Digging up the dirt on Mauna Loa.

How much time did it take to build Hawaii's massive Mauna Loa, the world's largest volcano? Much longer than scientists had thought, says a team currently drilling into the side of this fire-spouting mountain, most of which lies underwater.

Working near the city of Hilo on the big island of Hawaii, the research team has bored 1,100 meters into the ground to chart the history of lava flows that have poured out of Mauna Loa and its extinct sibling, Mauna Kea. Before starting the project, volcanologists had believed Mauna Loa's age might range up to 500,000 years. But the rocks pulled up during the drilling experiment show that the volcano spewed out lava far less often than previously thought, indicating that Mauna Loa and its neighbors on Hawaii may be more than twice as old as previously believed.

The drill hole first pierced lavas from Mauna Loa and then passed into the Mauna Kea flows. The big island also contains three other volcanoes: the extinct Kohala; the historically active Hualalai; and the currently active Kilauea. Because the drilling results indicate that these volcanoes grew slowly, they suggest that four might have erupted during the same span of time, says Donald M. Thomas of the University of Hawaii in Honolulu. He collaborated on this project with Donald J. DePaolo of the University of California, Berkeley, and Edward M. Stolper of the California Institute of Technology in Pasadena.

The big island's volcanoes and those on the other Hawaiian Islands formed as the Pacific plate passed over a vertical stream of hot rock rising from deep inside the planet. The ascending plume creates a volcano each time it burns a hole through the surface plate. But because the plate is moving, each volcano eventually dies out as it passes beyond the reach of this so-called hot spot. Then a new volcano develops over the spot. The youngest Hawaiian volcano, called Loihi, is slowly growing on the submerged flank of the island and will take many thousands of years to break the ocean surface.

The research team drilled into the Hawaiian lava flows to learn more about the plume of hot rock that feeds the volcanoes. While some geophysicists believe these streams of rock rise from the edge of the Earth's core, others believe they start much shallower in the planet, within the upper section of the mantle. By studying details in the successive lava flows that erupted from Mauna Loa and Mauna Kea, scientists hope to learn more about where plumes originate and how they rise through the mantle, DePaoIo says.