Mount Vesuvius Significant Threat to Naples Term Paper

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Vesuvius

The eruption of Mt. Vesuvius in 79 a.D. destroyed the city of Pompeii and also nearby Herculaneum. The volcano remains active, though it has not produced much more than steam for some time. However, a volcano is always a potential threat and could erupt at some time in the future. Over the centuries, people have moved back into the area, and today the city of Naples can be considered living under the threat of annihilation of Vesuvius erupts once more. The threat is real, and several times since World War II, the mountain has produced landslides and raise ash dust sufficient to cause concern that an eruption was imminent. Naples has a disaster plan in case of an eruption, one based on the level of damage seen in the eruption in 79 a.D. It also assumes a sufficient period of warning to put the plan into action, but if the eruption is more abrupt and larger than the one in 79 a.D., Naples could be destroyed and the loss of life would be massive.

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What happened in the eruption in 79 a.D. has been much studied and serves as a warning to the people of Naples today. The threat remains, and any plan may be limited by the speed and size of a given eruption: "Evacuation, however, would sometimes present enormous problems. About a million people live in the danger area if Vesuvius were to repeat its great exploit of AD 79" (Scarth, 1994, p. 240). Moving that many people in a relatively short time might be impossible. What happened at Pompeii in 79 a.D. is instructive. The eruption took place in two distinct phases, first a Plinian phase in which material was ejected in a tall column to spread in the atmosphere and fall to earth like rain, and a Pelean phase in which material flowed down the sides of the volcano as fast-moving avalanches of gas and dust, called pyroclastic flow. The pyroclastic flows of the Pelean phase at Pompeii were the reason for the volcanic damage to walls, while the air-fall pumice and ash fall during the Plinian made deposits that collapsed roofs and buried low structures, though also shielding them from the effects of the pyroclastic flow that came next.

Term Paper on Mount Vesuvius Significant Threat to Naples Assignment

It has been estimated that the roofs in Pompeii started to collapse when there was an accumulation of approximately 16 inches of pumice (Sigurdsson, 1985). In such an eruption, the pyroclastic flow is accompanied by an overriding ash cloud of fine material called a pyroclastic surge. The ash cloud (surge) layer separates from the ash-and-block (flow) layer, and the surge layer may separate from the flow layer by climbing hills and traveling greater distances (Fisher, 1995, p. 262).

The eruption over Pompeii started on August 24, and by the next day, all covered buildings in Pompeii were uninhabitable because of collapsed floors and roofs. By then, there would have been a mass exodus from the city. Pompeii had an estimated 20,000 residents, and only about 2,000 have been found in excavations, with most of those found on top of the pumice layer (Sigurdsson, 1985, p. 352). The Plinian phase produced a virtually deserted city of buildings without roofs or floors, setting the stage for the pyroclastic flow of the Pelean phase that began on the morning of August 25. It is clear that the pyroclastic surge and flow events during the Pelean phase inflicted significant structural damage and that the presence of bricks and roof tiles in the surge deposits prove this (Sigurdsson, 1982, p. 50).

Before the eruption, there were warning signs that were ignored or not correctly interpreted. Seven years before, there had been a major earthquake that had destroyed large parts of the city, which were still being rebuilt when the eruption came. A small earthquake earlier in August had shaken the town, and wells had gone dry. The only written report of an eyewitness came from Pliny the Younger, who saw a cloud of unusual size, a cloud that resembled the umbrella pine tree. This cloud was actually a column of hot gas mixed with tons of rock and ash, reaching skyward to about twenty miles. As the column cooled, it rained down in the form of ash, covering Pompeii. Those remaining behind in the city tried to flee and were often encased in ash as they ran, creating death statues that have been excavated from the ruins (Stewart, 2006).

Scientists today keep track of activity in the region and seek to know when an eruption might be imminent. Seismic activity in the region is monitored. One instance came on October 9, 1999 when an earthquake occurred about three kilometers beneath the central cone of Mt. Vesuvius, near Naples, Italy. This event had the highest magnitude recorded for at least 25 years and possibly since the last eruption of the volcano in 1944. It was not accompanied by other geophysical or geochemical changes. Seismological data had been collected at Mt. Vesuvius for 29 years before the October 9 earthquake till the end of 2001, and the data shows the time pattern distribution of seismic slip release. Pezzo, Bianco, and Saccorotti (2002) report on this data and investigate the source process through the scaling law of the seismic spectrum. They find that there is a two-fold pattern of stress release, with high values (up to 100 bar) for earthquakes occurring close to the top of the carbonate basement that underlies the volcano at 2-3 km of depth, and low values (down to 0.1 bar) for the shallow events occurring within the volcanic edifice. They also find reasons why the low-magnitude events substantially contribute to the overall cumulative seismic slip release. Because of the presence of extended aquifers, with their tops at about one kilometer beneath the crater, the authors believe this indicates support for the hypothesis of the triggering of the shallowest events by water-level changes, a hypothesis that is in agreement with the low values of the stress drop measured for the shallowest seismic events. They also find support for the hypothesis that the pre-fractured carbonate basement may be the site of tectonic stress release.

Troise et al. (2007) report that ground deformation data indicates that the Campi Flegrei caldera, near Naples, Italy, is undergoing renewed uplift. This volcanic area had its last eruption in 1538, and it started a new uplift episode in November 2004. This uplift began at a low rate, but since then it has slowly and steadily increased. Previous studies show that the 16th century eruption occurred after decades of uplift coupled with brief periods of subsidence. Over the past forty years, the caldera experienced a huge uplift phase until 1985. The new data shows that a subsequent period of subsidence has now ended, and the ratio of maximum horizontal to vertical displacement, determined from Global Positioning System data, suggests that the uplift is associated with input of magma from a shallow chamber. The authors offer this supposition and expect that future uses of this displacement method will help scientists monitor magma intrusion processes at this and other volcanoes and so help quantify volcanic hazards.

Scientists have recently noted that there is a massive layer of magma five miles below Vesuvius, a layer much larger than expected and extending beneath Vesuvius and neighboring volcanoes, such as the Phlegraean Fields closer to Naples. The researchers suggested monitoring the field for seismic clues about what might happen. The layer covers some 154 square miles, but scientists do not know how thick it may be. Efforts to ascertain its thickness continue (Noble, 2001).

Evidence emerged in the 1970s of an earlier eruption of Vesuvius, called the Avellino eruption and dating to the second millennium B.C. And showing a different sort of eruption that might occur. Unlike the eruptions often seen in which there is an eruption of lava flowing down the side in slow-moving streams, in Avellino-type eruptions, 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. Such an explosion is very violent and propels liquid rock into the air so fast that it breaks the sound barrier, creating a sonic boom. This is called a boato, and in Avellino it accompanied a blast that hurled nearly 100,000 tons a second of superheated rock, cinders, and ash into the stratosphere, reaching an altitude of about 22 miles (35 kilometers). This huge cloud then spread at the top, assuming the classic shape of an umbrella pine tree, the iconic feature of a plinian eruption. The prevailing winds carried the bulk of the initial fallout in a northeasterly direction, causing pumice and lapilli deposits to pile up as high as nine feet near the volcano in several hours. The column of ash may have remained in the air for up to twelve hours before collapsing, producing an apocalyptic sequence of events that makes a plinian eruption… [END OF PREVIEW] . . . READ MORE

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