Discontinuity under the Aloha State

Rachel Berkowitz

A surprise has been found 660 km beneath Earth’s crust. Geophysicists at MIT’s Earth and Planetary Sciences department have been using new seismic techniques to look for evidence of a mantle plume below Hawaii. They recently published their unexpected results in Science.

Traditional thinking assumed that a simple vertical plume originates at the base of Earth’s lower mantle (near 2900 km depth) and brings hot mantle material to the surface directly beneath the volcanic hotspot. However, Rob van der Hilst and Qin Cao found a striking lack of associated topography variation immediately beneath Hawaii. Instead, they found large perturbations in the mantle transition zone (between depths of 400 and 700 km).

“This doesn’t support the picture of a simple deep-rooted vertical mantle plume feeding the Hawaiian volcano,” says Cao. “But it is consistent with the scenario in which there might be a deep-rooted mantle plume coming from the lower mantle, being stopped by the 660 km phase transition [and] causing ponding of hot materials. . . . . From this pool, one or more secondary plumes might be generated.”

Not only does that complexity in the transition zone provide possible explanations for the western Pacific’s richly featured bathymetry, but it also changes geochemists’ ability to link hot-spot lava directly to deep mantle minerals.

The study reveals a discontinuity where a phase transition in mantle minerals occurs. The transition’s depth is influenced by temperature and pressure, which alter the mantle’s crystal structure. The reflection patterns of seismic waves emitted by earthquakes along the ring of fire surrounding the Pacific plate can be used to detect whether such phase transitions exist, and at which depths. “We can determine depth and pressure from seismic imaging,” says van der Hilst, “then use mineral physics data to estimate temperature.”

Seismologist Arwen Deuss at the University of Cambridge’s Bullard Laboratories notes that “the mantle is 60% olivine and 40% garnet and peroxine. Both [minerals] undergo phase transitions, but which phase transition shows up depends on the transition zone composition and temperature.” Disparities in transition zone thickness moving out from underneath Hawaii are indicative of different temperatures and thus mineral contents.

Mineral transitions from olivine to wadsleyite and from ringwoodite to perovskite (also known as the post-spinel transition) occur at depths of 410 km and 660 km, respectively. If a simple vertical mantle plume crossed those interfaces and created a temperature anomaly, the 410-km transition would occur at greater depth and the 660-km transition would occur at lesser depth. The transition zone would therefore become thinner. But that was found not to be the case, which argues against a simple vertical plume directly underneath Hawaii.

Van der Hilst and Cao propose that hot material from the lower mantle collects in pools at the base of the transition zone, near 660 km depth, where it forms a boundary between hot and cool material. At such a boundary, thermal instabilities may form smaller, secondary plumes or upwellings that carry hot material up from that surface.

Last year Cao and van der Hilst showed how inverse scattering of seismic waves from earthquakes can be used to gain higher-resolution images of structures beneath Earth. In conventional imaging, seismic waves are reflected off areas up to 4 million km² and different reflection points are added to provide an average image of the entire area. Inverse-scattering looks at complex waveforms that result from distortion and scattering of waves in a complex medium. From those waveforms, the point at which scattering actually occurs can be estimated, yielding topographic variations on a much more detailed scale.

The team used close to 200 000 seismic records from all over the world. The records were obtained from the Data Management Center of IRIS , the Incorporated Research Institutions for Seismology, located in Seattle, Washington.

“This project is a very good example of the possibilities afforded by open data access,” van der Hilst says, lauding the IRIS as an example of collaborative efforts that facilitate geophysical research.

With the number of seismographic stations increasing worldwide, inverse-scattering techniques provide the opportunity to create a more complete picture of the internal dynamics of Earth’s deep interior and to put constraints on mantle viscosity and plume dynamics in the mantle transition zone.