April 30, 2018 — Hawaii’s volcanos stand as silent sentinels. They guard the secret of how they formed, thousands of miles away from where the edges of tectonic plates clash and generate magma for most volcanos. A 2017 Nature study by Jones et al. found the best clues yet of the origin of Hawaii’s volcanos through simulation of a shift in the Pacific plate three million years ago. What remains elusive is conclusive evidence that mantle plumes exist.
The plumes are hypothesized, mushroom-shaped upwellings of hot rock from the deep Earth. They are hypothesized to form within the thermal boundary layer at the base of the mantle and are thought to carry heat from the Earth’s core that generates a volcano’s magma. Scientists have now made the best computational modeling yet of mantle plumes, according to a study made available online in January of 2018 ahead of its peer-review and publication November of 2017 in the American Geophysical Union’s Journal of Geophysical Research, Solid Earth.
The international science team showed through supercomputer simulations, for the first time, details of how plumes decelerate seismic waves and how plumes appear in seismic tomographic images of the Earth’s mantle, the layer beneath the crust. What’s more, the researchers say their work could help guide future experiments on the ocean floor with deep Earth imaging and help get to the bottom of mysteries like the origin of Hawaii’s volcanos.
“We found that mantle plumes are likely to be more challenging to seismically image than we previously recognized,” said study lead author Ross Maguire, formerly a PhD student who has recently graduated from the department of Earth and Environmental Sciences at the University of Michigan. “Our current picture of deep mantle plumes might be lacking,” Maguire said, pointing to a lack of seismic data coverage.
Seismic imaging can see rock structures thousands of kilometers below ground by listening to the echos of earthquakes. Networks of seismic stations sit on the ocean floor and measure differences in the travel time of seismic waves through rock, in essence taking a CT scan of the deep Earth.
“In order to constrain the role of mantle plumes in Earth dynamics as well as to understand the causes of hot spot volcanism, we need to focus on increasing the global coverage of seismic sensors, particularly in the oceans, which currently only have sparse coverage,” Maguire said. Oceanic deployments of seismic sensors are costly and tough to plan and execute, he added.
“In our study, we used computer modeling to find optimal imaging scenarios, so that we can recover the most detail of mantle plumes at the lowest cost,” Maguire said. “We hope that our results will help guide the design of future seismic deployments aimed at imaging the mantle beneath hotspots.”
“The thing that is probably new in this work is that we combine, maybe for the first time, actual numerical models of how plumes form and how they rise in the Earth with estimates of their seismic structure ” said study co-author Jeroen Ritsema, a professor in the Department of Earth and Environmental Sciences at the University of Michigan.
“Secondly,” he added, “we’ve also explored how various network configurations might change the way that we are imaging plumes. We’ve done extensive tests to figure out the optimal configurations of seismometers on Earth to see plumes. This is particularly important for Hawaii,” Ritsema said. “Hawaii is a place where we believe there is a plume responsible for volcanism on the Hawaiian islands. We’ve determined what might be optimal offshore deployments on the seafloor that could lead to best images of the deep mantle beneath Hawaii.”
“It is a big computational challenge to simulate wave propagation through mantle plumes,” Maguire said. They needed numerical codes that solve the elastic wave equationin Earth’s mantle at high frequencies and in three dimensions. “What that does is it allows us to accurately account for the effects of wave propagation phenomena such as wave diffraction around plume tails, which is very important for imaging plumes,” Maguire said.
XSEDE, the eXtreme Science and Engineering Discovery Environment, funded by the National Science Foundation, provided computational resources to the science team through access to supercomputers and experts in how to use them best. “We would not be able to do this type of work without supercomputing resources like those that are provided by XSEDE,” Maguire said. “They allowed us to run our wave propagation simulations on hundreds or sometimes thousands of computer cores in parallel.”