The drilling was being conducted for an existing geothermal power plant built to harvest heat from the world's most active volcanic zone, Kilauea volcano, which has been spewing lava continuously since 1983.
Don Thomas, a geochemist from the University of Hawaii's Center of the Study of Active Volcanoes, said it was just a matter of time until some drilling operation there struck hot magma.
But to have it actually happen is "tremendously exciting," said Thomas, who was not part of the discovery team.
In addition, researchers found that the magma is made of dacite, a type of rock that's a precursor to granite, rather than the basalt that forms most of Hawaii.
"If we had hit basalt, that would not have been a big surprise," said William Teplow, a consulting geologist at U.S. Geothermal, Inc., who is assisting on the project.
Scientists have long believed that dacite can separate from basaltic magma to form granitic rocks. But they'd never expected to see the process in operation.
"This may be the first time that the generation of granite has actually been observed taking place in nature," Teplow said. "This is important because it's the process that differentiates the continental granitic crust from the more primitive oceanic basaltic crust."
Volcanologist Marsh is excited by the prospects for further study.
"This is just the tip of the iceberg," he said. "We don't know where it's going to lead, but it's a golden opportunity."
It might even be possible to do experiments inside the magma.
"This could be the first magma observatory in the Earth," Marsh said. "This is a singular event of first contact with inner Earth, where magma lives."
With an estimated temperature of 1,900°F (1,050°C), the magma is also valuable as a high-quality heat source for geothermal energy production.
"But the first thing is to get the science," said Lucien Bronicki, Ormat's chair and chief technology officer.
Johns Hopkins's Marsh added that the magma body is big enough that tapping it for energy shouldn't interfere with future research.
"The drill hole is just a pinprick on an elephant's back," he said.
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