Along with megaplumes, midocean ridges feature steady geysers of raging, smokelike water.

Above the chamber are the relics of past intrusions of the magma to the seafloor’s surface. Directly overhead is a layer of hardened vertical lava pipes—imagine a batch of pencils clutched together—known as dikes. On top of the dikes lies cooled, fractured lava called pillow basalt, rife with cracks and voids through which seawater circulates. This loose collection of rock and water is what we generally think of as the solid seafloor.

 It’s hardly a stable situation. A hiccup from within the planet can drive more magma into the chamber, making it swell and send tremors through the rocks above. The pipe-shaped dikes can shift, suddenly making room for a new column of magma to force its way up through the pencil cluster. The new column of lava is hot—about 2200 degrees. The heat expands the water, percolating it through the loose layer above and shooting it through a seafloor laced with sulfur, methane, iron, and other chemicals. The geysers through which this water emerges are known as hydrothermal vents.


At particular hot spots, the magma breaches the surface repeatedly, forming a cone. Much as it does on land, the cone cools and turns solid, then a new lava flow breaks out from it, running down the flanks and beginning the process of making an undersea mountain. The process, repeated over thousands of years, creates volcanoes such as 5,000-foot-high Axial.

The Juan de Fuca Ridge has garnered a lot of scientific interest because it runs roughly parallel to the northwest coast of the United States only 300 miles offshore, making it relatively easy to get to. Axial lies on top of a segment midway along the ridge. In 1986 a research vessel carrying Ed Baker and his colleagues lay above a southern segment called Cleft.

 

Viewed from the side, a megaplume might initially look

like a gargantuan twirling mushroom, with a disklike top

that tapers down to a long tail.

“We wanted to map the ridge to see where vent fields were likely to be,” Baker says, riffling through the multicolored maps of the seafloor that clutter the desk in his Seattle office. A dark-haired man of medium build, Baker speaks deliberately, but his voice picks up speed as he tries to make a point. “We were doing something called a tow-yo, where you tow instruments behind the ship and send them up and down at the same time, in a sawtooth pattern. This lets you sample the water at different depths. All of a sudden, we started getting a bunch of plumelike signals up here”—the oceanographer raises his hands, indicating a point high above the seafloor—“in an area where there shouldn’t have been any plumes.

  

Despite the threats of boiling water and lava, midocean ridges support lush ecosystems that include animals such as this fish and crab.

“We were using an instrument that measures the optical clarity of the water. It’s very simple, really. It shines a light from point A to point B. And if there is less light at the end, it’s because there are particles in the water. All of a sudden it started going up and up and up when it should have been zero, zero, zero!

“At first we thought, ‘Well, what’s wrong with the instruments?’ It took us a while to realize there wasn’t a problem. There were just plumes where there weren’t supposed to be plumes. Instead of plumes rising 200 meters above the seafloor, we had plumes 1,000 meters above it.

“We spent about four days towing around the area, and we started drawing a picture of this great big circle,” Baker continues. “We’d start moving through the circle and the readings would get higher and higher, then we’d get to the middle and move out the other side and the readings would drop off again. It was a perfectly symmetrical circle around this one spot on the ridge.”

So Baker began to visit other eruption sites at various segments of the Juan de Fuca Ridge and saw megaplumes at all of them. “The plumes are pretty distinctive,” he says. “They’ve got a definite top and bottom. Their temperature and salt content are different from the surrounding water. They’re warmer, and they’re more fresh. In part that explains why they rise so high: hot fluids are more buoyant than cooler ones.”

Eventually the megaplume cools down just enough to stop rising. At that point it is about 2 degrees warmer than the water at the seafloor and three-tenths of a degree warmer than the water surrounding the plume. That difference may not seem like much, but water has tremendous heat capacity. Raising water temperature by 1 degree requires 4,000 times more heat than raising the temperature of an equal volume of air.

 Heat is a reason megaplumes maintain their shape. Heat causes water molecules to spread apart, making them much less dense than the surrounding water. Because fluids of different densities don’t like to mix, the megaplume hangs together. Another reason megaplumes live so long is that they spin. In 1996, at a segment known as the Gorda Ridge, Baker’s team spotted two megaplumes and dropped a sensor into one of them. Three months later the sensor popped back up to the surface and sent a description of its journey to a satellite. It had spun clockwise around the edge of the plume and then gradually moved toward the middle, as if caught in a giant eddy. Baker describes the plumes as similar to underwater hurricanes.

IS THE SEAFLOOR MORE LIKE A SPONGE OR A HOT PLATE?

Researchers are split about whether megaplumes form from lava sheets (left) or from suddenly opened fissures (right).

How does a megaplume get started? What prompts such a massive explosion of hot water in such a short time? No one knows for certain, but two competing views have gained ground. One sees the seafloor as a suddenly squeezed sponge; the other sees it as a large hot plate heating the water above.

Ed Baker believes in the sponge theory. Megaplumes are squeezed out by the deep crust because plume water contains helium 3, an isotope that’s rare at the surface but more common in deep Earth. So he and others envision a magma dike shooting up from below, squeezing a massive amount of fluid out the top of the loose seafloor. “Water would come erupting out everywhere,” he says.

