PETER CERVELLI/ COURTESY OF HAWAIIAN VOLCANO OBSERVATORY Hawaiian Volcano Observatory contract pilot David Okita set up a global positioning system receiver near Puu Oo in June 1999. The GPS unit in the yellow box records satellite signals received by the white antenna on the tripod. Five such GPS units monitor tiny ground movements on Mauna Loa.

Scientists watching
Mauna Loa swelling

The expansion is relatively
small but could be significant

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By Rod Thompson
rthompson@starbulletin.com

HILO >> Magma inside Mauna Loa volcano is causing the 2 1/2-mile high mountain to swell, signaling a possible future eruption, scientists at the Hawaiian Volcano Observatory say.

The swelling so far has been "small stuff, indeed," an observatory statement said, leaving scientists to wonder if an eruption will take place in six months or 10 years.

The mountain has a record of inflating and deflating, swelling and shrinking, since its last eruption in 1984. Mauna Loa's eruptions are not connected to the ongoing Kilauea eruption.

The mixed record of Mauna Loa leaves observatory scientist-in-charge Don Swanson at a loss to say what's probable.

"We can't tell. That's the hooker. The inflation and deflation cloud the issue," he said.

What scientists have seen since Mother's Day is a stretching of Mauna Loa's three-kilometer, 1.9-mile wide summit caldera by 2 centimeters, about four-fifths of an inch.

"It's tiny stuff," Swanson said. But it's a reversal of what had been going on.

From 1984 to 1994, the mountain inflated with magma. Geologist Peter Cervelli said new magma tends to collect in an underground area about three miles below Mauna Loa's summit, just a bit below sea level.

But from 1994 to Mother's Day, the mountain deflated. Scientists do not know why but maybe because magma there cooled and contracted, Swanson said.

The current 2-centimeter spread was spotted by global positioning system instruments on the mountain, similar to instruments fishermen carry in their boats, but more sophisticated.

The observatory had four GPS receivers on the mountain, is adding four more from Stanford University, and is keeping another four from Stanford in reserve, Cervelli said. It also has six tiltmeters on the mountain, which also show bulging and do it faster, he said.

Amid this preparation, one thing is missing: earthquakes.

Magma movement should create lots of mini-earthquakes, but apparently it is too soon for that. "Rocks bend before they break," the observatory statement said.

If an earthquake does come, the mini-quakes should give plenty of warning, perhaps several months-worth, Swanson said.

If lava does erupt, the most likely spot is the summit, he said. After that, the nervous guessing begins again.

As in the 22-day 1984 eruption, lava outbreaks could move away from the summit and toward Hilo. An 1880 eruption took 280 days to send lava to the edge of Hilo before stopping, said Mauna Loa geologist Frank Trusdell.

But lava outbreaks could also move toward steeply sloped South Kona. Lava from a 1950 eruption there reached the sea in just three hours.