sized volcanic event,'' Malahoff said at a recent news conference at the National Press Club in Washington. ``Pete's Dome, an area on the southern rim of the volcano that previously had been considered very stable, has simply vanished.''

He made dives into the volcanic depths late last month and early in October. The team dove in a Pisces submersible, which can carry three people down a little more than a mile. The team was limited to exploring the summit of the cone rising almost three miles from the ocean floor.

Malahoff is director of the Hawaii Undersea Research Laboratory at the University of Hawaii, and the dive series was financed by the National Oceanic and Atmospheric Administration.

Diving into the new crater, Malahoff found hot vents spewing a mixture of superheated water, dissolved minerals and microbes that thrive in the seabed's rocky substratum. Churning clouds of particles often limited the visibility of the scientific observers to 36 inches or less.

Tricky currents posed dangers. Currents flowed into the newly formed pit, percolated through the volcano's hot interior and rushed out over a lip on the western edge. The scientists had to avoid getting sucked down by the in-rush on one side and pushed up by the out-rush on the other.

They say the tumult was part of the volcano's halting upward growth. Lava builds it up, and avalanches and cataclysmic explosions knock it down and widen it, creating a larger base for the next stage of building. Tens of thousands of years are expected to pass before the volcano's fiery summit rises above the waves.

``The fight is between construction and destruction,'' Malahoff said.

Avalanches are well-known to have shaken the steep sides of Loihi, but no episode this violent has ever before been studied up close.

Scientists say the event sheds important new light on the dynamics of island building as well as a whole range of environmental issues, like the extent to which explosive releases of volcanic gases like carbon dioxide may contribute to greenhouse warming of the Earth.

Such releases, they say, may augment human ones and throw the warming debate into a broader context of natural ups and downs.

And it is aiding overall studies of the Hawaiian jet, the Earth's most dynamic zone of volcanic upheaval. Beneath the big island of Hawaii it powers the fireworks of both the Kilauea and Mauna Loa volcanoes.

``We think it is rather large, as much as 200 kilometers in diameter,'' or about 125 miles, James G. Moore, a geologist with the United States Geological Survey in Menlo Park, Calif., said in an interview.

``Loihi is the first manifestation of volcanic activity on that crust,'' added Moore, who has studied the Hawaiian volcanoes. ``It's the leading edge.''