Seismic experiment gives best evidence yet for mantle plumes.
Geologists have obtained the best image yet of a plume of hot rock that rises from Earth's deep mantle and fuels the volcanoes of the Hawaiian islands.
The study, led by geophysicist Cecily Wolfe at the University of Hawaii at Manoa in Honolulu, reveals the structure of the plume down to at least 1,500 kilometres. Critics have questioned in recent years whether such plumes even exist.
"This is a spectacular experiment that succeeded in getting data for putting the plume theory to the test," says Wolfe. The results are published this week in Science1.
The plume hypothesis, first proposed by geophysicist Jason Morgan in 1971, provides an explanation for the volcanic hotspots in Hawaii, Yellowstone and other regions that lie at the centres of tectonic plates. Theoretical models and studies of Earth's temperature and chemistry over the past 30 years seemed to support the idea, but direct measurements of the deep mantle remained out of reach.
So in 1999, Wolfe and her colleagues began to plan a US$4.5-million seismic experiment called the Plume-Lithosphere Undersea Melt Experiment (PLUME) to measure the velocity of earthquake waves using ocean-bottom sensors around Hawaii. Because waves move more slowly in hotter materials, researchers could use the data to visualize the structure of the plume. In 2005, the team tossed 36 seismometers off the side of a ship for their first one-year deployment. The following year they deployed a broader array.
Wolfe's team found that as the plume rises it gets squashed into a pancake shape at 200 kilometres beneath Earth's crust, where it melts and expands upward into fractures. The team also identified structures predicted by computer models, including a cold, downwelling curtain on the edges of the pancake and a tilt to the plume that is consistent with the northwest movement of the tectonic plate.
"Technically, this is a major accomplishment," says geophysicist Guust Nolet of Géoazur laboratory at the Côte d'Azur Observatory in France.
In 2004, Nolet published a paper2 that visualized at least half a dozen deep mantle plumes using global tomography, which integrates seismic data from around the world but lacks the high resolution of Wolfe's study. "If people are objective," he says of the new data, "there shouldn't be any more sceptics."
In fact, there's at least one remaining sceptic. Long-time plume critic Gillian Foulger, a geophysicist at Durham University, UK, remains unconvinced. "The present paper suffers from being obliged to find a plume when it can't really," she says.
Foulger says that, among other things, Wolfe's team released only one set of data — on shear waves — and not the data on pressure waves. By themselves, she says, the shear-wave data could be interpreted as meaning the changes in the mantle below Hawaii reflect not a plume but variations in melt or chemical composition.
Wolfe stands by the paper and says that the pressure-wave data will corroborate the current data set. Similar data are also coming from a group running a seismology array in Yellowstone.
In February, the PLUME data will be publicly available and researchers can decide for themselves whether plumes really exist. Accepting the new results is "going to take time for people," Wolfe says. "I'm optimistic, but in science you never know."
Wolfe, C. J . et al. Science 326, 1388-1390 (2009).
Montelli, R. et al. Science 303, 338-343 (2004).
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Geodynamics & Tectonophysics (2019)