How likely is it that three neighboring volcanoes would all erupt at the same time -- as the Kasatochi, Okmok and Cleveland volcanoes in the Aleutians did this summer?

About as likely as a storm that only appears once in a thousand years, says Anchorage volcanologist Peter Cervelli, who'll deliver a paper on the subject this winter to the American Geophysical Union.

In other words, seldom enough that Cervelli is now exploring the question of whether Alaska's triple eruption was only a coincidence involving three independent volcanoes or whether it was triggered by some common mechanism.

There's no question that the volcanoes are related in a broad geological sense, says Comelli, a numerical modeler at the Alaska Volcano Observatory. That's because all 40 active volcanoes in the Aleutian Arc -- a 1,500-mile-long necklace of volcanic peaks that stretch from Kiska Island in the west to Mount Spurr near Anchorage in the east -- owe their existence to the deep, subterranean collision of two tectonic plates.

As the northward moving Pacific plate, consisting of dense material from the ocean floor, dives beneath the North American plate, consisting of lighter material from the continent, friction between the two melts rock, which turns to magma, which eventually shoots skyward through a relief-valve network of volcanoes.

But the plumbing isn't necessarily connected, and the volcanoes don't all go off at once, Cervelli says. Each has its own combustion chamber, its own pressure issues, and one volcano under stress may well border another one that's quiet.

Since Alaska has so many volcanoes, it's not that unusual to have two that erupt the same year. In fact that's the average, according to a U. S. Geological Survey report posted online. And it's not unheard of to have two volcanoes erupting at the same time, especially since some volcanoes blow off steam and ash for weeks or even months.

Having three volcanoes in the 1,500-mile arc erupting all at once, however, is less likely, said Cervelli, who devised a computer calculation to determine the odds. "It turns out that something like this probably happens once every 100 years."

And it's even more unlikely that three volcanoes would all erupt at the same time within 300 miles of each other, as the three central Aleutian peaks did, more or less, in July and August. "When you take that into account, it might go up to once in a 1,000 years," Cervelli said.

That makes it harder to dismiss the triple-eruption as a chance occurrence. So Cervelli and other geologists -- including co-author Cheryl Cameron, based at the AVO office in Fairbanks -- are exploring other explanations.

One of them focuses on the fact that all three volcanoes lie within the rupture zone of the 1957 "Great Aleutian Earthquake," a magnitude 8.3 tremor that generated a tsunami that damaged buildings in far-off Hawaii. Because there was so much disruption of earth along the quake's fault line, Cervelli said, the ground around it was drastically distorted and compressed.

"That could create conditions that are favorable for eruption," he said. "That's speculative ... but certainly it makes some degree of sense."