Study sheds light on island’s birth

By COLIN M. STEWART

Tribune-Herald staff writer

Hawaii Island and the rest of the state was built mainly by extrusion — lava traveling up and spilling out onto the island’s surface, each level stacking itself atop the last like layers on a cake — according to new research performed by scientists with the University of Hawaii and University of Rhode Island.

Previously, researchers believed that the island chain grew primarily via a process of intrusion — magma infiltrating the interior of rock and then solidifying and expanding, blowing up the islands like an enormous balloon.

The new findings are the result of an extensive compilation of gravitational readings taken at various sites on land and in the ocean across the state to measure the density of the rock below, according to UH-Manoa Professor of Geology and Geophysics Garrett Ito, one of the co-authors of the study. Previous attempts to provide an overall picture of the state’s marine and land gravity data date back more than 45 years.

“Most modern methods of measuring the eruptions are by using GPS technology,” he said. “They have instruments on the volcano that monitor very small movements of the surface, so they can watch the expansion. We used a very different technique. We used gravity measurements.”

Known as gravimeters, the instruments the scientists used can detect variations in the pull of gravity “down to 1 part in 1 million or so,” he said. Greater pulls in gravity are detected above denser rock, so the gravimeter can essentially tell which areas have more density, thus telling the researchers how they were formed.

“The intrusive lava is stuck in the volcano, it’s very pressurized, it’s largely very dense,” he said Monday. “Meanwhile, the extrusive lava is less dense.”

The data revealed that there were far fewer areas of dense rock making up Hawaii’s volcanoes than previously believed. Less than 30 percent of the total volume of volcanoes throughout the state are the product of intrusive growth versus extrusive, according to the research.

The new data is a stark contrast to what was previously believed by scientists, according to Ashton Flinders, lead author of the study and graduate student at the University of Rhode Island.

“The discrepancy we see between our estimate and these past estimates emphasizes that the short-term processes we currently see in Hawaii (which tend to be more intrusive) do not represent the predominant character of their volcanic activity,” he was quoted as saying in a UH press release.

Much of the gravity data collection on the Big Island was done in concert with the U.S. Geological Survey’s Hawaiian Volcano Observatory, according to Ito. However, workers there, including Scientist-in-Charge Jim Kauahikaua, were unavailable to comment Monday due to the ongoing government shutdown.

“What he (Kauahikaua) found (on the Big Island) was the densest rock was relatively confined to the volcano summits and to the rift zones,” Ito said.

The findings could mean that Hawaii Island’s current eruption along the East Rift Zone, which has already been going on for 30 years, could continue for many more years than previously estimated.

“This could imply that over the long-term, Kilauea’s (East Rift Zone) will see less seismic activity and more eruptive activity than previously thought,” Ito was quoted as saying in the press release. “The three-decade-old eruption along Kilauea’s ERZ could last for many, many more decades to come.

Another question raised by the research will likely be how this ratio of intrusive to extrusive volcanism impacts the stability of the volcano’s flank, Flinders said.

“Collapses occur over a range of scales from as large as the whole flank of a volcano, to bench collapses on the south coast of the Big Island, to small rock falls,” he said. “If the bulk of the islands are made from these weak extrusive flows, then this would account for some of the collapses that have been documented, but this is mainly just speculation as of now.”