Second megaplume found in Pacific.

Second megaplume found in Pacific

While making routine measurements off the coast of Washington state two years ago, oceanographer Edward T. Baker and his colleagues found what they were hoping for: the second example of an intriguing phenomenon called a megaplume. A year earlier, in the same general area above the Juan de Fuca undersea ridge, Baker's group had discovered an extremely large "cloud" of slightly warmed water measuring some 20 kilometers in diameter and 700 meters in thickness (SN: 10/10/87, p.238). In the July 10 JOURNAL OF GEOPHYSICAL RESEARCH, the group describes its second megaplume find. And in the Sept. 10 issue of the same journal, British researchers attempt to explain how megaplumes form.

Baker and his colleagues at the Pacific Marine Environmental Laboratory in Seattle report that the new megaplume was about half the size of the first. Like the first, it carried concentrations of dissolved minerals and gases, and the temperature in its core was about one-quarter degree centigrade above the ambient sea temperature. The researchers discovered the second plume only 45 km from where they spotted the first one, but they say it is unlikely the two plumes are the same. Currents should have carried the first one much farther away in the 13 months separating the sightings, and minerals within the second plume indicate it formed only about a month before its discovery, Baker says.

The Juan de Fuca ridge has hydrothermal vents that spew out jets of mineral-laden, 350 [degrees] C water in a fairly continuous stream. Scientists believe the megaplumes also come from fields of these vents. But the megaplumes represent an explosion of fluids, like a giant underwater burp.

J.R. Cann and M.R. Strens of the University of Newcastle upon Tyne in England propose two scenarios for megaplume formation. In one, they envision plate tectonic forces fracturing the crust around the vents, providing a wider-than-normal passageway through which a large volume of hot water could escape into the ocean. Yet Cann and Strens favor a second theory, which they say better explains how vents could spew out a megaplume and then return to normal. They suggest that volcanic activity in the crust might pump excess heat or gas into the reservoir of hydrothermal fluids that feeds the vents. Pressure on the hot fluids would build until they broke through the partially clogged vent holes and surged into the ocean. Once the reservoir of hot water drained upward, the vents would clog up again and the flow would return to normal. The researchers say they must wait for more megaplume discoveries to test these hypotheses.