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volcanic eruption
In December 1631, Mount Vesuvius, a major stratovolcano in Southern Italy, erupted. The eruption resulted in at least 4,000 fatalities, making it the deadliest eruption of Vesuvius since 79 AD.
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In December 1631, Mount Vesuvius, a major stratovolcano in Southern Italy, erupted. The eruption resulted in at least 4,000 fatalities, making it the deadliest eruption of Vesuvius since 79 AD.
Before it became active again in 1631, Vesuvius had remained for about five centuries in a state of quiescence with its last significant eruption since 1169. A minor eruption was recorded in 1500 by a singular source from Ambrosio Leone, however this event was likely caused due to a phreatic event, increased fumarolic activity, or major rockfall.
Prior to the eruption, Mount Vesuvius was lush with dense vegetation. The lower slopes were lined with vineyards and orchards, while higher elevations housed groups of oak and chestnut trees. Inside the crater, forests thrived along with three lakes from which pasturing herds drank from, however they would ultimately disappear due to rising temperatures as Vesuvius began its erupting process.
As early as August 1631, visible signs of Vesuvius' reawakening could be seen on the North flank of the Vesuvian cone, where sightings of increased fumarolic activity and nocturnal incandescence were reported. Increased local seismicity would also begin to be perceived starting in December 1631 with the strongest tremors being felt as far away as the Italian city of Napoli, culminating in a grade VII MCS shake on 15 December, followed by an intense earthquake swarm.
Several unusual phenomena preceded the eruption, including acoustic disturbances, ground deformations, gas emissions, and changes in groundwater. From October to December, locals reported rumbling sounds and noises resembling thunder or rockfalls, particularly around Portici and Resina. These noises, likely indicative of volcanic unrest, were most prominent in the western sector of the volcano. Ground deformations, including landslides and soil fracturing, were also observed, particularly on the eastern and western slopes of the mountain. These shifts in the landscape were signs of internal pressure building as magma moved toward the surface. Local vegetation showed signs of distress, with drooping plants and reports of animal disappearances, likely due to the emission of toxic volcanic gases.
In addition to these signs, groundwater around the volcano showed unusual changes. Wells and springs began to produce muddy water, with some sources turning saline, indicating a disturbance in the underlying hydrogeological system. The most dramatic precursor was the intensifying seismic activity. Beginning with minor tremors in late November, the seismic activity escalated by 15 December, with stronger quakes and the eventual opening of an eruptive fissure on the morning of 16 December. These multiple signs of volcanic unrest clearly indicated that the eruption was imminent and provided critical insights into the complex behavior of Vesuvius before it erupted.
The eruption began on 16 December 1631, with the opening of a fissure vent on the western side of Vesuvius, close to the base of the volcanic cone. The explosion caused a giant eruption column, which, pushed by high pressure inside the volcano, reached about 13 km altitude. According to William Hamilton "Giulio Cesare Braccini measured with a quadrant the elevation of the mass of clouds that was formed over Vesuvius during the eruption, and found it to exceed thirty miles in height". The eruption column lasted around 8 hours. Fallout of vesiculated lapilli and lithic clasts occurred until the evening. During the night between 16 and 17 December the volcano produced discrete explosions accompanied by lapilli relapses.
In the morning of 17 December a violent earthquake occurred, lasting a few minutes. Modern volcanologists have used descriptions from the event to estimate a magnitude of between 4 and 5. The earthquake was immediately followed by the most violent phase of the eruption, characterized by pyroclastic flow activity. The Volcanic Explosivity Index was VEI-5, and it was a Plinian eruption that buried many villages under the resulting lava flows. Large quantities of ashes and dust were ejected and several streams of molten lava poured out of the crater and down the sides of the mountain, overwhelming several villages, including Torre Annunziata, Torre del Greco and Ercolano. Torrents of lahar were also created, adding to the devastation.
As part of the Campanian volcanic arc, the 1631 Vesuvius eruption occurred over the subduction zone formed by the collision between the African and Eurasian tectonic plates, where the oceanic African plate sinks beneath the continental Eurasian plate. In the lower portion of the African plate, geophysical studies reveal a unique tectonic feature: a slab window, created as the lower portion of the subducting African plate tore and detached from the upper portion. This gap in the plate enables upwelling convection currents to interact with enriched mantle materials from the subducted slab, resulting in Vesuvius' distinct volcanic characteristics compared to the other volcanoes in the Campanian arc.
