Lava crucible
beguiles scientists

Isle researchers hope seabed
seismometers will reveal the
source of Hawaii's volcanoes

An array of seismometers will be deployed on the ocean floor surrounding Hawaii in the next few years to explore mysteries of the so-called "hotspot" that some believe lies beneath the islands.

"We know we have a hotspot. It's just what does that hotspot represent -- a deep mantle plume or something else?" said Cecily Wolfe, geophysicist/seismologist in the University of Hawaii Institute of Geophysics and Planetology.

University of Hawaii Wolfe is one of the principal investigators for the $2.5 million, multi-institution National Science Foundation project called PLUME -- Plume-Lithosphere Undersea Melt Experiment.

The deployment of seismometers, beginning in July next year, will be the largest ever attempted on the ocean floor. The Scripps Institution of Oceanography in La Jolla, Calif., and Woods Hole Oceanographic Institution of Woods Hole, Mass., are developing special instruments.

The goal is to record seismic waves from earthquakes around the world to try to create a three-dimensional picture of the hotspot, much like magnetic resonance imaging (MRI) does of the interior of the human body, she said.

Seismic waves are slowed by hot temperatures, so scientists will be able to see the signature of a hot or boiling plume, she said.

"We have to get in close with lots of instruments to see the fine detail and conduit of upwelling creating Loihi, Mauna Loa and Mauna Kea." The findings may resolve doubts raised by some scientists about volcanic regions known as hotspots. Two articles in a recent issue of the journal Science reflect differing views.

Gillian Foulger of the University of Durham, England, and James Natland of the University of Miami said efforts to find evidence of hotspots with seismic waves haven't produced results.

In fact, they said, there may not be hotspots, that plate tectonics may explain volcanoes far from the edges of continental plates (such as Hawaii and Iceland) instead of lava rising from the Earth's mantle.

Another team, Donald J. DePaolo and Michael Manga of the University of California, Berkeley, agreed that seismological studies haven't produced proof of hotspots but said they expect it to be found.

They said there is supporting evidence of lava rising from a deep hotspot in the Earth under Hawaii, building islands as the Pacific Plate moves across it.

Wolfe said the two papers represent different interpretations of previous data and "tend to capitalize on uncertainties. My approach is to go out and get data to answer the questions rather than answer them before we get the data." UH geologist-geophysicist Michael Garcia said regarding the Science articles that it's "a good thing" to consider other possible origins of hotspots.

"We should not get locked into one mindset," he said.

At this point, however, he said, "Most of the (science) community is unpersuaded by their arguments."

Garcia acknowledged "some complications" because researchers "haven't been able to define a nice image of the plume under Hawaii.

"We have the hottest plume and are still unable to see it seismically. That is a nuisance. We're hoping with the experiment next year that we will be able to see the hotspot, that not seeing it is a matter of not having the right tools."

Many ideas have been thrown out over the years to explain why the underwater volcano Loihi is erupting off the Big Island, he added.

"The one we think explains it best is that it is a big hotspot."

Wolfe said there is clear evidence for a hotspot underlying Iceland, with several papers published by different scientists, including herself.

Hawaii's volcanic processes are well known, but the mantle plume generating the hotspot remains puzzling.

Scientists want to understand the processes creating the phenomena, Wolfe said, including the plume's depth in the Earth's mantle, its width and the temperature of the rising hot rock.

The depth is significant because of questions whether the plumes stem from the Earth's upper mantle or the deeper boundary between the lower mantle and the Earth's core, she said.

Another theory is that the deep upwelling at the hotspot could be caused by a crack in the lithosphere (earth's crust) or weaknesses allowing the material to rise, she said.

Some people think the hotspots are very narrow upwellings of solid rock from the mantle, she said, pointing out the rocks are buoyant and rise up because of hotter temperatures than normal.

As the material gets near the surface, it melts and the melt generates the lava that creates the volcanoes, she said.

"We're looking for a narrow anomaly, a narrow feature of the upwelling, if it is a mantle plume," she said.

Plans are to place 35 instruments on the seabed surrounding the Big Island in the first 15 months of the project, then expand the network around the entire chain in the following 15 months. Some instruments also will be placed on the islands.

Wolfe said the instruments will be left about a year before they're retrieved to collect the data.

"We have to leave these down long enough to have enough earthquakes to get a signal."


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