Subsea volcanoes found near Hawaii.

Subsea volcanoes found near Hawaii

Using a high-resolution sonar to shoot pictures of the ocean bottom, geologists have discovered extensive young lava flows several hundred kilometers from the Hawaiian islands. The find may force scientists to revise their ideas about the volcanic system that formed this island chain in the mid-Pacific.

"These were totally unknown until last April," says Mark Holmes of the U.S. Geological Survey (USGS) in Seattle, Wash. "The surface area of the flows that we've mapped is twice the size of the subaerial [above sea level] portions of the major Hawaiian islands, so it's really a phenomenal amount of material that's erupted."

These findings emerged from a continuing USGS project to map the Exclusive Economic Zone, the 200-mile-wide border extending from all U.S. coast-lines. At its heart, the project relies on the GLORIA sidescan sonar, which uses sound waves to create detailed pictures of the seafloor. Holmes and his colleagues reported their discoveries last week in Denver at the meeting of the Geological Society of America.

The lava flows appear on the sonar images because they are young and covered only by a thin veneer of sediment. The relatively rough lava surface reflects more sound energy than the surrounding seafloor, which is topped by a thick sediment layer.

Based on samples dredged from the bottom as well as the lack of cover on the lava, researchers believe some of the flows may date back only 50,000 to 200,000 years, which is younger than most eruptions along the major Hawaiian islands, Holmes says.

According to widely accepted theory, the Hawaiian islands and the chain of Emperor seamounts to the northwest all formed as the Pacific plate passed slowly over a stationary "hotspot" in the Earth's upper mantle, which is fed by a plume of molten rock rising from deeper in the mantle. The island of Hawaii is now leaving the hotspot while a new volcano, Loihi, is starting to grow through underwater eruptions southeast of the "big island."

Until now, though, scientists thought the hotspot fed a small geographic area that corresponds to the main axis of the islands, so new islands would pop up all in a line, says Holmes. If these recently discovered lava flows are linked to the hotspot, then volcanic plumbing in the area reaches outside the classical hotspot boundary. By pulling up samples of the basalt erupted in the flows, scientists will be able to date the flows and compare this material to that found on the islands.