Recent geophysical data collected on the surface of the 13,500-foot volcano revealed that Mauna Loa's summit caldera has begun to swell and stretch at a rate of 2 to 2.5 inches a year, according to scientists from the U.S. Geological Survey (USGS) and Stanford University. Surface inflation can be a precursor of a volcanic eruption, the scientists warn.

"Inflation means that magma is accumulating below the surface, but at this point we don't have the kinds of sophisticated models that would be required to tell us if or when an eruption will occur," said Paul Segall, a professor of geophysics at Stanford who has collaborated with USGS volcanologists in Hawaii since 1990.

Located on the Big Island of Hawaii, Mauna Loa - or "Long Mountain" in Hawaiian - is the largest volcano in the world. Its last eruption occurred in spring 1984 - a violent three-week event that produced fast-moving lava flows that came within 4 miles of the city of Hilo. The volcano has remained silent for the past 18 years - in sharp contrast to its neighbor, Kilauea, which has been erupting continuously since January 1983.

"After the 1984 eruption, Mauna Loa went through nearly a decade of inflation, followed by almost 10 years of deflation," said Peter Cervelli, a geophysicist with the Hawaiian Volcano Observatory (HVO).

The deflationary period abruptly ended around Mother's Day, May 12, when HVO's global positioning system (GPS) network revealed that the summit had begun to rise and swell. May 12 was the same day that Kilauea's most recent active lava flow began - a discovery that scientists say is far from coincidental.

"This clearly indicates that there is a connection between the two magma systems," Segall noted. "That's the great thing about Hawaii: It's so incredibly active that just about every year we learn something new."

GPS stations

HVO maintains several GPS stations on Mauna Loa that continuously record their positions using information transmitted from orbiting satellites. The around-the-clock satellite data allow scientists to measure how far the GPS stations have moved - and thus determine if the volcano is expanding or contracting. Cervelli, who earned his doctorate at Stanford last year, said the university has loaned HVO eight additional GPS stations to monitor the volcano. Because of the remote mountaintop location, each instrument can cost up to $20,000 to install. Segall's research on the Big Island is funded through a National Science Foundation grant.

"Until recently, Stanford's research in Hawaii has been primarily on Kilauea, but when Mauna Loa started to show renewed activity in late spring, Paul [Segall] agreed to lend us four of his continuous GPS receivers," Cervelli explained. "We are holding four more Stanford instruments in reserve to be deployed as conditions warrant."

Cervelli and his USGS colleagues will work with Segall to interpret the new GPS data as they become available.

"We see this as an opportunity to watch the volcano evolve through an entire eruptive period - from early awakening to actual eruption," Cervelli said. "If the recent activity does culminate in an eruption, this will be the first time that a Mauna Loa eruption is imaged with precise clarity. Without Stanford's help, this would not be possible."

History of destruction

Mauna Loa has erupted 33 times since 1843, spewing out enough lava to cover 40 percent of the Big Island. The most destructive eruption in recorded history occurred in 1950, when lava raced to the sea at speeds up to 5 miles an hour - destroying homes, businesses, roads and ranches along the way.

Despite the volcano's destructive potential, the USGS estimates that more than $2.3 billion has been invested in new construction along Mauna Loa's slopes since the 1984 eruption.

"Mauna Loa is capable of erupting huge volumes of lava in a relatively short period of time, and the flows can reach great distances," Segall observed. "It presents a more significant safety hazard than Kilauea."

Cervelli echoed that concern: "There has been a substantial amount of development on what has historically been the most hazardous part of Mauna Loa - its southwest rift zone above South Point. Though lava flows can reach Hilo on the eastern side of the island and the Gold Coast resorts of Kona in the west, flows are much more likely to inundate the subdivisions in the southwest rift zone - and possibly without much warning."

Increased earthquake activity is another indication that magma is rising to the surface. "Seismicity does seem to be picking up," Cervelli noted, "but at this point we are not issuing a public warning. Instead, we are asking that the people of Hawaii remind themselves that they live among the world's most active volcanoes."

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