More than 120 years after it became a household name by blowing itself to smithereens, Krakatau (a.k.a. "Krakatoa") and its ever-growing replacement, Anak Krakatau, have become persistent sources of scientific discovery.

To geologists and biologists, the new volcano is a rare window into Earth's mantle and an ongoing natural experiment in catastrophe and resurrection. To tsunami scientists and climate researchers, the 1883 blast is revealing how all killer waves are not created equally and just how much volcanoes effect global climate.

DOUBLE DANGER ZONE

When Krakatau exploded in 1883, scientists of the day were at a disadvantage. First of all, they did not see it explode. They lacked a theory that was up to the task of explaining the disaster. Naturally, they could have done nothing to prevent the death of tens of thousands on nearby islands.

Twelve decades later, that's all changed. Take what's happening underground, for instance. Seismic imaging and geochemical analysis of lava rocks have been used to peer into crust under the volcano and find their source.

"There's a pretty vigorous magma source under Krakatau," said geologist and Krakatau specialist Charles Mandeville of the American Museum of Natural History. Magma (molten rock) is plentiful under the Sunda Strait, which separates Sumatra from Java, because of a rare double dose of volcanism, he says.

"The Sunda Strait is kind of a rift zone," said Mandeville. The movement of large tracts of crust along regional faults in the region has thinned and lowered the crust at the Sunda Strait, creating a rift zone.

Deep, hot rocks in the crust there feel the rifting as decompression, which can cause the rocks to melt into magma. That magma usually buoys upward and erupts as volcanoes — in this case, Krakatau.

The second source of Krakatau's magma is one of the world's largest tectonic plate collision zones nearby. To the south of Sunda Strait, the Australian Plate is being shoved beneath the Eurasian Plate.

As the "subducted" Australian Plate is pushed deeper into the earth under Sumatra, Java and the Sunda Strait, it's baked by heat from Earth's mantle. Seawater trapped in that rock boils free and percolates upward. When the water reaches hot rocks of the right pressure — which are more easily found in the Sunda Strait rift zone — the water helps the rocks to melt into magma, Mandeville explains.

In ancient times, "plumes" of Krakatau's magma slowly moved upward, like bubbles in nearly frozen honey, and eventually made a gaping, four-mile-wide by three-mile-deep caldera "supervolcano," of which Anak Krakatau is just a little piece.

"Anak Krakatau is essentially growing as we speak on the periphery of the [ancient] caldera," said Mandeville. Still, the magma Anak spews out is important to science because it has taken the express train from the deep earth, he says. "It's a very, very intriguing place to study because it's a window into the mantle."

SHORT-ARMED TSUNAMI

It's also a window into disaster. When the 1883 eruption flung more than three cubic miles of magma over an area of almost 10 square miles, it also created a very unusual tsunami that killed more than 36,000 people.

"Krakatau was a classic case of a not very big volcano making an awfully big mess," said tsunami modeler Charles Mader, formerly of Los Alamos National Laboratory and author of a report on the latest numerical model of the Krakatau explosion in the journal Science of Tsunami Hazards.

According to the model, the gaseous explosion of water being instantly superheated by magma in 1883 generated air pressures 60,000 times normal sea level air pressure, said Mader. That's probably why Krakatau's blast was detected by air pressure gauges around the world.

The tsunami unleashed by Krakatau's "hydrovolcanic" blast was a strange, abbreviated sort — not at all like wide, undulating monsters created by earthquakes, said Mader. According to the model, the Krakatau tsunami was more like the kind that would be created by a meteor impact or a submarine nuclear blast, he said.

"It's a short-period wave," Mader explained. It starts big and mean looking, "but it doesn't get very far." That's good news for people who were worried about asteroids or enemies making man-made tsunamis with bombs. For the inhabitants of the narrow Sunda Strait in 1883, however, the tsunami didn't need to travel far to find tens of thousands of victims.

SUPERSONIC HEAT BLAST

When the magma hit the seawater, as modeled by Mader, the seawater flashed violently into steam — a gigantic, hotter version of what happens when water hits a very hot skillet. The resulting supersonic heat blast pounded nearby settlements.

Another aspect of the eruption that might have laid waste to nearby islands was speeding flows of superhot gases and frothy molten rock, called pyroclastic flows. These may have raced atop the water of the Sunda Strait to the nearby islands.

One German researcher has even done tank experiments of simulated pyroclastic flows over water — showing how they can shoot quite a few miles over water before mixing and dying out, says Mandeville. Similar flows over water have been witnessed at Monserrat in the Caribbean, he said.

LIFE RETURNS

All that destruction hasn't been entirely in vain. Anak Krakatau, which is coughing and sputtering higher and higher all the time, is a gold mine to biologists wanting to learn how life recovers from utter annihilation.

"It's been periodically denuded and repopulated," said University of Washington biologist John Edwards of Anak's ongoing eruptions. Among the surprises discovered by scientists: plants did not return to Krakatau first.

"It was the spiders that came first," said Edwards. They blew in on silk sails from other islands. The spiders served essentially as compost, adding nutrients to the nitrate-poor volcanic soil, which is a lot more suitable for plants. The same pattern of animal and plant return has been seen, with local variations, at Mount St. Helens, and today there is quite a bit of plant life on Anak Krakatau, he said.

KRAKATAU VS. GLOBAL WARMING

There are even hints about the future in Krakatau's past. Climate modelers at Lawrence Livermore National Laboratory compared the global cooling effects of the 1883 eruption to that of the comparable Mount Pinatubo eruption in 1991.

The particles lofted high into the atmosphere from both eruptions filtered sunlight and cooled ocean surface waters. Oddly, however, Krakatau's cooling effect lasted for decades and even slowed global warming.

Pinatubo's cooling effect, on the other hand, faded very fast. The researchers suspect that because late 20th-century ocean waters were a lot warmer than in the 1880s, the cooling effects of Pinatubo had a lot more heat to counteract than did Krakatau, and it just couldn't do it for very long.

So, in effect, Krakatau may be the last volcano to deliver a global climate effect for centuries to come. Perhaps it's a fitting motto for the scientific legacy of this small violent island: She always delivers.