It's been nearly 25 years since Mauna Loa, Hawaii's most dangerous volcano, last erupted—but researchers warn that another eruption may be on the horizon. It's nearly impossible to pinpoint the exact date or time the mountain may blow next, but a new technology allows scientists to determine the eruption's location on the slopes of the giant volcano, thereby helping them determine where the lava it spews will go.

Mauna Loa, the largest volcano on Earth, is a so-called shield volcano, which means that lava can rush either from its central crater or its slopes—and in some eruptions from both. If lava shoots out of Mauna Loa's southern or northern rifts, two neighboring villages are at risk of being scorched. When the mountain blew in 1950, lava ran down its southwest side, prompting the evacuation of 75 people and destroying 15 homes near the village of Kona, about 25 miles from the mountain. When it last erupted in 1984, rivers of fiery-hot lava flooded the northeastern side of the mountain, stopping just short of the island of Hawaii's largest city, Hilo (population: approximately 150,000).

Falk Amelung, a geophysicist, along with colleagues at the University of Miami and Stanford University used satellite imagery known as interferometric synthetic aperture radar (InSAR) to monitor the growth of two bulges on the flanks of Mauna Loa. The lumps indicate subterranean magma (molten rock) pressing against the surface. InSAR uses numerous images taken from space to determine changes in the shape of Earth's surface over very large areas. The researchers reported in Science last year that the bump on the southwestern side of the mountain had risen eight inches (20 centimeters) in just three years.

The researchers speculate that a 1983 earthquake under Mauna Loa and the 1984 eruption shifted the ground underneath the southwestern side of the volcano, allowing magma to push into that new open area. They believe pressure from this flow caused the bulge to grow, and may trigger a series of earthquakes that could cause a rip in the southwestern rift. Should their forecast prove accurate, ejected lava would likely head toward the K'au district on the southeastern side of the island.

The U.S. Geological Survey (USGS) considers the prediction to be an "important scientific contribution," but experts at the Hawaiian Volcano Observatory (HVO) in Hawaii Volcanoes National Park say a better predictor of the future is the volcano's past behavior. As the HVO staff scientists wrote on the USGS Web site in May 2007:

"Since 1843, nearly all of Mauna Loa's eruptions have started at its summit. About half of them have stayed in the summit, and the other half have moved about equally into either rift zone. Therefore, the chance of the next eruption of Mauna Loa moving into the southwest rift zone is about 25 percent. Amelung's model predicts a 100 percent probability, and Mauna Loa's history says he has a one in four chance of being correct.

One thing is for certain—Mauna Loa will erupt again. For the last few years, HVO has been reporting that Mauna Loa has been inflating at slow rates; HVO continues to watch closely. Studies like Amelung's may prove very useful once tested and verified. We all continue to gather clues, analyze patterns, and propose predictive models."

Read more about other prediction models and a review of devastating eruptions, in our In-Depth Report, A Guide to Volcanoes.