In Search Of The Elusive Megaplume – an underwater phenomenon caused by volcanoes – includes related article on the sea floor
Imagine volcanoes that erupt with giant twisting plumes filled with microbes and other life that spin like a discus for months. Welcome to the strange, almost completely unknown life of undersea eruptions
A little more than a year ago, on the night of February 9, the 170-foot research ship Wecoma headed west from the Oregon coast into darkening rough seas. Within hours, the ship was trying to make its way into a gale that drove rain horizontally across the deck. By the next day 45-knot winds were jacking waves up to heights of 30 feet, conditions that could easily have smashed smaller vessels. The relentless pounding continued for days. Oceanographer Ed Baker remembers February 14 as the worst night of all. Waves crashed and roared over the deck, blurring sky and sea. Then the wind would suddenly change direction without warning, propelling rollers into the ship from an entirely new angle. The Wecoma pitched and bucked violently in unpredictable jerks. “What you can’t appreciate until you’re out there is the whipping motion,” says Baker. “If you’re not holding on, you can get thrown across the deck.”
Despite the storm, Baker, a researcher at the Pacific Marine Environmental Laboratory in Seattle, and his colleagues were determined to launch a half-ton deepwater probe into the Pacific from the roller-coaster decks. They knew that a mile below the Wecoma a volcano had blown its top, and only the most unholy of storms would keep them from collecting evidence of a newly discovered side effect of such an eruption: giant underwater twisters of hot water, called megaplumes, loaded with minerals and strange life-forms.
As the deck crane cavorted wildly in the wind, the researchers gingerly lowered the probe–a cluster of sampling bottles held by a wire tether–over the side. Once the probe was in the water, they let the tether unspool for 40 minutes until the bottles were a mile underwater. They worried the whole time that the corkscrewing motion of the ship would snap the wire. Then they had to haul the load back aboard. “Swinging is a bad thing,” Baker says, remembering the gyrations of the crane. If the bottles slammed against the hull, they would be destroyed. If they slammed against scientists and deckhands, someone would die.
Conditions deteriorated and the captain of the ship banished the scientists inside, where they passed hours trying to drink coffee that frequently flew out of their cups or trying to rest in tossing bunks that thrust them to the floor. Sleeping, Baker says, “was always an adventure.”
The night of the fourteenth was memorable not only because the seas were so rough but also because the researchers had to deploy an acoustic beacon the size of a telephone pole. In the dim glow of deck lights, the crew struggled on the Wecoma’s fantail. The hook of the crane crashed into the device. Although they didn’t know whether the beacon had been damaged, they set out a chain of fragile glass globes that would later bring the device back to the surface. Then they raised the 30-foot-long aluminum shaft. “It’s very heavy, yet very delicate as well,” Baker says. As the ship roared up the face of one wave and plunged down the other side, the beacon began swinging back and forth. The crew grappled with the heavy lines. One deckhand lost his grip but then grabbed on again before the shaft could crash into anyone. Finally the crane lowered the instrument into the water. Baker remembers, “It went off without a hitch, but that was probably the scariest part of the trip.” Several days later, everyone was relieved when the beacon began broadcasting data.
The voyage to the volcano and back took ten days, “probably the worst weather for ten days straight that I’d ever had at sea,” Baker says. But when the Wecoma docked in Oregon, he was a happy man, and not just because he was standing on terra firma. The data he and his colleagues had collected painted a picture of a far more violent spectacle on the seafloor than the scientists had experienced in the storm on the surface.
The first rumblings of that unseen spectacle had begun weeks earlier, on January 25. A network of sensitive hydrophones in the North Pacific, deployed by the U.S. Navy to listen for Soviet submarines, picked up a swarm of earthquakes at the summit of an undersea volcano called Axial. The quakes were a lot less subtle than the quiet hunters of the cold war: they sounded like freight trains rumbling by. Over the course of a day the quakes traveled 30 miles from the volcano, following a scar in the seafloor where two plates of the planet’s crust are pulling apart. The quakes shook loose the rock that had stopped up the volcano’s plumbing. Through the newly cleared rocky pipes came an inferno of molten rock and boiling water driven upward by the intense pressures and temperatures of the inner Earth miles below.
