When such a column falls upon itself, it creates a pyroclastic surge, meaning a boiling, turbulent avalanche of debris that shoots out sideways from the slopes of the volcano. This cloud can travel for many miles (Hall, 2007).
The material in a pyroclastic surge is baked in a subterranean magma chamber to temperatures of up to 1650°F (899°C), making the initial surge instantly lethal. This produces a hot, choking wind that can advance at about 240 miles per hour, reaching temperatures of at least 900°F (482°C). Under 200 degrees Fahrenheit (93°C), a person can survive for several seconds if the wave passes quickly, but the person would then suffocate on the fine powder in the air. The entire countryside surrounding Vesuvius was covered by several feet of this powder, 65 feet deep at a distance of three miles from the crater to about ten inches thick at a distance of 15 miles (Hall, 2007). If there is a massive eruption at Vesuvius in the near future, scientists know what to expect:
The testimony derived from geological studies also indicates the kind of activity that usually prevails, and therefore what materials may be expelled. Obviously the behavior of a cinder cone eruption is much easier to forecast than that of a stratovolcano, and the effects of lava-flows may be assessed far more accurately than those of Plinian ashfalls (Scarth, 1994, p. 239).
In addition, scientists will be able to tell how much damage may result once they know the type of eruption involved:
Different types of activity present different potential threats. Fast-moving, sudden, ground-hugging or wide-spreading products of eruptions, such as ashfalls, jkulhlaups, lahars, toxic gases or nuees ardentes, are obviously the most dangerous and destructive. A nuee ardente killed 28000 people in St. Pierre in two minutes, the inhabitants of Nyos village were gassed without warning, and Pompeii was buried in a day, but the lava-flow that erupted in 1669 on the flanks of Etna took five weeks to travel 12km to the walls of Catania (Scarth, 1994, p. 240).
Nadeau (2006) notes growing concerns that Vesuvius might erupt and destroy Naples, though she also finds that the people in the region remain largely complacent in the face of this threat. In truth, the people might not be able to evacuate the mountain itself in an emergency, especially when there is snow on the sides. Authorities hope to have a warning 27-days in advance, and they believe that they can get the people off the mountain in a week However, there is no certainty that scientists can make a prediction that far in advance. Nadeau (2006) notes that no Italian volcano has erupted in the last few decades without some sort of pre-eruptive seismic activity. In some cases, this activity served as a warning, while in other cases, the signs were missed. As Nadeau writes,
If Mount Vesuvius were to erupt as quickly as Mount Etna, it would put Naples, a city of 1 million, in danger. Right now the official plan to evacuate Naples is based on an eruption that occurred in 1631, which didn't even come near the city. Officials fear that most Naples residents would take off in their cars northbound on the A1 highway, the so called Autostrada del Sole, where traffic, even in the best of times, is often bumper to bumper (Nadeau, 2006).
It is clear that Naples is threatened by the possibility of a large eruption from Vesuvius, and it may not be possible to forecast such an event in time to evacuate the region, including the city. The plans that are in place are insufficient for doing so, and one problem is that the people and the government have not faced up to the real possibility of the complete destruction an eruption could cause. A much more effective plan must be developed and implemented to assure that the public can be moved out in time.
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