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Vesuvius
SEPTEMBER 2007
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Vesuvius
By Stephan S. Hall Photographs by Robert Clark

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Despite the loss of these sites, Mastrolorenzo, Petrone, Pappalardo, and American volcanologist Michael Sheridan triggered worldwide fascination when they summarized these findings in the spring of 2006 in the Proceedings of the National Academy of Sciences (PNAS). But their research went beyond mere archaeological documentation. The Avellino event, they wrote, "caused a social-demographic collapse and the abandonment of the entire area for centuries." The new findings, along with computer models, show that an Avellino-size eruption would unleash a concentric wave of destruction that could devastate Naples and much of its surroundings. In the world before Hurricane Katrina and the Indian Ocean tsunami, these warnings might have sounded as remote and transitory as those prehistoric footsteps. Not anymore.


THERE ARE MANY ways for a human being to die after a volcano erupts, and a blast like the one Vesuvius unleashed in 1780 B.C. provides a grim inventory of almost all of them. "In the first hours of the Avellino eruption, material like this fell," Mastrolorenzo explained, dropping two transparent bags of volcanic material on the desk in his office at the Osservatorio Vesuviano. One of the bags contained a fine white powder, the ash that blanketed the fallout zone; the other was full of small rocks, no more than an inch or two in diameter. Some of the rocks were pumice, pebbles honeycombed with bubbles and nearly as light as Ping-Pong balls; others were dense and hard. "These are lighter than water; they float," he said, picking up a piece of pumice. "But these," he continued, picking up one of the harder rocks, called lapilli, "these were falling at about 90 miles (145 kilometers) per hour."

The first hint of the Avellino eruption of Vesuvius emerged in the early 1970s when volcanologists identified pumice deposits underneath the A.D. 79 residues. But in recent years Mastrolorenzo, Pappalardo, and their colleagues, analyzing everything from meters-thick ash deposits visible in road cuts to micron-thin slices of volcanic crystals viewed in a scanning electron microscope, have reconstructed the Avellino event in harrowing detail.

Some eruptions ooze lava in picturesque, slow-moving streams. But in an event like Avellino, the conduit of the volcano is so tightly corked by solid rock that it takes an enormous amount of pressure building up from below, in the magma chamber, to blow a hole to the surface. When it does, the violence of the explosion—the boato, Italian for the enormous roar—propels liquid rock into the air so fast that it breaks the sound barrier, unleashing a sonic boom. During the Avellino eruption, the boato accompanied a blast that hurled nearly 100,000 tons a second of superheated rock, cinders, and ash into the stratosphere. It reached an altitude of about 22 miles (35 kilometers)—roughly three times the cruising altitude of commercial airliners. As this incredible cloud of material rose, it spread at the top, assuming the classic shape—classic ever since Pliny the Younger first described it in a letter to the Roman historian Tacitus about the later eruption that buried Pompeii—of an umbrella pine tree, the iconic feature of a plinian eruption.

Prevailing winds out of the west carried the bulk of the initial fallout in a northeasterly direction, toward Nola and Avellino, where pumice and lapilli deposits piled up as high as nine feet (three meters) near the volcano in several hours. The column of ash may have hovered in the air for up to 12 hours. Then it collapsed, producing an apocalyptic sequence of events that makes a plinian eruption one of the most lethal natural disasters on Earth.

When a plinian column falls upon itself, it creates a pyroclastic surge—a boiling, turbulent avalanche of debris that shoots out sideways from the slopes of a volcano. This searing cloud can travel for many miles, initially at great speed. Not too many humans have seen (much less survived) a pyroclastic surge at close quarters, but many of us have an image of its horrifying power burned into our memories: It shares many physical properties with the huge clouds of powder and ash produced by the collapse of the World Trade Center towers in 2001.

Unlike the collapsed towers, the material in a pyroclastic surge is baked in a subterranean magma chamber to temperatures of up to 1650°F (899°C). The initial surge of the Avellino eruption, especially in the zones closest to Vesuvius, was instantly lethal. Hot, choking wind, advancing at about 240 miles (386 kilometers) an hour, reached temperatures of at least 900°F (482°C), and retained enough heat to bring water to a boil ten miles (16 kilometers) from the vent. "Below 200 degrees Fahrenheit (93°C), you can survive for several seconds, perhaps, if the wave passes quickly," Mastrolorenzo pointed out. "But even if you survive the temperature, you will suffocate on the fine powder in the air. The entire countryside surrounding Vesuvius was covered by foot upon foot of this powder, 65 feet (20 meters) deep at a distance of three miles (five kilometers) from the crater to about ten inches (25 centimeters) thick at a distance of 15 miles (24 kilometers). Eight inches (20 centimeters) of ash is enough to cause modern roofs to collapse."

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