The huge mudflow that buried the city of Herculaneum at the foot of Vesuvius in 79 A.D.
still today displays peculiar geological features which make identification a certainty even after so much time. Involved are consolidated volcanic ashes and welded blocks which were already hard as cement a few months after the event. Digging through the hardened ashes the remains of the victims and objects that belonged to the community were found. Nearly everything from small everyday items to children’s toys was conserved due to the relatively low temperature of the flow (not as hot as molten lava).
Here a city with its dead was completely covered over with mud originated from Vesuvius’ volcanic ashes, standing witness to a civilisation. In different contexts the same sort of tales have been handed down: volcanic mudflows (whether freshly erupted or as “fossils”) are among the most frequent and destructive phenomena on Earth.
The explosive eruptions – which in Italy are typical of Vesuvius, of the Campi Flegrei and of Vulcano, and which are possible in limited form at Etna and Stromboli as well – leave sometimes thick layers of ashes and scoriae along the slopes of the volcano and in the surrounding area. By its very nature volcanic matter is unstable and not very consistent, meaning that it is easily set in motion in the form of mudflows. This is especially true for naturally rainy climates, but they can also be triggered where rains are fed by just the steam vented during eruptive activity. If the rains then exceed the volcanic terrains’ ability to absorb it, these will melt and slide down below in an avalanche of water, mud and detritus. Mudflows are a terrible danger, so much so that they take more lives than any other cause associated with volcanoes, the main reason being that they can strike tens of kilometres away from the volcano itself. No one in the area is out of danger in light of lahar speed, which is over 40 metres per second down the sides of the volcano and from 5 to 15 metres per second even as far as 50 kilometres away. All ashes deposited around Vesuvius have been thoroughly re-elaborated and removed numerous times after being first deposited at the time of the original eruption. Beginning in the 1920s up to the present rainwater has always played a decisive role in these later movements, even though these days ever less water seems to suffice to “activate” ever greater chunks of terrain. A mudflow is difficult to predict; it may occur at the time of eruption, a few days later (such as at Herculaneum), or even after weeks or months. In the case of the Campania Region where Vesuvius is located flows of ashes generated in 79 A.D. still occur thousands of years after the eruption, affecting even far distant areas. Those nearby engaged in agriculture are well aware of this: the age-old practice here is to build dry walls to defend vineyards from the disastrous black torrents with the idea of diverting rather than attempting to contain them which would prove impossible.
The powerful 1631 eruption caused major mudflows which reached all the way to the San Giorgio a Cremano locale, and obviously Portici and Resina as well. Some of the flows were at least partially diverted by the hand of man, giving rise to interminable property disputes.
It bears mentioning that the term “lava” in Neapolitan dialect was not used to indicate, strictly speaking, a flow of non-gaseous magma over the Earth's surface, but rather the flow of torrents of mixed water and mud down the slopes of Vesuvius following the eruption of ashes. Moreover, there is a distinction made peculiar to the Naples area between hot lava (magma) and cold lava (lahar); for centuries a Naples district has been known as “lavinaio”.
Many of the survivers of the 1998 mud flows a few kilometres from Vesuvius in Sarno were heard to shout “Lava, lava”, as they raced through the streets. As confirmed by the reconstruction of events in Sarno, the mechanism is almost invariably the same: a blanketing layer of matter covering carbonaceous mountain ridges and often comprised of various types of detritus in addition to ashes altered by the pedogenic process, gets crumbled and eaten into by streets and houses, so that with the first rains it gives way in various points and rushes downhill, including in deep gullies previously considered dry for ever so long. Involved here is a type of flow that is especially rapid which sets in motion material already affected on many previous occasions. In brief, Vesuvius’ ashes have never come to rest once and for all, even though deposited in a coating on elevations around Naples thousands of years ago.
The principal cause lies in the particular geological formation – subjection to pressure carbonatic pressure from low due to contact between a lightly compacted pyroclastic top layer and the carbonous substratum, as geologists would say. In other words, intense pressure generated by rainwater just below the surface causes detachment from underlying limestone and rapid avalanche of volcanic soils. This separation generally comes about due to siphoning caused by excess infiltrated rainwater “regurgitated” by the limestone substratum once having reached the saturation point, thus causing the slide. Limestone is a type of rock which acts as a sponge in the true sense of the word, primarily because it is shot through with a myriad of microscopic fractures which allow water to deeply penetrate. Technically speaking, then, the triggering cause of such slides is pressure exerted at the interface between surfaces different in composition. We are dealing here with true and proper landslides, cave-ins and, above all, mudflows, which involve ancient volcanic ash changed into soil and the detritus of various nature deposited in contact with the carbonous ‘skeleton’ (the substratum). These flows move at high speed (3-20 metres per second) and come without signs of warning in the medium term. But another important cause – decisive in recent years – is anthropic change that has taken place on the hillsides. The increased number of buildings increase risk, and indeed cause new risk. Moreover, fires, uncontrolled deforestation, the flight from the countryside, the replacement of traditional crops by others with higher yield but less suited to the environment and new road construction all join together to worsen the situation day by day. Two thousand years of history, or better, of dealing with lahars seems not to have taught much. Perhaps it is an acceptable price for those who enjoy the privilege of dwelling on the hills of the gods.