Lava cracks the seafloor-spreading code.

Lava cracks the seafloor-spreading code

For nearly 30 years, geologists have studied seafloor spreading, the volcanic process that creates the deep ocean floor. But for all their effort, they still know little about this important phenomenon, which churns out more than half of the Earth's surface crust.

That may change in the next few years. Researchers have now identified an actual eruption believed to be part of a seafloor spreading episode. Their finding opens up new possibilities for studying these planet-shaping events.

"This is the first documented case of a deep-water eruption on the midocean ridge system," says Robert W. Embley, a marine geologist with the National Oceanic and Atmospheric Administration (NOAA) in Newport, Ore. Embley discovered the eruption along with NOAA's Christopher G. Fox and William W. Chadwick Jr. of Oregon State University in Newport.

The three scientists made their finding while comparing two maps of the same area produced at different times. Information collected by deep-water cameras in 1989 revealed a low hill of young lava along the Cleft segment of the Juan de Fuca ridge, which runs offshore of Oregon and Washington state. But bathymetric sonar data from 1981 showed no hill at that time. This suggests an eruption occurred in the region sometime between the two surveys, the researchers assert in the April 4 NATURE.

Geologists have identified many other young volcanic deposits on the midocean ridges, but have been unable to tell whether the lava erupted last week or last century. The mapping data for the Cleft segment enabled Embley's group to bracket the region's eruption date between 1981 and 1989. Using other seafloor surveys, the Oregon scientists believe they can further constrain the dates to between 1983 and 1987.

The first mound they discovered measures 35 meters high and a kilometer in width. New eruptive features extend in a line from that mound for 16 kilometers.

The young rocks are pillow lavas, created when molten rock comes in contact with cold water. The lava has a shiny appearance and lacks a sediment cover. In some places, tubeworms have already established residency.

The researchers suggest that the recent eruption indicates an episode of subsurface seafloor spreading. According to theory, the spreading process occurs when two oceanic plates separate by several meters and molten rock rises to heal the crack.

The Oregon scientists think their findings also support speculations that seafloor spreading creates huge plumes of mineral-rich, slightly heated water. Oceanographers discovered these "megaplumes" in 1986 and 1987 while cruising in the Cleft region (SN: 10/10/87, p.238).

To catch any future action, Embley and his colleagues plan to install several types of instruments on the Cleft segment. Among other things, scientists want to know how often the ridge spreads and how large an area opens at once. "It seems like we're finally getting to the point where we'll get some answers to these questions," says Chadwick.