Yes, this is the stuff that exploded and rained down on Pompeii in AD 79.

Geologist James Webster gathered pumice from the slopes of Vesuvius and now is cooking and squeezing the material to the same temperatures and pressures it had in the seething belly of the volcano before the eruption.

Like some diabolical brewmaster, Webster has spiked the mixture with the one original ingredient the pumice was lacking: the volatile gases that had been dissolved in the Vesuvian magma, the gases whose explosive release threw ash into the stratosphere and sent deadly lava racing down the slopes of the volcano.

The piece of magma is far too tiny for the experiment to pose a threat. Still, the juxtaposition with the oblivious people outside is thought-provoking: Some half a billion people worldwide live within 100 kilometres of historically active volcanoes, according to the Cascade Volcano Observatory of the U.S. Geological Survey.

Around the slopes of Vesuvius alone, which erupted as recently as 1944, 2.5 million people live in the city of Naples — one reason Webster's closest collaborator on the work is Benedetto DeVivo of the University of Naples.

This little chemistry experiment is part of a worldwide effort to understand what makes volcanoes — darkly famous names like Mount St. Helens and Pinatubo, Krakatoa and Mount Etna — go bang.

Other researchers are studying the dynamics of the hot, pressurized bubbles that form in the magma before it erupts, much like shaken seltzer or champagne. Still others are analyzing the ominous seismic signals — called long-period oscillations — that ring out from the frothy magma as it moves inside the beast.

The view of a volcano's innards most people carry in their minds looks something like a vertical tube drawing magma from a balloon-like well, which obligingly burbles out through a crater at the top.

But the geologists' new work reveals frothy, viscous masses that crack and groan as they rise through irregular fissures.

The chemistry and dynamics are so complex that the same magma, in the same volcano, can either create a tremendous explosion or ooze forth sedately in an "effusive" eruption — depending on slight differences in how fast the magma rises.

"We all cringe when we see the balloon and straw because that's everybody's view of a volcano," says Steve McNutt, research professor of volcano seismology at the Geophysical Institute of the University of Alaska.

"We think it's quite a bit more complicated than that."

Volcanologists are not ready to say theirs has become a predictive science, like meteorology. But the new science of volcanoes has taken great strides, they say.

They cite the subtle seismic blips that were picked up from Galeras, a volcano in Colombia, before it erupted and killed nine members of a research party at the summit in 1993.

Standing next to a tall cabinet filled with drawers of mineral samples at the museum, volcanologist Charles Mandeville holds two large rocks.

The charcoal-coloured one in his left hand is heavy, jagged and pitted with tiny, round holes.

In his right, is a pale gray, friable, blob of about the same size that feels light enough to float on water.

The rocks are at the centre of a volcanological version of the nature-nurture debate.

"These two samples have identical chemistry," says Mandeville, making them the geologic equivalent of identical twins. And the pieces were drawn from the same magma supply and ejected from the same volcano, Mount Mazama, which exploded and formed Crater Lake in Oregon some 7,600 years ago.

The dark, denser sample, he says, emerged in an effusive event that preceded the cataclysmic eruption by a few weeks or months.

Like sociologists looking for environmental factors to explain criminal behaviour, volcanologists around the world want to understand these striking differences.

They start with a general grasp of how explosive volcanoes form. They are most common around the "Ring of Fire," which follows the Pacific coastline from the western limn of South America, Mexico, the United States and Canada, across the Aleutian Islands and down eastern Russia through Japan, Indonesia and New Zealand.

In those places, the ocean crust, laden with water, is slowly diving beneath the much thicker crust of the continents in a process called subduction.

The ocean crust heats and releases its water, which diffuses upward and soaks into Earth's hot mantle.

Because water alters the chemical structure of the mantle rock and causes its melting point to fall, parts of the rock melt and, like enormous hot-air balloons in slow motion, rise through fissures in the mantle.

As the magma rises, the confining pressure on it drops and the water in it begins to form bubbles. So many bubbles form that they can occupy as much as 90 per cent of the volume of the magma as it moves toward the top of a volcano, says Donald Dingwell, director of environmental sciences at the University of Munich.

And when the volcano erupts, he adds, "the basic explosive energy is coming from the high-pressure gas in the bubbles."

Although volcanologists agree with that general picture, it leaves open the crucial question of why some eruptions are explosive and others — often in the same volcano, as with Mount Mazama — are merely effusive, spitting up magma like overheated oatmeal on a stovetop.

Dingwell notes that, as a volcano's water outgasses and forms bubbles, the magma itself becomes drastically more syrupy.

A drop by a factor of 10 in the amount of water in the magma can cause its viscosity to soar by a factor of 1 million.

The magma can become so viscous that it acts like a solid, plugging its conduit and causing terrific pressure to build.

The bubbles also have a hard time expanding and dissipating in the syrupy mass, meaning that they stay inside the molten rock like bits of dynamite waiting to explode.

If the magma rises slowly enough, the bubbles, like air in honey, have time to expand, coalesce and leak away through fissures in the Earth. A little faster, perhaps because the subterranean forces are slightly greater, and the trapped energy remains inside the magma, now as rigid as glass.

Additional gases rising from fresh magma below can also make the system more unstable and prone to a huge eruption.

The brittleness of the magma may help explain why seismic jolts called long-period oscillations are indicators of an eruption, says Geological Survey volcanologist Bernard Chouet.

The magma can crack as it flows, releasing gases that set off resonances — the oscillations — within cavities and conduits in the mountain. Then, because it is ultimately a liquid, the cracks can close up before the magma ruptures again, producing an insistent signal of disaster.