By Sid Perkins
SAN FRANCISCO — Engineers drilling a new well at a geothermal site in Hawaii recently struck liquid gold — a mass of molten rock that is giving geologists an unprecedented peek at how magma cools today and insights into how continents might have formed billions of years ago.
The Puna Geothermal Venture, which sits on the slopes of Hawaii’s Kilauea volcano, has been producing at least 25 megawatts of power since 1993. By taking advantage of the volcano’s immense source of subterranean heat, the facility produces about 20 percent of the power now needed on the Big Island, says William Teplow, a geologist and consultant at U.S. Geothermal Inc. in Boise, Idaho.
While expanding the facility in 2005, drillers encountered increased resistance at a depth of about 2.5 kilometers. They decided to raise the equipment slightly and stop drilling for a while. When they resumed, the tip of the drill struck solid rock at a depth about eight meters higher than where they’d previously stopped. Soon thereafter, the geologist on duty noticed that the rock cuttings brought to the surface by the drill — the geological equivalent of sawdust — were clear bits of mineral, not the normal black bits of chewed-up basalt. “Nothing like this had ever been seen,” Teplow says.
Subsequent chemical analyses of the clear cuttings indicated that the mineral bits were 67 percent silica, Teplow and his colleagues reported December 16 in San Francisco at the fall meeting of the American Geophysical Union. That percentage is distinctly different than the 50 percent silica content typically found in basaltic rocks, he notes, but chemically similar to the granitic rocks that make up continental crust. Also like granite, the minerals were high in potassium and sodium but low in iron and calcium.
Teplow and his colleagues quickly realized that they’d struck a pocket of molten rock, some of which had oozed up the 13.65-centimeter drill hole and then solidified. “This is the first time scientists have encountered magma in its true natural habitat” deep below ground, says coauthor Bruce Marsh, a geologist at Johns Hopkins University in Baltimore. “This is like Jurassic Park for magmatic systems,” he adds.
The researchers estimate that the molten material in the pocket is about 1,050° Celsius and has a viscosity like that of cold molasses. That magma is the uncrystallized portion of a mass that flowed into its subterranean chamber during a previous eruption, probably one that occurred in 1955, Marsh says.
Unlike a pure substance such as water, which freezes at a single temperature, a mixture of minerals dissolved in magma typically solidifies over a broad range of temperatures, Marsh explains. In general, the basaltic minerals in this magma are the first to freeze to the walls of the chamber, and the dacitic minerals — the ones that make up granite — are the last to solidify. While geologists often find lumps and tubes of granitic rock inside large masses of long-solidified basaltic rocks, scientists have never caught such a crystallization process in action.
The team’s serendipitous discovery “is tremendously exciting but not surprising,” says Don Thomas, a geochemist at the University of Hawaii at Hilo Center for the Study of Active Volcanoes. Researchers have been drilling holes in this area for years, and it was only a matter of time until someone struck molten rock, he suggests.
Analyzing the masses of granite that gradually solidified within masses of basalt long ago provides some clues about how that process unfolded, says Thomas. However, those masses often have been partially eroded or otherwise altered by chemical weathering. Monitoring how the newly found mass of magma crystallizes in the future may yield detailed insights into how such solidification happens, both on the small scale and in the larger version of the process that occurred when Earth’s continents formed billions of years ago.
