Textbook mantle plume theory may need revision

by Mary Caperton Morton
Tuesday, December 9, 2014

Every basic geology textbook has a section on mantle plumes, which have long been thought to underlie certain island chains and other volcanic hot spots, but hard evidence in support of narrow columns of magma upwelling from deep within the Earth’s mantle remains scant. Now a new study suggests that the long-held plume theory should be abandoned altogether.

Textbook mantle plume theory holds that convection deep within Earth’s core produces narrow streams of superheated magma, no more than 300 kilometers wide, that rise up through the mantle to the surface, where they produce hot spots. These hot spots appear to be long-lived and stationary, remaining in position while tectonic plates move over time, producing island chains such as Hawaii and Samoa.

“People have been using seismological techniques for decades to look for plumes, but there have been no sightings,” even despite advances in seismic technology and increasing network coverage, says Don Anderson, a geophysicist at Caltech and lead author of the new study, published in Proceedings of the National Academy of Sciences.

Nonetheless, seismology offers the most useful tools for studying Earth’s interior structure. As seismic waves produced by earthquakes move through the planet, the types of materials the waves pass through influence the speed and other properties of the waves, giving earth scientists a glimpse of Earth’s mantle and core. Over time, various studies have suggested that mantle plumes are simply too narrow to be detected by seismic waves, or that the plumes are tilted by convection in the mantle in a way that masks their appearance in the upper mantle. “At some point, we need to seriously consider the fact that we haven’t been able to find these narrow plumes because they’re just not there,” says James Natland, a geoscientist at the University of Miami and co-author of the new study.

A small but vocal contingent supports scrapping the mantle plume theory altogether. “When it was first proposed, the mantle plume theory was intriguing, but over time exceptions kept arising and people kept modifying the theory to fit the exceptions; now we’ve gotten to the point where the emperor has no clothes,” says Gillian Foulger, a geophysicist at Durham University in England who was not involved in the new study. “Given the lack of data in support of plumes, we’ve got to face the fact that the [mantle plume] theory may be wrong.”

Based on a review of the latest geophysical data, Anderson and Natland propose an alternative hypothesis: Instead of narrow jets, they suggest that broad upwellings up to 1,000 kilometers across may be delivering hot material up from the mantle. This convection is driven not by heat from the core, as purported in the plume theory, but by cooling at the surface. As cold slabs descend at subduction zones, they displace hotter, more buoyant material deep in the mantle, forcing it upward in a model known as “top-down tectonics.”

This version of top-down tectonics would still produce hot spot chains and calderas like Yellowstone National Park, Anderson says. But the erupting magma would come from within the upper 200 kilometers of the mantle, not thousands of kilometers deep as the mantle-plume theory suggests. As the wide plume wells up to the surface, it follows fissures and cracks in the Earth’s crust to arrive at surface features, like volcanoes.

This idea has been floating around the geophysics community for some time, Foulger says, but the new study is among the first to lay it out in detail. “[Anderson] has a very good reputation among geophysicists for seeing angles that other people don’t see. This paper will be considered with great care, although I’m sure there will be resistance to change,” she says.

Natland says the resistance reminds him of the plate tectonics revolution that took place in the 1960s. “Some people have built their entire careers around the mantle plume model, and it will be difficult to get those people to consider new ideas,” he says.

The new model would have some wide-reaching implications, Natland notes, for a range of subjects from island chain formation to isotopic ratios. “This is a big deal for earth science. The interior structure of Earth underlies so many other theories.”

What’s needed at this point isn’t more data, but more discussion, Foulger says. “This subject certainly doesn’t suffer from a lack of data,” she says. “What’s needed is more talking. More people to open their minds and consider where the weight of evidence falls.”

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