Local school kids have all heard the theory in science class that a plume of heated rock rooted deep in the Earth led to the formation of the Hawaiian Islands.

And now that theory has moved closer to fact with the findings of a new study that offers the best evidence yet of the origins of the "Hawaiian hot spot."

The five-year Plume-Lithosphere Undersea Melt Experiment, or PLUME, deployed a large network of seafloor and land-based seismometers to measure the timing of seismic waves generated by major earthquakes around the world as they pass beneath Hawai'i.

The seismic waves are sensitive to temperature, traveling more slowly through heated material, said Cecily Wolfe, a geologist at the University of Hawai'i's School of Ocean and Earth Science and Technology and lead author of the study.

"We looked at the timing of these waves across the network from earthquakes in a multitude of regions recorded on a multitude of stations and were able to construct a set of measurements we could use to image the regions around and beneath Hawai'i down to 1,500 kilometers, into the Earth's lower mantle," she said.

The three-dimensional image shows an elongated low-velocity area beneath the Islands indicating hot material from an upwelling plume. The area extends down from the upper mantle into the Earth's transition zone, at a depth of 255 to 410 miles, and even deeper into the lower mantle down to at least 932 miles, or 1,500 kilometers.

"A prediction is one thing. To be able to see it so clearly and so deep as we did was a surprise," Wolfe said. "People have proposed that hot spots are formed by mantle plumes. This is the real evidence for it."

PLUME TILTS A BIT

Wolfe said the image also shows the plume tilting toward the southeast, "which is very consistent with the idea of a Hawaiian plume."

"It's really exciting to actually have this snapshot of the process that generated the entire state of Hawai'i," she said.

Most volcanic activity on the planet occurs near subduction zones, where tectonic plates meet, one riding over the other. But there are isolated "hot spots," such as Hawai'i, where surface volcanoes are believed to be fed by upwelling plumes originating deep inside the Earth.

The theory goes that as the Pacific plate moves over the stationary plume under Hawai'i, toward Japan, one volcano dies out and a new one is formed, creating a chain of islands.

Wolfe said the relatively rapid movement of the plate drags the upwelling plume with it — much like wind bends smoke rising from a chimney — accounting for the southeast tilt.

The PLUME project, funded by the National Science Foundation, marked one of the first large-scale deployments of seafloor seismometers. Four research cruises were conducted over 2005 and 2006 to set and recover the seismometers at 73 sites across 600-plus miles. The voyages were headed by lead PLUME investigator Gabi Laske of the Scripps Institution of Oceanography and John Collins of the Woods Hole Oceanographic Institution.

Sean Solomon of the Carnegie Institution for Science provided a concurrent deployment of 10 land seismometers on the main Hawaiian Islands.

HIGH TECH GOES DEEP

Hawai'i's geographic isolation from the most active Pacific earthquake zones and other factors hampered earlier efforts to study the deep structure beneath the Islands, according to the study. Use of seismometers was limited to land sites that didn't provide sufficient coverage for high-resolution imaging, and earlier versions of the seafloor devices lacked battery power for long-term deployment.

"This pioneering experiment combining large numbers of broadband seismometers on the seafloor with instruments on land has provided the most persuasive evidence yet for the existence of a mantle plume extending into the lower mantle beneath Hawai'i," study co-author Robert Detrick said in a news release.

Detrick conducted the research while at Woods Hole but now works at the National Science Foundation's Division of Earth Sciences.

Wolfe said the results of the project make a strong case for the existence of a deep mantle plume, with implications not just for Hawai'i but for studies into how the Earth evolved.

Their research findings are published in the Dec. 4 issue of the journal Science.