ON THE RIM OF PU'U O'O, HAWAII — Volcano National Park, Hawaii - On its face, the grapefruit-sized rock Donald Swanson unwraps and sets on his desk looks like an unremarkable chunk of earth. Yet to Dr. Swanson, it speaks volumes about a violent eruption 1,200 years ago that is earning Kilauea - Hawaii's "drive-in" volcano - a new measure of respect.
Swanson and colleagues at the US Geological Survey's Hawaiian Volcano Observatory are amassing evidence showing that Kilauea can erupt with greater force than previously believed.
Until now, the volcano's explosive eruptions have been attributed to lava welling up within a chimney blocked by a collapse of crater walls or filled in by lava flows from nearby vents. When the upwelling magma hits the water table, it generates a steam explosion that blasts away the earthen "plug" at a relatively shallow depth.
The team now holds that Kilauea's explosive eruptions also can be driven largely by an influx of carbon dioxide. It can originate miles underground, blast material to altitudes of up to 20 miles, and send a potentially lethal rain of rock falling miles from the summit.
Moreover, such eruptions can occur as a series of events over centuries; they don't have to occur as a single event or as several within a week, as some researchers had held.
If Kilauea were to enter another period similar to the one the team has been studying, "you might have to worry for decades or centuries that something would be popping out every few years" with potentially devastating effects, Swanson says. "All this at a volcano where we're comfortably sitting now and feel safe, and everybody thinks of it as a drive-in volcano."
The story has taken nearly 15 years to piece together. Initially, Swanson says, the team wanted to reconstruct the history of faulting and eruptions in one region of Kilauea's flank. But as they dug and searched, they were drawn to one layer of ash sandwiched between two young lava flows. The layer cropped up everywhere. It consisted of five major sublayers, dated from 1,000 to 1,500 years old. Their chemical composition was unusual. Of special interested was a sublayer dating back 1,200 years. Rocks among the ash appeared to have originated two to three miles underground.
Where exposed lava flows were old enough, scientists found rocks of similar composition strewn over the surface. The rocks grew smaller the farther from Kilauea's crater the scientists looked. They found rocks the size of grapefruit six to seven miles from the summit and rocks the size of golf balls much farther afield. "In the world of volcanoes, that's incredible," says Richard Fiske, a Smithsonian Institution volcanologist, referring to the amount of energy needed to hurl those rocks.