Large lava field discovered in Pacific.

Large lava field discovered in Pacific

At the ocean bottom, along the mountain chain known as the East Pacific Rise, scientists have discovered an immense field of fresh lava they think may be as young as 25 years old. Aside from garnering size records, such as "the largest lava field created within recent times," this discovery is forcing geologists to reconsider theories about the creation of the ocean's floor.

Ken Macdonald of the University of California at Santa Barbara and his colleagues detected the lava field about 1,200 kilometers southwest of the Galapagos Islands while using side-scan sonar to map the East Pacific Rise -- part of a worldwide network of seafloor spreading centers where molten basalt from the mantle rises to form oceanic crust. What the researchers actually found is a 53,000-acre bright spot on the sonar images, which they say corresponds to fresh lava, still free of the sediment cover that dulls the floor's reflectivity.

"We have continuous coverage of 3,500 kilometers, which is most of the East Pacific Rise from Baja California to Easter Island, and this is the only area that was so bright in t erms of reflectivity, and this was the largest area of high reflectivity," says Macdonald, who presented the finding at the spring meeting of the American Geophysical Union in Baltimore.

The eruption appears to have started near the central axis of the spreading center. It then flowed downslope, over cliffs and valleys, for 20 kilometes. By estimating the height of the cliffs the lava covered, the researchers calculated the volume of erupted material to be 15 cubic km. While some prehistoric basalt flows were thousands of times larger, this flow -- if confirmed -- would capture the title for largest during historic times, says Macdonald. In comparison, the current record-holder, the 1783 eruption of the Icelandic volcano Laki, netted only 12 cubic kn. According to Macdonald, the volume of the new field is also "enough to pave the entire U.S. interstate system to a depth of 10 meters."

"What's really starting," he says, "is that the way in which new oceanic crust is formed may be much more episodic than we had thought. People have been thinking about [seafloor spreading] as a wound that never heals -- a slow oozing of magma." Conversely, this flow or series of flows seems to represent a period of prodigious, punctuated ridge activity. Geoscientists have only recently started to consider this kind of activity possible, in the wake of discoveries of large "megaplumes" of heated water near spreading centers (SN:10/10/87,p.238). Such plumes could be produced only by short periods of intense volcanic activity.

The researchers suspect the field may be related to earthquake swarms recorded in that area of the Pacific in 1964, 1965 and 1969. Moreover, in 1972 oceanographers found to the southwest of the lava field a 2,000-km-long plume of helium-3, which serves as a tracer for undersea volcanic eruptions. One of the original discoverers of the helium plume, Santa Barbara's John Lupton, calculated that if the eruptions at the newly discovered field had occurred during the mid-1960s, known ocean currents could account for the plume's 1972 position.