Mega-volcano, magma map mark seismic advances

Discovering a mega-volcano on the floor of the Pacific Ocean is about as sexy as volcanology gets. But more often the scientific advances come from the drab confines of computer labs.

Both happened this week – Tamu Massif, a shield volcano the size of New Mexico, was mapped and described by a research team led by Texas A&M;, based on a scientific cruise off the coast of Japan.

Meanwhile, a team from UC Berkeley and University of Maryland stayed home and ran a model on a supercomputer. They crunched seismic wave data from about 200 earthquakes to refine a model for how magma may flow and contribute to the volcanoes that create island chains and seamounts in the oceans worldwide. Their results were published Thursday in the online journal Science Express.

The resulting three-dimensional map suggests that magma plumes radiate in “fingers” thousands of miles long in Earth’s upper mantle, in patterns consistent with volcanic activity far from plate boundaries and spreading ridges. It is the most sophisticated worldwide magma map using data from multiple earthquakes.

Both teams were chipping away at the same conundrum: how magma flows and contributes to volcanic eruptions. The simplest model, ubiquitous in high school textbooks, shows balloons of hot, buoyant magma rising from Earth’s molten core as part of a large-scale convection cycle that creates a conveyor belt of tectonic plates creeping along over millions of years. Volcanoes erupt where these plates collide and slide under and over each other.

There are plenty of volcanoes that form far from such zones, however, and some geologists have offered that these are explained by “hot spots” of convection. But even adherents to that plume model argue that it can’t be that simple. The Berkeley-Maryland team, among others, suspected that this magma was deflected into channels, and some previous work had suggested as much.

“It’s kind of hard to pinpoint a simple model that would generate all of them,” Scott French, a UC Berkeley seismologist and coauthor of the study, said of the oceanic volcanoes. “Something more complicated must be happening when these plumes get to the upper portion of the mantle.”

Interpreting seismic waves is somewhat akin to reading a sonogram or CT scan – anomalies created by the way waves travel through different material allow researchers to sketch that material’s contours and infer its composition. An earthquake’s shear waves – which propagate like those on the ocean surface – slow as they move through a less dense medium, such as one that is hotter than surrounding material.

Researchers found that these low-velocity “anomaly” zones, about 120-215 miles below Earth’s surface, tended to form bands more than 600 miles wide, some of which extended thousands of miles in the direction of plate travel. Deeper into Earth, the patterns came together in more column-like shapes – suggesting the traditional model of upwelling “hot spots.”

“It’s the first time that anyone has observed this expected channelization process on global scales, and especially seen it aligned with plate motion,” French said. “It helps us fill in the gaps in our understanding of what happens to the plumes as they make it to the upper mantle and eventually impact the plate.”

Intriguingly, there are signs from other research that measured anomalies in Earth’s gravitational field, which is sensitive to mantle densities, also align with the fingers mapped by the UC Berkeley team, which plans to analyze higher-frequency waves next.

Unfortunately, they’ll do so without plying the ocean waves.