Undersea hot spot


POSTED: Friday, December 04, 2009

The mystery of the “;hot spot”; that created the Hawaiian Island chain over the last 75 million years has been unraveled with an experiment that “;opened a window into the earth.”;

The findings, reported in a cover story in today's Science magazine, provide strong evidence of a mantle plume beneath Hawaii, University of Hawaii geophysicist Cecily Wolfe said in an interview.

“;We live in Hawaii and enjoy it, but we forget how special and unique it is,”; she said. “;It is one of the most outstanding volcanic features on Earth, and scientists want to understand it.”;

Wolfe, lead author and principal Hawaii investigator of the PLUME project, was able to construct a three-dimensional image of the Hawaiian mantle by combining timing measurements of seismic shear waves from earthquakes greater than 5.5 magnitude around the world.

Seismic waves travel at different speeds that are sensitive to things like temperature, melt, water content and composition, Wolfe said. “;They move more slowly through warm material than cold.”;

The images showed evidence of an upwelling of hot material from Earth's deep mantle that partially melts as it nears the surface and feeds magma to the volcanoes that created the Hawaiian Islands, she said.

As the Pacific plate moves above the plume, activity dies on a particular volcano, and a new one is formed — such as Loihi, a submarine volcano about 20 miles off the Big Island growing into Hawaii's next island.

The long Hawaii-Emperor chain of seamounts can be traced to the Northwest Pacific, where the oldest seamount, an extinct submarine volcano, is 80 million years old, scientists said.

The deep-mantle plume beneath Hawaii suggests islands still will be forming in the chain for millions of years, Wolfe said. “;There are no indications it will end any time soon. It is a big feature down there.”;

Wolfe said the project, involving a team of scientists, “;has been a decade in the making. Now we have data and answers.”;

The evidence is “;a promising start”; toward ending a debate over the origin of mantle plumes and understanding hot spots such as the Hawaiian Islands, she said. “;There are doubters out there, plume skeptics who didn't believe there was a plume under Hawaii.”;

She expects the paper — “;Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot”; — to be controversial but said she thinks scientists worldwide will be using the data “;to get a better picture or find alternatives to our interpretation and imaging.”;

The development of deep ocean bottom seismometers that could be deployed for a long time made the experiment possible. Four research cruises were conducted to install the seismometers at 73 sites on the sea floor and recover them for the data after a year.

Gabi Laske of Scripps Institution of Oceanography led the experiment, and she and John Collins of Woods Hole Oceanographic Institution conducted the research cruises.

“;We also had land seismometers at sites all over the islands, in schools, in a community college and even in peoples' back yards,”; said Wolfe. Leading that effort was Sean Solomon of Carnegie Institution of Washington.

Other investigators and authors were E.H. Hauri of Carnegie Institution, J.A. Orcutt of Scripps, R.S. Detrick of Woods Hole and D. Bercovici of Yale University.

“;Mapping the seismic speed variations helped to form an image of processes that formed the islands,”; she said. “;It's like when you go to a doctor's office and get an X-ray, a CAT scan or MRI. We do that in the earth equivalently.”;

One surprise was how deep they could see, she said, “;all the way down to 1,500 kilometers depth in the mantle (about 930 miles) — about halfway through the earth's mantle.

“;We were never expecting to see something that deep, but with different types of (seismic) waves, we were able to extend imaging quite deep and see low-level velocity.”;

Another big surprise: The plume is tilted toward the southeast of Hawaii as it extends downward, which was predicted by theoreticians, Wolfe said.

“;Think of it like the wind. Like rising smoke on a windy day, the Hawaiian plume gets deflected in the mantle wind as it rises upward.

“;We're starting to get a picture. It's pretty exciting,”; she said, noting the sea floor experiment worked so well, “;it was a technological feat in itself.”;

“;The National Science Foundation took a big chance on us, and it paid off and it's just beginning.”;