Like geologists, most visitors to Hawaii know from their travel guides that the majestic tourist destination is a product of (relatively) fresh magma rising from somewhere beneath the island chain. But even geologists and geophysicists are mystified about exactly where the magma is generated deep inside the earth.
A common assumption holds that Hawaii’s magma pushes up directly beneath the islands. Not so, says a scientific team that includes Gabi Laske and Peter Shearer of Scripps Institution of Oceanography at UC San Diego, which recently found clues about where the magma is likely coming from. Their answer: Look to the west!
The new study, published in Nature Geoscience, was led by Catherine Rychert, a former Scripps Oceanography postdoctoral researcher who is now at the University of Southampton. The paper explains how the researchers analyzed a data set generated by the Scripps-led plume-lithosphere undersea mantle experiment, or PLUME, a project that instituted the first large-scale, long-time deployments of ocean bottom seismometers (OBSs). Such instruments, which measure the fine details of earthquake motions, allowed the scientists to not only locate the vicinity of Hawaii’s magma, but establish its boundaries to trace back its root sources.
Using the seismometer data, “we now have strong evidence that the plume is diverted to the west of Hawaii when it ascends from the lower mantle,” said Laske, a professor-in-residence at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics at Scripps. “We speculate that when Hawaii formed, so much magma was produced that a solid protrusion formed from the rock left behind. The ascending plume material now has to migrate around this protrusion, and is diverted to the west before it finds a pathway through fractures in the Pacific plate to feed Mauna Loa, Kilauea, and Loihi, Hawaii’s youngest volcano.”
The researchers dissected so-called “S-to-P” seismic signals recorded by OBSs to create an unprecedented image of the base of the magma zone at 110 to 155 kilometers (68 to 96 miles) beneath Hawaii.
“The results are a tribute to the quality of the PLUME data and the ability of modern processing techniques to extract new information about old problems, in this case the location of the mantle plume feeding the Hawaiian volcanoes,” said Shearer, a Scripps professor of geophysics.
Although they have provided valuable information about Earth structure around Hawaii, the current seismometers used in the PLUME project can only retrieve data for up to a year at a time due to the life span of their lithium batteries. Laske believes even more vital information could be captured with permanent OBSs deployed around Hawaii. Such devices could transmit real-time seismic data through a communications hub such as a seafloor cable, mooring, buoy, or even a wave glider. Scientists would have instant access to further details about Hawaii’s volcanoes and Earth’s structure around them.
Such a system would also provide further information about where and how often earthquakes that had previously gone undetected occur around Hawaii, and related motions on the Pacific plate.
“It is important to understand why some volcanoes are explosive and some are not,” said Laske. “Hawaii is currently one of the most benign volcanoes on the planet, attracting a lot of tourism. To fully understand volcanism, we need to find out how they work and how they are fed. Where does the magma come from? How is it formed? Are benign volcanoes always benign or can they become violent? How does a volcano become violent? Our study is only one piece of the big puzzle but it’s an important one.”
In addition to Rychert, Laske, and Shearer, the study also included Nicholas Harmon of the University of Southampton. The Natural Environment Research Council provided support for the study.
-- Mario C. Aguilera
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