**6. The Somma-Vesuvius history after the AD 79 eruption**

The history of ancient stratovolcano that suffered a caldera-like collapse, Somma [8, 10], ended with the AD 472 eruption. In Middle Ages, from AD 500 to AD 1100,

*Forecasting Volcanic Eruptions*

dead*"* (Pliny's letters).

effects of the pumice fall phase tried to return to their homes by walking over the pumice deposits which had become as high as the rooftops. As they made their way home, accompanied by some soldiers, hot toxic cloud swamps departed by the hot avalanche that are invading Herculaneum, and kills them (**Figures 16** and **17**).

October 26 (the day after the eruption): Pliny the Elder's body was found dead on the beach "…when the light returned, which was not till the third day after his death, his body was found untouched by the fire, without any visible hurt, in the dress in which he fell, appearing rather like a person sleeping than like one who was

The destructive action of pyroclastic products on constructions is exerted in two main ways. Fall products, which are emplaced grain by grain, falling down from the eruptive cloud, blanket roofs with a layer of granular deposit, whose thickness is a function of both the intensity of the eruption and the distance from the vent. The pyroclastic fall layer loads the roofs with an extra weight that, when the maximum sustainable load threshold value is exceeded, can cause the roof to collapse. Pyroclastic density currents (PDC), on the other hand, generally originating from the collapse of the eruptive column, are gas clouds rich in both lithic and juvenile fragments that, running over the ground, exert a dynamic overpressure on the obstacles encountered on their way and are capable to destroy the buildings (**Figure 18**) [13]. Actually, the first damage was made by white pumice fall deposits,

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**Figure 18.**

*Effects of AD 79 eruption on Roman buildings: Villa 6 at Terzigno.*

### *Forecasting Volcanic Eruptions*

the mediaeval interplinian period occurred. During this period, the volcano alternates quiescence periods to effusive phases with low energy Strombolian style eruptions. The last reliable historical reports about this long time span concern the effusive activity that took place in 1139. The mediaeval interplinian activity was responsible

#### **Figure 20.**

*Engraving showing Somma-Vesuvius during a repose period before the 1631 eruption.*

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**Figure 22.**

*(a) December 1760 eruption, Fabris. (b) October 20, 1767, Fabris (c) June 1794 eruption.*

*The Somma-Vesuvius Activity with a Focus to the AD 79 Eruption: Hazard and Risk*

for the construction of a new stratovolcano, the Vesuvio, which grew within the Somma caldera [14–19]. Since 1550, many coeval chronicles and the pictures of the Atlas by the cartographer Gerardus Mercator (1512–1594) make it possible to deduce that in this time span the Vesuvius was quiescent and that Gran Cono was covered by a closely cropping vegetation. On December 16, 1631, Vesuvius resumed its activity after 500 years of quiescence period. The eruption, with both effusive and explosive products, was not expected by the inhabitants, who were not aware that Vesuvius was an active volcano (**Figure 20**). Famous label was placed at Portici by Spanish viceroy after the AD 1631 eruption to warn the future generation about the volcanic risk of the Vesuvius (**Figure 21**). From 1631 to 1944, the last interplinian phase occurred. The typical volcanic activity, effusive and weak explosive, was developed according to a succession of volcanic cycle, called vesuvian cycle. Between the end of eighteenth century and the first half of the nineteenth century, in the absence of photographs,

*DOI: http://dx.doi.org/10.5772/intechopen.89989*
