The 1631 eruption
Compared with the AD 79 eruption, the event of 1631 was of minor proportions regarding eruptive magnitude and erupted volumes but not in terms of destruction and fatalities. Beginning on 16 December 1631 and culminating the day after, it destroyed all towns and villages around the volcano and killed at least 3000 and maybe up to 6000 persons. It was thus the worst volcanic disaster in the Mediterranean during the past 1800 years.
Like the AD 79 and 472 eruptions, the 1631 event appears to have been purely explosive (although Rolandi et al. 1993a state that lava flows did occur in 1631) but was characterized by the emplacement of devastating pyroclastic surges and flows. The eruption occurred after a repose period lasting at least 130 and probably almost 500 years. Very detailed but in part confusing or even misleading information is available about the 1631 eruption from contemporaneous or near-contemporaneous sources. Only very recently (starting in the late 1980's) has there been modern volcanological research on this important event that has significant implications for volcanic hazard assessments.
When it became active again, Vesuvio had not experienced any significant eruptions since 1139; an eruption recorded for the year 1500 may actually have been something like a minor phreatic event, increased fumarolic activity, or a major rockfall. Furthermore, this event has been mentioned only in one source (Ambrosio Leone 1514, cited in Rosi et al. 1993), in a period of which numerous other more or less contemporaneous documents are available.
Before the eruption of 1631, Vesuvio was densely vegetated except at the summit of the active cone which by then had an elevation of about 1187 m, about 100 m less than its present elevation, and 55 m higher than Monte Somma. The crater had a diameter of about 480 m, it was funnel-shaped and had sa few fumaroles on the rim and in its deepest part. Some contemporaneous sources mention that small ponds were present in the crater, but they probably existed on the caldera floor rather than within the active crater.
During the 24 hours before the eruption, earthquakes were felt more and more frequently. The population must have become extremely uneasy, but there was no major exodus from the area.
Chronology of the eruption
The eruption rapidly gained in vigor as more vents opened on the flanks of the cone ejecting pyroclastics at an ever increasing mass eruption rate. Shortly after the beginning of the eruption, a large eruption column rose up, attaining the famous shape of a pine tree already described from the AD 79 eruption. The height of the eruption column at this stage exceeded 20 km and may have reached up to 28 km, thus the eruption was Plinian. Ash began to fall around the volcano about one hour after the onset of the activity, but heavy block and scoria fall began at about 1000 in the direction of Ottaviano (NE flank of Monte Somma), a village that later was to suffer from numerous other eruptions of Vesuvio.
Also during the forenoon of 16 December, a continuous tremor began to be felt in Napoli, it did not cease until 8-10 hours later. Darkness fell over the area around the volcano and reached Napoli at 1600 on that fatal day.
The main portion of the eruptive plume was however blown towards E, causing darkness and tephra falls over southern Italy as well as over the Balkan. Slight asfalls are reported to have occurred as far as Constantinople, W Turkey, about 1250 km from the volcano. Detailed contemporaneous descriptions of the ash fall from various locations are cited in Rosi et al. (1993). The proximal maximun thickness of the initial pumice deposit is 1.5 m at Canale dell'Arena.
After the initial Plinian
phase, between 1900 and 2200 on 16 December, the eruption assumed a pulsating
character. This was accompanied by strongly increased seismicity. During
the night of 16-17 December, strong earth shocks occurred at intervals
lasting 1-15 minutes.
During the morning and early forenoon of 17 December, the activity fluctuated with occasinal surges of sub-Plinian to Plinian activity that caused tephra falls around the volcano. As day broke on the 17th, the summit of the volcano was seen to have already been partially destroyed by the activity.
At about 1100, after a strong and continuous series of earthquakes, a large mass of ash, gas and stones shot out of the crater and spilled down on all sides of the erupting cone, covering it almost completely. Contemporary reports speak of the apparent disintegration, or liquefaction, of the mountain. The swift movement of the material resembled to many the flow of water, thus many sources actually speak of water that came from the volcano at that stage. In fact, the phenomenon was again, like in the AD 79 and 472 eruptions, pyroclastic flows.
The emplacement of the pyroclastic flows was accompanied (not followed) by a retreat of the sea around all of the Gulf of Napoli, amounting to 3-4 m, and in places to 6 m. Some 10 minutes after, a tsunami with runup heights of 2-5 m hit the shores.
After reaching the base of the cone, the pyroclastic flows sped further downward along the main ravines and valleys, destroying all vegetation and buildings in their path and killing all living beings who did not manage to escape. Rosi et al. (1993) distinguished at least trhee major and several smaller pyroclastic flow lobes, the largest on the W and S sides of the volcano where they entered the sea in 7 or 8 locations. Numerous successive flows cascaded down in that area, obliterating the towns of Torre Annunziata (7.5 km S of the crater) and Torre del Greco (6.5 km from the crater) along with numerous smaller villages and settlements. Interestingly, only slightly elevated areas were often spared from the lethal flows, such as the 185 m high Camaldoli cone with its Monastry, but even garden walls served to divert the hot avalanches in some cases. Evidently, the flows were very dense and did not carry significant hot clouds ("nuèes ardentes") above them near the coast. Their deposits remained very hot for many days after the eruption.
Where the pyroclastic flows entered the sea, small headlands were formed, extending up to 570 m from the former coast.
This culminating phase of the eruption may have lasted for several hours, then there was a marked decrease in the strength of the activity. At 1800 on 17 December, light colored clouds rose from the crater and ash fall continued around the volcano. This activity continued with declining intensity for several days.
When the main phase of the eruption was over, at least 3000 and maybe up to 6000 people were dead. In a daring manoeuvre, rescue teams saved thousands of survivors on 19 December who had been lucky to stay in areas spared by the pyroclastic flows. Besides the towns destroyed or damaged by the pyroclastic flows, other towns were severely stricken by heavy tephra falls, mostly on the NE and E sides of the volcano.
The volume of tephra emitted during the 1631 eruption is given by Rosi et al. (1993) as between 7 x 107 m3 and 2.1 x 108 m3 for airfall tephra alone while these authors give no figure for pyroclastic flow deposits. Rolandi et al. (1993a) computed about 1.1 km3 (corresponding to 5.5 x 108 m3 of magma) for the combined 1631 pyroclastic deposits.
The cone of Vesuvio underwent dramatic morphological changes. While some sources give the reduction in height of the summit as only 168 m, recent reconstructions by Nazzaro (1989) and Rosi et al. (1993) arrive at the conclusion that the cone lost at least 450 m in height. The crater left by the event is estimated to have had a diameter of more than 1.6 km. The mechanisms of destruction of the summit are not well constrained but may have been similar to those observed and vividly described by Perret (1924). He was witness of the 1906 eruption of the volcano that destroyed a smaller portion of the summit. That event occurred at the climax of the eruption and was described by Perret as resembling "the falling of the petals of a flower" in all directions.
Lava flows in the 1631 eruption?
Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"
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