Geotimes - March 2003 - Geophenomena

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Geophenomena

Conversations between volcanoes
Bending thoughts about the Hawaiian chain

Conversations between volcanoes

It started last May: Mauna Loa, the world’s biggest volcano, began to swell amidst a swarm of deep earthquakes beneath its summit. After 18 years of quiet, the giant appeared to be waking. At about the same time, neighboring Kilauea, which had been inflating slowly for months, turned up the volume on an eruption that has flowed nearly continuously since 1983. In the so-called Mother’s Day event in 2002, a new vent broke out on the volcano’s southeast flank, and lava output jumped to 30 times the previous average flow. Now, geologists say the two volcanoes, long viewed as solo acts, may commune with each other beneath Earth’s surface.

Westward view across the crater of Kilauea volcano, with Mauna Loa in the background. Photo by Dorian Weisel.

Although a Hawaiian volcano generally begins life on the flank of an older sibling, each volcano owns distinctive lava chemistry. Volcanologists have interpreted this uniqueness to mean that each volcano maintains a separate plumbing system that delivers magma from a single enormous hotspot or mantle plume anchored in Earth’s depths. When the exceptional volcanic events of last spring occurred so closely together, Asta Miklius and Peter Cervelli, volcanologists at the U.S. Geological Survey Hawaiian Volcano Observatory (HVO), suspected it was more than pure chance. “If it’s not coincidence, this is kind of the first line of geophysical evidence that shows the two volcanoes are communicating,” Cervelli says.

To evaluate the significance of the close timing of the events, the researchers analyzed the monitoring data from the satellite-linked GPS (Global Positioning System) receivers and tiltmeters that continuously monitor the volcanoes. The GPS instruments capture slow, long-term changes in the volcano’s shape. And the tiltmeters, which are housed in boreholes and work like a highly sensitive carpenter’s level to record changes in ground slope, detect fast changes — such as abrupt ground movements that indicate where an eruption will break the surface.

Researchers fed the instrument data into a computer programmed to calculate the probability of Mauna Loa starting to inflate within one week of an abrupt, voluminous increase in lava output from Kilauea. Such effusive outputs have occurred only six or seven times since 1983, Miklius says. “However, we used the estimate of three events per year as an extreme upper bound, which gives us an upper limit on the probability. Similarly, we used a high estimate for the occurrence of Mauna Loa re-inflation.” This makes the probability estimate very conservative, she says.

Finding the chances of a random coincidence to be, at best, one in ten, the researchers derived a couple of hypotheses to explain the chain of events. The first is that the magma supply to both volcanoes increased, causing Mauna Loa to begin inflating and Kilauea to pop open the new lava-gushing vent. However, this scenario contradicts the well-established idea of separate lava chemistries and conduits for each volcano.

The other, more favored hypothesis is that a slug of magma entered into Mauna Loa’s underground plumbing, passing under Kilauea’s flank. The magma may have squeezed the younger volcano and caused it to spring a leak, in effect bursting open the vent that ushered in the Mother’s Day event. “The idea that one inflating volcano can squeeze the other means they can interact without commingling magma,” Cervelli says.

Veteran Hawaii volcanoes researcher David Clague of the Monterey Bay Aquarium Research Institute says, “The idea that Mauna Loa squeezed an already inflated Kilauea and compressed its engorged magma chamber, triggering the effusive lava event, makes sense to me.” If the effect continues, there are likely to be more repercussions, he says. For example, the squeezing could increase the rate of movement of Kilauea’s south flank toward the ocean, increasing the likelihood of a large earthquake. “In any event, changes at Hawaii’s active volcanoes are rapid enough that the causes and effects should become clearer in a relatively short time,” he adds.

Don Swanson, chief scientist at HVO, remains skeptical of a link between events on Mauna Loa and Kilauea. He points out that both the lavas and the eruption patterns of the two volcanoes are entirely different. But there could be another possible explanation, Swanson says. “Some other force, such as regional stresses in the Pacific tectonic plate, could squeeze both volcanoes at the same time.”

Noreen Parks
Geotimes contributing writer

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Bending thoughts about Hawaiian chain

At 19 degrees north latitude, a persistent hotspot continues today to fire up the Big Island of Hawaii. Millions of years ago, the stationary hotspot acted in much the same way. As the Pacific Plate moved northward, it carried the new volcanic islands beyond the hotspot, away from the magma source. And so, one by one, the Emperor Seamount chain formed. Then, around 43 million years ago, the Pacific Plate dramatically shifted from a northward direction to a more westerly direction. The ancient seamounts took a bend as the plate dragged across the fixed Hawaiian hotspot, forming the Hawaiian volcano ridge, which now extends some 2,000 miles northwest across the Pacific Ocean. So the geologic story of Hawaii’s creation goes … or maybe not.

New results from last year’s Ocean Drilling Program (ODP) Leg 197 are suggesting that the Hawaiian-Emperor bend may be the result of motion not of the plate, but of the hotspot. Paleolatitude data from the Detroit, Nintoku and Koko seamounts show that they did not form at 19 degrees north.

