Skaftafell - Just north of here, on the far side of the impenetrable Vatnajokull ice sheet, lava is spewing from a crack in the earth on the flanks of Bardarbunga, one of Iceland's largest volcanoes. By volcanologists' standards, it is a peaceful eruption, the lava merely spreading across the landscape as gases bubble out of it. For now, those gases - especially sulfur dioxide, which can cause respiratory and other problems - are the main concern, prompting health advisories in the capital, Reykjavik, 150 miles to the west, and elsewhere around the country.

But sometime soon, the top of Bardarbunga, which lies under as much as half a mile of ice, may erupt explosively. That could send plumes of gritty ash into the sky that could shut down air travel across Europe because of the damage the ash can do to jet engines. And it could unleash a torrent of glacial meltwater that could wipe out the only road connecting southern Iceland to the capital. All of that could happen. Then again, it may not.

Such are the mysteries of volcanoes that more than four months after Bardarbunga began erupting, scientists here are still debating what will happen next. The truth is, no one really knows. Volcanic eruptions are among the Earth's most cataclysmic events, and understanding how and when they happen can be crucial to saving lives and reducing damage to infrastructure and other property.

Scientists have several powerful tools to help, but in the end, they are often reduced to analyzing possibilities within possibilities, chains of potential events that could unfold in multiple ways. "Volcanoes are really difficult to predict because they are so nonlinear," said Pall Einarsson, a geophysicist at the University of Iceland. "They can suddenly decide to do something very different." For now, the eruption remains what volcanologists call an effusive one - the lava, consisting primarily of molten basalt, is thin enough that the gases bubble out with little explosive force.

And the amounts of sulfur dioxide and other gases, while a concern locally, are nowhere near the amounts produced by an eruption at a fissure called Laki in the 1780s. In that event, the gases poisoned livestock across Iceland, leading to a famine that killed about a quarter of the country's population and had other effects in Europe and elsewhere. One possibility is that the current eruption will eventually peter out as the source of magma is depleted. "Maybe the most likely scenario is something similar to what we've been seeing," Sigmundsson said.

But that could take a while; although the volume of lava has declined, it has done so only very gradually, he said, suggesting the eruption could continue for many months. But there are many other possibilities. Bardarbunga sits at the heart of a complex system of volcanoes and "has a history of affecting its neighbors," Einarsson said. Were the dike to continue moving to the northeast, he said, it could set off an eruption at the nearby Askja volcano, although that seems less likely. Of greater concern is what is happening at Bardarbunga's caldera, the wide, deep valley at the top of the mountain that is filled with hardened magma from past eruptive activity.

Earthquake data and GPS measurements show that this hardened magma, which acts like a plug, is sinking, probably as the hot magma below it escapes through the fissure to the north. The subsidence is astonishingly rapid, about a foot a day, and the question is how much more of this the plug can take before it breaks up. "As of now, the system seems to be relatively stable," Einarsson said. "But it's almost certain that this can't last very long, and that's what people are worried about. Because this plug is bound to disintegrate as it moves so much."

If the plug cracks apart, the hot magma below would have a new, easier path to the surface - straight up - where it would combine with ice to cause a steam-magma explosion. Such an eruption could create a large plume of ash that could disrupt air travel, as the eruption at another Icelandic volcano did in 2010. Its effects on the surrounding region could be catastrophic as well, with glacial meltwater collecting in the caldera until it overflows, causing a vast flood.

That has happened countless times in Iceland's geological history, and it is what created the eerie skeidararsandur, the vast delta west of Skaftafell that resembles the surface of the moon, as floodwaters brought huge quantities of black volcanic sand down from the mountains. The skeidararsandur could take the brunt of a flood again, although it would depend on precisely where the eruption occurred.

A short distance this way or that, and the floodwaters might flow to the north, or even to the west - an especially troubling possibility given that several hydroelectric dams responsible for much of Iceland's electricity could be damaged or destroyed. "One can never be absolutely certain about predicting," Einarsson said. "So we have to line up all the possible scenarios and stretch our imaginations to figure out what could possibly happen." - Alaskan Dispatch