Published online 26 May 2011 | Nature | doi:10.1038/news.2011.327

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Hawaiian hot spot fuels volcano debate

Textbook picture of Earth beneath Hawaii may be wrong.

volcanoHow Hawaii's volcanoes formed has long been the subject of debate.Frans Lanting/Corbis

The plumbing of Hawaii's volcanoes might not be as straightforward as scientists thought. Rather than being fuelled by a vertical plume of magma rising from deep within the Earth, a new study suggests that volcanic activity in the region is fed by a massive hot pool of rock lurking in Earth's mantle some 1,000 kilometres to the west of the islands.

"The simple, canonical textbook picture of Hawaii is probably wrong," says Robert van der Hilst, a seismologist at the Massachusetts Institute of Technology in Cambridge and a co-author of the volcano study, published today in Science1.

For decades, scientists have puzzled over the origin of Hawaii and its volcanoes, which are located in the middle of a tectonic plate and not at the seams, where such volcanism is expected. Diagrams of the islands' formation, based on the mantle plume theory developed in 1971 by US geophysicist Jason Morgan2, depict Earth's crust and tectonic plates sliding above a deep, stationary, bottle-shaped plume of molten rock. Upwellings of lava from the plume build huge volcanoes on the ocean floor that eventually become islands. Over time, the moving plate extinguishes the volcanoes, and the older volcanic islands erode and sink beneath the Pacific — leaving behind the Hawaiian–Emperor seamount chain, which stretches as far north as the Aleutian Trench.

Now, van der Hilst and his team suggest that Hawaii's volcanic plumbing might be considerably different: they think that the islands draw their fire from a huge hot pool of material as much as 2,000 kilometres wide, festering nearly 700 kilometres beneath Earth's surface.

"Hot materials seem to be trapped near the top of the lower mantle," says first author Qin Cao, a graduate student with van der Hilst. "There might be secondary plumes stemming from this hot pool and fuelling current volcanism."

Plumbing the depths

But peeling back Earth's crust wouldn't reveal a massive, glowing lake of magma. Instead, pressures that far down are so high that the mantle is solid and rocky. The rocks are squeezed to the point that minerals abruptly reorganize themselves, forming 'discontinuities', which act as mirrors for seismic waves. Seismic signals reflect off these discontinuities and can pinpoint deep subterranean structures such as plumes. Mapping the depths at which seismic waves are reflected can provide temperature readings of Earth's interior.

textbookThis classic diagram of the plume beneath Hawaii may not be correct.GARY HINCKS/SCIENCE PHOTO LIBRARY

Cao, van der Hilst and their colleagues used such subsurface features to identify the massive hot spot. They gathered seismic data from some 4,800 earthquakes recorded at stations around the Pacific Ocean, and included nearly 170,000 reflected seismic waveforms in their analysis, which incorporated a technique commonly used in oil and gas exploration. Next they applied mineral-physics models of how different minerals behave at different pressures and temperatures to predict the temperature at the subsurface regions that reflected seismic waves, and inferred the existence of a huge, hot region west of the islands.

"Our results suggest a new picture of the internal dynamics of our planet," Cao says, adding that there are implications for the recycling of materials within the Earth, planetary heat transfer and formation, and geochemistry.

"This is a really clever way of processing the data," says seismologist Arwen Deuss at the University of Cambridge, UK. Confirming the results using different methods is the next step, she adds, with the caveat that working on subterranean structures in the deep ocean is complicated by the lack of local sensors.

Subterranean signals

But seismologist Edward Garnero, of Arizona State University in Tempe, points to inconsistencies in the strengths and patterns of the team's seismic reflections, which differ from the results of other studies in the area3. And Thorne Lay, a seismologist at the University of California at Santa Cruz, worries that the authors did not select data that were clean enough for their analysis, and that noisy signals may introduce errors. "Careful data selection leads to robust models," Lay says. "There aren't 170,000 good-quality waveforms."

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Van der Hilst argues that the same technique applied to a nearby region produces the same patterns in the mineral layers, and that using carefully chosen data might bias results. "There is no doubt that we include data that seismologists looking at individual traces would find too noisy," he says. "But we can exploit the noise reduction of very large data sets."

Seismologist Cecily Wolfe, of the University of Hawaii in Honolulu, has also studied the Hawaiian hot spot by deploying an array of ocean-bottom seismometers around the islands4,5. Using a different technique, she and her team imaged something resembling a plume beneath the islands that extended into the uppermost part of the lower mantle.

"This is an interesting and worthwhile study," Wolfe says. "But the results are puzzling. I think there are still questions with both of our results. It is nice when you have multiple people doing multiple analyses that lead to a consensus — but we haven't reached that point yet." 

  • References

    1. Cao, Q., van der Hilst, R. D., de Hoop, M. V. and Shim, S.-H. Science 332, 1068-1071 (2011). | Article | ChemPort |
    2. Morgan, W. J. Nature 230, 42-43 (1971). | Article | ISI |
    3. Schmerra, N., Garnero, E. and McNamara, A. Earth Planet. Sci. Lett. 294, 143-151 (2010). | Article | ChemPort |
    4. Wolfe, C. J. et al. Science 326, 1388-1390 (2009). | Article | ISI | ChemPort |
    5. Wolfe, C. J. et al. Earth Planet. Sci. Lett. 303, 267-280 (2011). | Article | ISI | ChemPort |
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