“The whole issue of mantle plumes and reference frames is going through a rebirth of debate with these new results,” says Bob Duncan of Oregon State University, who served as a geochemist aboard the ODP cruise. “The big conclusion from our Leg is that the Hawaiian hotspot was moving south at a fairly high velocity in the period of Emperor Seamount production about 80 to 45 million years ago.” Then the hotspot stopped moving and plate motion took over, creating the Hawaiian-Emperor bend.

Ocean Drilling Program Leg 197 drilled into the Emperor seamounts to gain insight into formation of the Hawaiian-Emperor bend millions of years ago. The research team’s findings suggest that the bend is a result of a change in movement of the Hawaiian hotspot and not the Pacific Plate. Image by Rory Cottrell.

Ever since J. Tuzo Wilson proposed in 1963 that the bend formed as a result of a moving plate over a fixed hotspot, researchers have questioned whether the hotspots themselves move, says Rory Cottrell, a member of the paleomagnetic research group at the University of Rochester. Hand-in-hand with this discussion has been the idea of true polar wander: that fixed hotspots rotate together globally with respect to Earth’s spin axis as a result of moving mass. For the bend to have formed, a large change in motion must have occurred, either in the Pacific Plate, the hotspot or Earth itself, as true polar wander suggests.

In 1995, Cottrell’s advisor, John Tarduno, went to the ODP core repository at Texas A&M University and looked at basalts from the Cretaceous-aged Detroit Seamount. Basalts serve as excellent records of past magnetic fields, trapping magnetic minerals. The Detroit basalts seem to indicate that the seamount did not form at 19 degrees, the present-day location of Hawaii.

“So we went on Leg 197 with the supposition that the Emperor Seamounts actually represent motions of the Hawaiian hotspot under a nearly stationary Pacific Plate,” Cottrell says. Their objective was to drill as many of the seamounts as possible and determine their paleolatitudes. “We had a lot of data and all of it reinforced that the Detroit Seamount did not form at 19 degrees north,” Cottrell says. “And it was that way with each site.”

David Scholl, a geophysicist at Stanford University and research scientist emeritus with the U.S. Geological Survey in Menlo Park, Calif., was onboard, eagerly awaiting the paleomagnetic results. His interests lie in the larger regional-scale tectonic processes that affect the rims of the Pacific, particularly near the Bering Sea and Alaska. If the Hawaiian-Emperor bend formed from plate motion, the Pacific Plate would have been moving very fast toward the northern rim of the Pacific.

“It has a real impact on reconstructing the kinds of plates present in the north Pacific in the early Tertiary,” Scholl says. “If the bend turned out to not be a change in the motion of the Pacific Plate, then I didn’t have to contend with that big tectonic hammer.”

If the bend resulted from Pacific Plate motion, the entire Pacific basin would have felt the effects at the same time — possibly creating the Aleutian Islands, which Scholl studies. But the Aleutian Arc formed some 12 million years earlier than the bend. Scholl was baffled. “If you look at the fabric of the magnetic pattern in the North Pacific, you can see a very big rip and tear in the fabric at about 55 [million years ago], but that didn’t make any sense with the timing of the bend.”

When Scholl saw that the paleolatitude data supported a moving hotspot, he kicked the bend and the Pacific Plate out of the picture. Now convinced that the formation of the Aleutians is not due to Pacific Plate motion, he is refocusing his energy on ancient subducted plates.

Not all researchers, however, are embracing moving hotspots. Geophysicist Loren Kroenke of the University of Hawaii at Manoa cites work by Warren Sharp at the Berkeley Geochronology Center suggesting that the Hawaiian-Emperor bend actually formed 46 to 49 million years ago, not 43 million. Much initial support for a moving Hawaiian hotspot came from the idea that there is no physical evidence for significant Pacific Plate movement 43 million years ago. If the timing for the bend changes, however, the evidence for Pacific Plate movement also changes. “Now that the bend is dated much earlier, the timing fits numerous plate reorganization events around the Pacific basin, and as such the Hawaiian-Emperor bend is no longer a ‘nonevent,’” Kroenke says. Even if the paleomagnetic data are correct and verified on other seamount trails, Kroenke says that true polar wander is a better explanation for the bend.

Numerical models for mantle convection may support a combination of a slowly moving hotspot with true polar wander, Duncan says. “True polar wander is much easier to turn on and off; it’s a response to moving mass around to new positions in the Earth.” The models, he says, calculate that mantle convection alone could move around enough mass to induce true polar wander. “They get rates of true polar wander which are a little bit less than the southward motion of Hawaii but about in the same direction.”

Tarduno, however, stresses that true polar wander is calculated based on a fixed hotspot reference frame. No independent evidence for large-scale true polar wander exists. “We have found very small true polar wander over the last 130 million years based on global tests of paleomagnetic data,” he says.

Researchers must also look at other seamounts in the Pacific and other basins, Duncan says. But, he adds, “All these different approaches are honing in on the same answer that, yes, hotspots, and the mantle plumes that are supplying them, are moving around as a consequence of mantle convection, and this is producing some of the paleolatitude evidence for southward motion in Hawaii.”

Lisa M. Pinsker

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