Mount St. Helens may be following the example of Kilauea in Hawaii with magma being replaced from a reservoir beneath the volcano as fast as it emerges as lava at the surface, scientists say.

While the two volcanoes are different in many respects, St. Helens appears to have become an "open system" as its dome building eruption that began in the fall of 2004 continues at a pace that has been unchanged for the past year, said Daniel Dzurisin, a geologist at the U.S. Geological Survey's Cascades Volcano Observatory.

Analyzing of digital elevation models made from high-resolution aerial photographs, scientists have kept close tabs on the rate at which lava has been pushing into the crater.

At first it was about a dump truck load per second. A year later it was down to slightly more than one-eighth that amount per second, and since last April it has been fairly constant about 0.6 of a cubic yard per second - still the equivalent of about nine truckloads every two minutes.

Lava channel likely

The longer the eruption continues at that rate, the more likely it is that a direct pathway has developed for molten rock to emerge from deep within the planet's crust, Dzurisin said, adding that it will take another year of data to reach a more definite conclusion.

"That situation could go on for a long time," Dzurisin said. "The ongoing eruption [at Kilauea] in Hawaii, for example, started in 1983."

Other evidence indicating the development of an open system at St. Helens is the slowing of deformation around the flanks of the volcano, indicating the magma chamber beneath the surface is being refilled rather than deflating, which would cause sagging.

Rate of deformation subsided

At Johnston Ridge Observatory, eight kilometres north of the crater, a global positioning system monitor has moved toward the volcano by about 2.5 centimetres since the eruption began, with most of the movement coming in the first year and a half.

Since then, the rate of deformation has subsided considerably while lava continues to emerge, indicating the magma is being replenished.

It took about four centuries to build the symmetrical, cone-shaped peak that led St. Helens to be compared with Mount Fujiyama in Japan before a massive blast on May 18, 1980, removed the top 400 metres, flattened miles of southwest Washington forests and left 57 people dead.

"We know that St. Helens is capable of dome eruptions lasting decades," Dzurisin said.