SAN FRANCISCO--The "hot spot" of volcanic activity that created Hawaii, long regarded as a fixed plume of magma from deep inside Earth, might have swayed like chimney smoke in a gentle breeze between 80 million and 50 million years ago. Although research described here last week at a meeting of the American Geophysical Union strengthens that view, other reports stoked the debate.
The Hawaiian-Emperor seamounts form a prominent "V" in the Pacific Ocean. This chain arose in the last 80 million years as one volcano after another popped up over the persistent hot spot. Geophysicists think that the chain is V-shaped because the Pacific plate changed direction as it drifted over the stationary hot spot. However, others suspect that the hot spot itself has moved.
To test that idea, researchers with the Ocean Drilling Program collected samples of volcanic rock from the older seamounts in summer 2001. A quick shipboard analysis of magnetic orientations frozen in the rocks hinted that those volcanoes erupted farther north than Hawaii's current latitude, implying that the hot spot had moved south over time (Science, 11 January, p. 260). Now, a detailed look at the magnetic signatures from the lab of paleomagnetist John Tarduno of the University of Rochester in New York confirms the suspicion that the hot spot drifted southward 3 to 5 centimeters per year--faster than many crustal plates move. These results suggest that currents of warm material in Earth's mantle buffeted the plume of magma. "The hot spot is not strong enough to be a stationary blowtorch," Tarduno says.
In related work, geologist Warren Sharp of the Berkeley Geochronology Center in California analyzed isotopes of radioactive argon in the rocks to reexamine the ages of seamounts at the chain's bend. He found that the volcanoes erupted about 50 million years ago, 7 million years earlier than previously thought. Sharp then created a mathematical model that included the rate of volcanic activity before and after the bend. His model explains the bend by the movement of the hot spot alone, without any shifting of the Pacific plate.
But others think that the new seamount dates bolster the moving plate hypothesis instead. There were high levels of crust movement elsewhere along the Pacific plate 50 million years ago, points out geologist Paul Wessel of the University of Hawaii, Honolulu. Several other Pacific seamount chains also show similar bends at about the same time, he reports. Wessel believes that both lines of evidence point to a shift in the entire plate's motion.