Yet Baker also realizes this theory has a problem: nature abhors a vacuum. “For every gallon of water leaving the crust, there’s got to be a gallon of water entering the crust. Water won’t get out unless something comes in to fill the vacuum. How does that happen?”

CRACKING THE SEAL

William Wilcox, a researcher at the University of Washington, has an answer: forget the vacuum. Wilcox notes that the fluid and gases such as helium and methane percolating through the seafloor are already under great pressure. An intruding magma dike might crack the seal at the top, like loosening the valve on a pressure cooker. Reduce the pressure and the water-gas mixture will expand, vastly increasing its volume. There would be enough to fill all the voids and cracks in the seafloor, eliminating any vacuum problem, while the excess would come boiling out to form a megaplume.

But when chemist Dave Butterfield looks at the fluid inside a megaplume, he sees something that doesn’t jibe with the big- sponge theory. “The chemistry of megaplumes and regular plumes is really different,” Butterfield says. “So it doesn’t look as if they are coming from exactly the same place.”

What stops Butterfield is the ratio of manganese to iron in megaplumes. Manganese leaches from seafloor rocks much more slowly than does iron. Because normal plume water spends a lot of time circulating through the loosely packed seafloor, it has time to pick up extra manganese, and the iron has time to settle out. Typically, this produces water with an iron-to-manganese ratio of 4 to 1. But in a megaplume, the ratio is closer to 10 to 1. The disparity led Butterfield to wonder whether the megaplume water wasn’t spending sufficient time within the seafloor but was instead rushing by it too quickly to pick up enough manganese.

He began envisioning megaplumes formed by the geologic hot-plate theory. “Now, I’m a chemist, not a modeler,” he cautions, but it’s not hard to imagine a magma dike breaching the seafloor and laying out 1200-degree lava over half a square mile. That would expose a lot of seawater to heat, but only once. The water would be warmed as it passed over the molten rock, like a pot of water on a hot plate, and start to rise before it had enough time to pick up manganese.

Baker doesn’t buy it. “I’m not convinced you can get the heat out fast enough that way,” he says. “These guys envision the megaplumes forming in a week or so. I think they happen in a much shorter time, days or hours.” And the hot plate doesn’t have enough heat to do it.

The voyage to Axial last February was supposed to test both theories. If researchers found a megaplume but no detectable large flows of lava, Baker could toss out the hot-plate idea permanently. So he set out sensors designed to track water flow along the ocean floor, changes in temperature, and the movement of the crust. The sensors were supposed to record the data, then bob up to the surface where they could be gathered.

RETURN TO AXIAL

On the return visit to Axial to retrieve the data during a calmer August, however, there were few clues, and the debate still rages. The instruments on the sides of Axial that measured current could have supported the squeezed-sponge model if they had measured a strong flow of water back toward the volcano to replace the expelled fluids. But one instrument couldn’t be found at all, and another was buried under new lava.

These frustrations are typical for underwater research. “You know, it’s not like being a chemist in a lab where you can say, ‘Okay, I want to do this kind of experiment to prove my theory.’ We’re sort of at the mercy of Mother Nature here,” Baker says. “And it’s not like being a geologist on land. We can’t necessarily get to where we want to go when we want to get there. So it takes longer to figure these things out than you might expect.”—J. F.

Their shape and movement result from a devilishly complex interaction between the rising plume and the sideways rotation of the planet. Moving up or down while going sideways produces a twist known as the Coriolis effect. It’s the reason bathtubs drain in a spiral, hurricanes rotate in a spiral, and megaplumes spin round and round. The megaplume Baker tracked was spinning at about 6.5 feet a minute, taking eight days to make a full rotation. (Some megaplumes can spin three times faster.) Viewed from the side, he says, a megaplume might initially look like a huge twirling mushroom, with a disklike top that tapers down to a long tail. Eventually the tail disappears and the megaplume takes on the form of a giant Frisbee.

On his most recent voyages, Baker has been trying to figure out how megaplumes form (see page 115) and where they go. He wonders whether megaplumes carry the gases of an eruption, such as carbon dioxide and methane and helium 3, as well as minerals such as sulfur and iron, to upper layers of the ocean where most plant and animal life resides. Plankton, for example, thrive on iron. An infusion of iron makes plankton bloom, starting a vast domino effect on the sea surface that alters food chains and possibly the atmosphere as greenhouse gases such as carbon dioxide are absorbed.

To find out the answers to these questions, Baker’s colleague Bob Embley is laying the groundwork for a long-term observatory at Axial that will be put in place early in the next century. He envisions a network of sensors and cameras on the seafloor, transmitting data to buoys near the surface, which in turn bounce the signals off satellites to Baker’s office in Seattle. This New Millennium Observatory (NeMO, for short) will also employ an autonomous submarine at a permanent mooring on the site so that a scientist, after hearing the rumbles of an upcoming eruption, can tell it to “please do survey number 5.”

Then, Baker says, with the memory of ten horrible days at sea still fresh in his mind, “I could dial up the results on the Web from the comfort of my home.”