The slab window formed due to differential subduction velocities along the plate boundary, causing stress-induced tearing. This feature is bounded by two faults that accommodate horizontal movement between adjoining sections of the subduction zone. Arc-related magmatism also played a significant role in the lithospheric detachment, with the interaction of convection currents and varying subduction dynamics believed to have driven the slab tear. These processes have distinctly influenced the lithology of Vesuvius.
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The 1631 eruption of Mount Vesuvius produced a variety of volcanic materials, including tephra falls, pyroclastic flows, and lava flows, each with distinctive characteristics that have been extensively studied. These deposits provide critical insights into the dynamics of the eruption.
The tephra fall deposits from the 1631 eruption are divided into three distinct layers, each reflecting changes in eruptive style and magma composition:
Granulometric analyses reveal a systematic increase in grain size from Layer 1 to Layer 3, with a subsequent decrease in the finer-grained topmost portion (Layer 3').
Pyroclastic flows were a significant component of the 1631 eruption, depositing thick, poorly sorted layers in the proximal areas. These deposits, characterized by lithic fragments and scoriaceous pumice, show evidence of high-energy emplacement. Surge deposits, found in localized areas, consist of well-sorted ash and pumice, indicating turbulent flow conditions.
Lava effusions occurred during the later stages of the eruption, emerging from fractures on the southern slopes of the volcano. The lava flows are predominantly tephrite-phonolite in composition, containing minerals such as leucite, augite, and plagioclase. These flows extended towards the coastline, contributing to the reshaping of the landscape.
The 1631 eruption of Mount Vesuvius has been analyzed quantitatively to estimate parameters such as eruption column height, mass discharge rates, and the physical properties of the deposits. These analyses provide a clearer picture of the eruption's scale and dynamics.
The height of the eruption column was reconstructed using historical accounts and lithic isopleth maps. The column reached its maximum height of approximately 28 kilometers during the eruption's peak phase. The eruption was characterized by three distinct phases, each associated with different column heights and discharge rates:
The variations in column height and discharge rates reflect changes in eruptive dynamics, including transitions from phreatomagmatic to magmatic activity.
Detailed granulometric studies of the tephra deposits revealed trends in grain size and composition:
Chemical analyses showed a progression from phonotephritic to tephrite-phonolitic magma, with the upper layers of the deposit containing denser, less vesiculated pumice.
Using the dispersal and fragmentation indices, the deposits were classified into different eruptive styles:
These indices also demonstrate the influence of wind and column dynamics on the dispersal of tephra, with fallout predominantly affecting the northeast sector of the volcano.
The 1631 eruption was considered to be of minor proportions regarding its eruptive magnitude and erupted volumes compared to the AD 79 eruption. However, there was a lot of damage that the region sustained due to the eruption. It is estimated that around 4,000 people were killed by the eruption, making it the highest death toll for a volcanic disaster in the Mediterranean in the last 1800 years. Pyroclastic flows, ash falls, and toxic gases caused a majority of the deaths in nearby villages, including Torre del Greco, as many people ended up being suffocated or buried under debris. The list of casualties might have been higher still, had it not been for a rescue mission conducted three days later to save stranded villagers.
In addition, the infrastructure of the region was severely damaged by several factors. Fast-moving pyroclastic flows and lahars not only swarmed and killed villagers but quickly destroyed many buildings as well. Many buildings were also weakened by seismic activity occurring throughout the eruption process, and eventually collapsed under the weight of accumulated ash or were wiped out by succeeding lava flows. The town of Naples was not directly damaged by the eruption, but food supply from the Vesuvius neighborhood was interrupted. Ash falls contributed to the destruction of farmlands, killing plant life and causing land infertility on a large scale. The accumulation of ash and debris also blocked ports, which limited the trade that Naples had once thrived from. The loss of trade coupled with the loss of agriculture not only stopped the food supply but ruined the economy as well.
Additionally, many townsfolk living in towns surrounding Vesuvius had lost their homes and their livelihoods. Seeking a way to rebuild their lives, over 20,000 refugees rushed into the town of Naples, the heart of the kingdom. This created problems of public order that worsened the existing issues that plagued Naples.
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