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Sitting right on top of an eruption.

Oceanographers have captured the first close-up measurements of a subsea volcanic eruption, thanks to a mixture of careful planning and geologic good luck. Their observations offer new clues into how outpourings of lava create the seafloor and season the oceans with important chemicals.

The eruption started on January 25, 1998, at Axial Seamont, a broad-shouldered mountain 300 kilometers west of Cannon Beach, Ore. Researchers with the National Oceanic and Atmospheric Administration (NOAA) had previously surveyed Axial and set out several instruments there in 1997, hoping to record any unrest. In the late summer of 1998, they returned to examine the seafloor and see if their instruments had survived the 10-day volcanic episode.

Using a robotic sub, the team recovered three of the five equipment moorings attached to the seafloor near the eruption site. One sensor, which gauges water depth, indicated that the crater at the top of the volcano dropped by 3 meters during the episode, says Christopher Fox of NOAA in Newport, Ore.

Fox suspects that before the eruption, the volcano's summit swelled up as molten rock gathered beneath it. During the eruption, the magma forced its way to the southern flank of the volcano and erupted out that side. The summit then sank like a cooling souffle.

The NOAA team also recovered two strings of temperature sensors. According to these instruments, which floated above the ocean floor, the water temperature spiked about 3 hours alter earthquakes had started rumbling, says Edward T. Baker of NOAA in Seattle. The lag reflects the time it took magma to work its way up to the seafloor and erupt at the surface.

For all their success, the team may have been a little too lucky in choosing sites for their instruments. The sensors were so close to the action that one disappeared without a trace and another became embedded in a lava flow. The NOAA crew tried to extricate the trapped instrument using the robotic submersible, but the gambit failed.
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Publication:Science News
Date:Jan 2, 1999
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