The intricate interplay of rocks, gases, and water that occurs inside a volcano as it begins erupting is becoming clearer now as scientists study how magma behaves when intense pressure is suddenly released.

"The ascent of magma can be astonishingly fast" once the huge load of rock that serves as a cap falls away, said volcanologist Donald B. Dingwell. "Decompression is all you need. There's a lot of energy there, and there's a lot more energy in the gas."

Dingwell, speaking Sunday at the AAAS Annual Meeting, heads the Munich Magma Group at the University of Munich in Germany. And he is also secretary general of a new research support organization, the European Research Council.

In a 45-minute lecture on volcano science, Dingwell said that the"gas" inside volcanoes includes huge amounts of compounds like sulfur dioxide, nitrogen species, and water vapor, which remain dissolved in the molten rock as it lies cooking deep inside the Earth's crust. And as a result of recent findings, including data from laboratory experiments, "we now have a lot simpler view about how a volcano works."

Once the lid is off, he explained, the magma comes roaring out, regardless of what else happens.

Although that new view is important toward understanding how volcanoes work, it's still not possible to accurately forecast when a volcano will erupt. Educated guesses based on a given volcano's history of repeated eruptions are possible, but good accuracy is still to be achieved.

"Prediction is a heavy word," he added, "but 'forecasting may, in a few decades, be reasonable."

Dingwell also pointed out that volcanic activity has always been a dominant player in the  history of the Earth, and it's responsible for the creation of life through the recycling of  chemicals it constantly spews onto the land masses and into the air and drops into the seas. Volcanic activity is also evident on the surfaces of other planets, including Mars and Venus. But in both cases the results are different, reflecting the unique physical conditions that have evolved on each planet.

As for volcanism on Earth, Dingwell said that "most people think of it as something that occurs somewhere else," usually far from Europe and the United States. But it's clear that somewhere on this planet there's a volcano going off about once a week. And the activity is constantly adding new chemicals to the planet's surface.

"It's over your head, [and]  it also blows out over the ocean" and sinks to the seafloor, Dingwell explained. "Most of it is (volcanic) glass, which is highly unstable and highly reactive" in the air and water. "It's the source of iron for life in the seas; it's a fundamental nutrient for life." Indeed, he added, "if there were no volcanoes, there would be no life."

What's also evident from geological studies is that some terrifically large volcanic eruptions have occurred in the Earth's long history, including many thousands of years ago. Among the tremendous eruptions are those known to geologists as Tambora, Laki, and Yellowstone. And a more recent event, the eruption of Mount Pinatubo in the Philippines, in 1991, pushed enough dust and gas into the air to block sunlight and cool the Earth's surface, as measured in Europe, for several years.

When it comes to the dangers involved in eruptions, "there are various ways that a volcano can kill you," he said. One of the most lethal is the phenomenon called a pyroclastic flow, in which the huge column of broiling hot gases collapses and comes sweeping down the mountain's flanks. This deadly cloud can move at speeds of 100 to 200 miles an hour, carrying gases and ash as hot as 500 degrees Fahrenheit, and spreading them for miles.

Dingwell also discussed the various types of lava that are known to flow from volcanos, including some that are thick, pasty and almost rock-solid, or those that are runny and flow readily across the ground. The difference lies in how much nitrate is in the lava--the more nitrate, the thicker the goo.

But perhaps such analysis distracts from the big picture. And on that point, Dingwell lets history guide him to a general prediction. "Eventually there will be another big one," he said.  "They seem to recur on a time scale of about half a million years."