Near-Death on the Ocean Floor

Alive and well. The instrument that recorded a submarine volcanic outbreak.

When an oceanographic instrument was hit by a scorching lava flow in 1998, it might well have been fried. But the instrument serendipitously survived to tell its tale. In this week's issue of Nature, researchers report that it captured the most complete picture of an underwater eruption ever. Scientists say the data confirm existing models of underwater volcanic eruptions.

Geophysicist Christopher Fox and his colleagues at the University of Oregon have maintained instruments at the Axial Volcano 500 kilometers off the Oregon coast for 15 years to study the dynamics of volcanic activity on the seafloor. In October 1997, they deployed a "rumbleometer," an instrument that precisely measures ocean depth, current, and temperature near the volcano. They intended to use it to watch movements of the seafloor prior to eruptions. But when Axial came to life in what is known as a sheet flow on 25 January 1998, the rumbleometer unexpectedly got caught in the lava's path.

Luckily, sheet flows take place gradually. A large, thin layer of lava spills out from the volcano, covering several square kilometers, before its edges harden and stop it from expanding. The rumbleometer's legs held its instruments a meter above this fiery blanket, recording temperatures only 4ºC above normal due to the extraordinary capacity of seawater to absorb heat. Subsequently, the top of the initial sheet hardened, forming a pocket of solid rock filled with molten lava. The top layer trapped the rumbleometer's legs, allowing the instrument's sensors to chart what happened next.

As more lava flowed out of Axial, the pocket inflated like a balloon. In a little over an hour, it rose 3.5 meters. Then it began to shrink, which the researchers attribute to fracturing of the pocket's surface and flow of lava back into the volcano. Two and a half hours after the inflation began, the seafloor level stabilized at about 1 meter above its original position.

The researchers rescued the instrument from the ocean floor in mid-1999, using the powerful winch on the remotely operated underwater vehicle ROPOS. "It was an extraordinary set of events," says volcanologist John Sinton at the University of Hawaii. Sinton says the data confirm expectations about undersea sheet flows and give scientists more confidence in their models of underwater eruptions.

Related sites

Rumbleometer Web site
Christopher Fox's home page

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