Deep-sea searchers.

Using sonar, scientists discover the largest cluster of volcanoes on Earth--at the bottom of the ocean! Second in a series on Earth's Oceans

Check out Dan Scheirer's favorite place on a map and you won't see much: just a wide, featureless square of blue in the middle of the South Pacific Ocean. The last time he was there, though, Scheirer helped discover the biggest known cluster of volcanoes in the world. He found them thousands of meters below the surface.

"We had a pretty good idea that we'd see some unusual stuff here," says Scheirer, a geophysicist at the University of California at Santa Barbara. "But this was just astounding."

The undersea volcanoes, called seamounts, "just kept coming and coming," Scheirer says. "It's like, you're going over an area no one's ever seen before, and you don't think it can continue, but the instruments keep showing more and more seamounts."

From November 1992 to January 1993, Scheirer's team charted 1,133 new volcanoes in an area the size of Washington State.

Active volcanoes. As you read this, a few of them may be erupting, silently spouting hot lava that will cool into a glassy black rock.

MID-OCEAN RIDGE 

To locate the newly discovered seamounts on a map, use latitude and longitude markers. The volcanoes are in a remote area that stretches from 109 [degrees] to 118 [degrees] west longitude, and 15 [degrees] to 19 [degrees] south latitude. The nearest land, Easter Island, is about 950 kilometers southeast.

What interested Scheirer and his coworkers in this isolated patch of seafloor? Weird satellite data. Satellites had picked up some odd fluctuations in Earth's gravity in this part of the South Pacific. Perhaps it was worth looking into. . . .

The scientists also knew that this area was near a very unusual place: the East Pacific rise. The rise is part of a giant undersea mountain range, named the mid-ocean ridge, that runs the length of Earth's major oceans.

This elevated crack in Earth's crust marks the boundary between large pieces of crust, called tectonic plates. Molten rock from Earth's interior regularly oozes up between the plates. In fact, geologists call the ridge "the wound that never heals" because of the hot lava that is always pouring out.

The lava pushes apart the plates--and the continents they carry. Then it cools and hardens to become new ocean floor--new crust.

At the East Pacific rise, new crust grows faster than anywhere else in the world--at a rate of 15cm a year, or about three times as fast as your fingernails grow.

DATA DIVING

With all that gushing lava and pushing of tectonic plates, it's no surprise that there are volcanoes (and earthquakes) near the ridge. But that doesn't make finding them easy. Scheirer and his coworkers spent 62 long days aboard the research vessel Melville, cruising slowly over the area, as sonar and other instruments on board scanned the seafloor. These instruments allow scientists to "see" the bottom without actually diving down.

Each scientist stood watch for two four-hour shifts, monitoring the instruments, looking for features such as volcanoes and earthquakes. Some of the work was very repetitive. "It's boring as heck," says crew member Don Forsyth, "unless you're looking at new data coming in. That's what you do for excitement."

The highlight of each day: the "data dump," when the scientists entered the day's data into the computer. After that came "playtime," as Scheirer calls it, when the scientists manipulated the data and made maps. That's when they actually made their discoveries.

SONAR MAPS

By making maps from sonar readings, for example, the scientists discovered that the submarine volcanoes look a lot like volcanoes on land.

The scientists could also use their data to predict what they might find the next day. Sometimes they were wrong. Says Scheirer, "We'd predict, 'Well the seamounts will form a line in this direction.' Then sure enough, the next day, the seamounts would turn up at right angles to what we said."

Such unexpected discoveries are part of the fun of exploring uncharted waters, Scheirer says. "You can find really interesting things just by going out to a new area . . . and make a significant contribution."

And there's plenty of uncharted territory out there. Though oceans cover two-thirds of the world's surface, only about five percent of the ocean floor has been explored. We know more about the landscape of the dark side of the moon than we do about the landscape beneath the seas, says geophysicist Ken Macdonald. "Here we thought we knew our own planet," he says, "and we didn't even know it well enough to know that these volcanoes were there."