The movement of tectonic plates over a volcanic hotspot typically results in a narrow chain of seamounts, such as the Hawaiian-Emperor Seamount Trail. This hotspot trail is fed by a deep-mantle plume, which scientists believe drifted 15 degrees southward over millions of years because of a Pacific Ocean-wide "mantle wind."
But a new study that analyzes the Louisville hotspot in the southern Pacific Ocean found very little drift of its mantle plume, suggesting it is moving independently of its Hawaiian counterpart, and not as part of a large-scale mantle wind.
Results of the study, which was funded by the National Science Foundation through the Integrated Ocean Drilling Program (IODP), were published in Nature Geoscience.
"Our findings suggest that the Louisville hotspot experienced very little latitudinal drift between 70 and 50 million years ago, an interval when the Hawaiian hotspot moved significantly southward," said Jeff Gee, a professor of geophysics at Scripps Institution of Oceanography at UC San Diego whose research focuses on the history of the earth's magnetic field and a coauthor of the paper.
Estimates of the latitude at which each of the volcanoes in the chain formed are deduced from the orientation of the magnetic signal in the rocks. As the lavas cool the rocks become magnetized in the direction of the earth's magnetic field, which is horizontal at equatorial latitudes and becomes steeper toward the poles.
Gee was one of four magnetics experts on board the drilling vessel Joides Resolution and played a significant role in the interpretation of the magnetization signal in the rocks recovered by drilling.
The 4,000 kilometer (2,485-mile) long Louisville seamount trail is the southern hemisphere analog of the better-studied Hawaiian-Emperor chain.
"Having direct estimates of the latitude of formation of both of these chains provides critical information on possible motions of the Pacific mantle," said Gee.
The site survey for this drilling program was conducted aboard the research vessel Roger Revelle in 2006 in an expedition led by Scripps marine geologist Peter Lonsdale, former Scripps research scientist Anthony Koppers and Gee.
"The earth is a unique planet because of its plate tectonics and there is much we still do not understand about the dynamic processes happening in its deep interior," added Koppers, an associate professor in Oregon State University's (OSU) College of Earth, Ocean, and Atmospheric Sciences. "We're just starting to document how some of these major volcanic chains formed, and how their mantle sources moved over geological time."
In 2003, a study by Robert Duncan of OSU and led by John Tarduno of the University of Rochester concluded that the Hawaiian Islands had drifted 1,600 (994 miles) to 1,800 kilometers (1,118 miles) from north to south - in a direction and at a speed that wasn't consistent with the movement of the tectonic plate. Most of the drift occurred 50 million to 80 million years ago. They attributed it to the mantle wind or the influence of a nearby mid-ocean spreading center that can capture a plume and divert it from its track.
It has taken nearly a decade for scientists to analyze a second volcanic hotspot, Koppers said, because the logistics of such a study are daunting. To analyze and date the volcanoes, the researchers had to drill through as much as 1,400 meters (.9 miles) of hard basement rock located some one-two kilometers (.62-1.2 miles) beneath the surface of the Pacific Ocean in the Southern Hemisphere.
"It was," said Koppers, "a record for the Integrated Ocean Drilling Program."
Koppers specializes in the study of how the inner Earth moves. He was able to date the volcanoes' ages by looking at the argon isotope ratios of gas trapped within the rocks. He and his colleagues found that the Louisville and Hawaiian chains formed new volcanoes every one million to three million years.
"What is interesting is after a time, the mantle wind more or less disappeared for Hawaii," Koppers said. "Most of the drift took place during the first 30 million years then it looks like the hotspot moved far enough away from the spreading center to lose that influence."
Koppers said the researchers hope to next study the Walvis Ridge volcanic chain in the Atlantic Ocean off South Africa - another long-lived seamount trail, but in another ocean.
"This is fundamental yet important research because it is becoming apparent that no one theory explains the formation of all of Earth's 125,000 or so seamounts," Koppers said.
Original text provided by Oregon State University
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