A Deeply Hawaiian Hotspot

A new study by a multi-institutional research team has opened up a window into the earth to reveal Hawaii’s deep roots and the best picture yet of a plume originating from the lower mantle. The findings suggest that the Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle.

The National Science Foundation-funded project, called the Plume-Lithosphere Undersea Melt (PLUME) Experiment, deployed a large network of seafloor seismometers at 73 sites off Hawaii during four research cruises to record data over more than two-years. It represented the largest ocean-bottom seismometer (OBS) experiment in the world, reaching deeper into the lower mantle than previous experiments.

“The PLUME team has unambiguously traced the Hawaiian plume from the seafloor through Earth’s mantle transition zone,” said Scripps Institution of Oceanography at UC San Diego geophysicist Gabi Laske who led the PLUME experiment. The transition zone was previously thought to be a layer that obstructs whole-mantle convection between the core and the surface of the planet.

Laske’s research involves long-term, ocean-bottom earthquake monitoring to find and image the plume that gives rise to the spectacular Hawaiian island chain of volcanoes and to better understand how they are fed by a super-heated plume from deep in the earth.

The seismometers were used to record the timing of seismic shear waves from earthquakes around the world with magnitudes greater than 5.5. This information can be used to determine whether seismic waves travel more slowly through hot rock as they pass beneath Hawaii. Combining the timing measurements from many earthquakes recorded on many seismometers allowed researchers to construct a sophisticated three-dimensional image of the Hawaiian mantle.

"The data collected by the PLUME experiment helped pioneer an entirely new class of marine seismology research," said Scripps professor John Orcutt and a co-author of the paper. "The state-of-the-art broadband OBS instruments developed at Scripps represent a great advancement for seafloor seismology."

The images from the PLUME experiment provide support for the existence of a mantle plume beneath Hawaii, which strikingly, tilts toward the southeast of Hawaii as it extends downward. Results of the project strongly indicate the existence of a deep-mantle plume, which has fundamental implications--not just for Hawaii, but more generally for the form of convection in the solid Earth, Earth's composition with depth, its evolution over geologic time, and how the earth releases heat.

The Hawaiian volcanoes are a classic example of the unique type of volcanic activity that occurs in the interior, rather than at the edges, of the earth's tectonic plates. The location of the Hawaiian Islands in the middle of the Pacific Ocean had hampered past seismological efforts to resolve the plume's deep structure. Scientists turned to a more technologically advanced marine seismology approach by placing temporary instrumentation on the seafloor to record seismic waves for two one-year deployments.

The large, 1,000-kilometer (621-mile) wide aperture of the seafloor ocean bottom seismometer network, developed at Scripps Oceanography and Woods Hole Oceanographic Institution yielded unprecedented precision and resolution in a remote oceanic region. The inclusion of SKS waves, a type of wave that travels through the earth's core, was critical for extending the imaging down to 1,500-kilometer (932-mile) depths.

“This experiment was first conceived by our team a decade ago.  Fortunately, the results have been worth the wait and exceeded all expectations,” said Cecily Wolfe, professor at the University of Hawaii and the paper’s lead author.

The study, titled “Mantle Shear-wave Velocity Structure beneath the Hawaiian Hotspot,” was published in the Dec. 4 issue of Science magazine.