“It would have been pretty spectacular if you’d been down there in the early going and seen, really, a boiling cauldron of water popping up out of the seafloor,” Baker says. A typical volcano on land, such as Mount St. Helens, fills the sky with a plume of ash when it erupts. But Axial belched forth an equally massive eruption of superhot water. That was followed by streams of lava, some as deep as six feet, flowing down the sides of the volcano. Along Axial’s flanks, fields of geysers suddenly pierced the ocean bottom, shooting up superheated jets of water darkened with heavy concentrations of minerals from Earth’s crust. “We estimated, roughly, that there were ten gigawatts of energy coming out of the volcano’s caldera,” Baker says. “By comparison, Bonneville Dam on the Columbia River puts out about one gigawatt, and that can provide energy for 3 million homes.”
The trip on the Wecoma was launched to find out what happens when all that energy breaks loose. In the last few decades, geophysicists have sketched the broad outlines of plate tectonics–how Earth’s molten mantle of lava rises up to the surface and turns into plates of rigid crust–but many mysteries remain. Answers are not easy to come by because they must be found at places like Axial, obscured by deep ocean. But in recent years researchers have learned how to place sensors on or near volcanoes like Axial. The instruments dropped by Baker and his fellow oceanographers, for example, measured water temperature, current flow, and the chemicals released during the eruption.
Baker was particularly interested in learning more about megaplumes, observed only seven times before. Hot geysers of water, such as those on Axial’s flanks, rise only a few hundred feet above the seafloor, then diffuse like a pall of low-hanging smoke. Megaplumes, however, can soar more than several thousand feet off the ocean floor and spin like slow but gargantuan tornadoes. They can be 12 miles across and travel hundreds of miles. “It’s a bomblike event,” says Baker, who was among a group of oceanographers who discovered the phenomenon in 1986. Shortly thereafter, the researchers dubbed the underwater cyclones megaplumes.
The plumes have attracted interest for three reasons. First, they’re new. “You don’t often get a chance to find something that’s not known to exist,” Baker says. “And this isn’t some tiny feature, like a little rock This is a whole giant process that no one knew about.”
Second, megaplumes stir up huge amounts of ocean, carrying minerals and gases and heat almost to the sea’s surface. Vertical mixing doesn’t happen easily in the ocean. Cool, dense water tends to stay near the bottom and warmer buoyant water near the top. As they rise from the ocean’s depths, megaplumes may bring energy and food to animals in shallow water. “They could be doing things to the energy of the ocean that we don’t even know about,” says David Butterfield, a chemist at the marine environmental lab.
Third, megaplumes move great distances horizontally too. In their travels, they may have helped some of the oddest and oldest beasts in the world colonize the planet. A hydrothermal hellhole like Axial may seem like a poor choice for a home, but it supports vibrant communities. Bacteria that feed on methane and other noxious chemicals provide sustenance for three-foot-long hollow tube worms and tiny dams. These critters can live only in hot spots on the seafloor, yet they have been found all over the world. How do they cross vast nutrient-free expanses of the sea to find new places to live? Megaplumes, which remain intact, spinning and drifting for months and traveling hundreds of miles, could be mobile ecosystems. “Maybe they’re like express buses,” Baker says.
If so, the bus depot is a mid-ocean ridge such as the Juan de Fuca, which runs right below Axial. The ridge’s hot material rises from deep inside Earth, cools at the top, then slides off on both sides to become yet more planetary crust. At various hot spots, the ridge crosses over vertical channels that rise from the mantle. Molten rock rises to within a mile of the surface of the seafloor and forms a magma chamber. There it lies, slowly bubbling away.
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.
“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 towyo, 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.
“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.”
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.
Their shape and movement result from a devilishly complex interaction between the rising plume and the side ways 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 often horrible days at sea still fresh in his mind, “I could dial up the results on the Web from the comfort of my home.”
RELATED ARTICLE: Is the Seafloor Like a Hot Plate?
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 watergas 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 Pacific Marine Environmental Laboratory chemist David Butterfield looks at the fluid inside a megaplume, he sees something that doesn’t jibe with the sponge theory. “The chemistry of megaplumes and regular plumes is really different,” he 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 iron does. 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.
Butterfield 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, the researchers found 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 a fresh layer of 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.
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