Scientists have now made the best computational modeling yet of mantle plumes, hypothesized, mushroom-shaped upwellings of hot rock from the deep Earth. They plumes are hypothesized to form within the thermal boundary layer at the base of the mantle and are thought to carry heat from Earth's core that generates a volcano's magma. (Credit: Ross Maguire et al.)

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.

Numerical simulation of a mantle plume rising from the core-mantle-boundary. The left half of the plume shows the temperature anomaly and the right half illustrates the amount which S waves are slowed. The top of the image shows the time that has elapsed since the beginning of the simulation, in millions of years.

"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.

Ross Maguire, Department of Earth and Environmental Sciences, University of Michigan.

"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."

We found that mantle plumes are likely to be more challenging to seismically image than we previously recognized.

Jeroen Ritsema, Department of Earth and Environmental Sciences, University of Michigan.

"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."

Dynamic simulations of plumes used in sensitivity tests. The plumes are symmetric about the vertical axis at x = 0. For each plume, the excess temperature is shown on the left and the reduction in shear velocity (delta)VS relative to the Preliminary Reference Earth Model is shown on the right. The plume structures R1a, R1b, and R1c are snapshots of the same dynamic simulation at 45 Myr, 55 Myr, and 175 Myr, respectively. (Credit Maguire et al.)

The workflow management proved daunting, with many simulations that produced hundreds of gigabytes of data. "The XSEDE team was really helpful in answering all my questions about how I can optimize my workflow, for example how I can spend the least amount of time waiting in the queue for my jobs to run; or how I can efficiently transfer large amounts of from Stampede onto my local machine," Maguire said.

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.

Another useful resource, said Maguire, was access to an allocation on XSEDE Science Gateways through the Computational Infrastructure for Geodynamics with the help of Lorraine Wang. "Science Gateways enabled us to test our code and really figure out how computationally demanding our project would be," Maguire said.

(a) Ray geometry for S (in blue) and SKS (in red) waves traversing plume R1c. The distance D between the earthquake and plume is 50 deg for S and 100 deg for SKS. X is the angular distance beyond the plume along the great circle path. (b–d) Sensitivity kernels K(x) for cross correlation travel time delays measured over the frequency band 0.04–0.10 Hz. Kernels are shown for an S wave at an epicentral distance of 80 deg (in Figure 2b), a P wave at 80 deg (in Figure 2c), and an SKS wave at an epicentral distance of 100 deg (in Figure 2d). The yellow star indicates the earthquake and the red triangle indicates the receiver. Earthquakes are 400 km deep. The black lines in the center of the kernels are the geometric raypaths. (Credit: Maguire et al.)

"Our study focuses primarily on lower mantle plume tails because it's really one of the only ways forward in terms of settling the debate on the existence of mantle plumes," Maguire said. This relates to hotspot volcanism, caused by an anomalously hot mantle away from plate boundaries. Mantle plumes that rise from the core boundary interest geoscientists because they play a role in Earth's total heat budget by moving heat from the core to the surface.

"Whole mantle plumes, meaning plumes that rise from the core-mantle boundary, are also the most challenging to image seismically because our resolution is very poor in the deep mantle and deep plume conduits are likely to be thin," Maguire said.

Supercomputers might finally be starting to catch up with long-standing scientific questions and help provoke new questions. "I think that the challenge remains in understanding exactly what we are looking for," said Ritsema. "In Maguire's work, we have defined a mantle plume as a purely thermal upwelling in the deep Earth. In this particular case, the plume is a fairly narrow structure -- it has a fairly narrow tail, with its complications in imaging. But there has been other work by other groups who have in fact argued that plumes might be much thicker than what we have investigated in our work. The very nature of plumes, whether plumes are purely thermal or temperature-driven, or whether there is also a compositional component to their formation, are issues that are now being addressed in geophysics," Ritsema said.

The computational demands of simulations in this study of wave propagation limited the number of plume structures, which can be more varied in shape, size, composition, and temperature than the solely thermal plume cases they considered.

Source-receiver geometries used in synthetic tomography experiments. Yellow stars indicate earthquakes and red triangles indicate receivers. (a) A scenario in which earthquakes are recorded on a rectangular network at distances D between 30 deg and 120 deg and with uniform azimuthal coverage. The width L of the network is 6,000 km, and the spacing between stations Δx is 100 km. (b) The network geometry shown is identical to the PLUME geometry (Wolfe et al.,2009). The earthquakes are larger than magnitude 6 between 2012 and 2017. (c and d) Hypothetical deployments in the Pacific Ocean. The earthquake locations are taken from the historical seismicity record of events greater than Mw 6 over the previous five years. The network in Figure 4c comprises three intersecting linear arrays with Δx = 200 km. The arrays-of-arrays network shown in Figure 4d is similar to the proposed Pacific Array. The insets in Figures 4b and 4d show a close-up of the stations near Hawaii. (Credit: Maguire et al.)

Said Maguire: "Understanding Earth dynamics is of fundamental importance, because we all live here and are affected by what goes on beneath our feet. The existence of mantle plumes and the role that they play in our planet is still a big question mark. Additionally, plumes have been linked to some of the biggest volcanic eruptions in the history of the Earth. And they're thought to potentially play a role in the largest mass extinction events that we have on geologic record. There's still a lot that we don't understand about them. Doing research into probing the nature of mantle plumes is of fundamental importance."

We would not be able to do this type of work without supercomputing resources like those that are provided by XSEDE.