Eruptions from A.D. 79 to 1631

Following the A.D. 79 eruption, Vesuvio remained in an active state (an eruptive cycle) for maybe up to 1060 years. Documentation for that period is poor and therefore very little is known about how Vesuvio behaved like in that period. From what little information is available, though, it seems that the patterns of eruptive activity differed considerably from those of the most recent active cycle, 1631-1944:
1) - eruptions occurred at longer intervals;
2) - the eruptions were more explosive;
3) - fairly large eruptions occurred during the cycle (in particular, that of A.D. 472);
4) - if the entire interval between A.D. 79 and 1139 (the date of the last certain eruption before 1631) is taken as one eruptive cycle, then it would be more than three times longer than the following one.

Volcanoes of the World (1994 edition) lists eruptions from Vesuvio on the following dates:

• 172
• 203
• 222 until 235
• 379 until 395
• 5-6 November 472
• 9 November 505
• 8 July 512
• 536
• February(?)-March(?) 685
• 787
• November or December 968
• 1007
• 27 January 1037
• 1049
• 1073±5
• 1-9 June 1139

Questionable eruptions are further listed for 991, 999, 1150, 1270 and 1347. An eruption in 1500, listed as a phreatic event in Volcanoes of the World, is believed by many authors to have been a very minor event, if an eruption at all.

Of these eruptions, only seven unequivocally erupted lava, the first occurring in 968. At least two eruptions produced pyroclastic flows and surges. Average repose periods seem to be considerably longer than during the 1631-1944 cycle, but this may be an enhanced effect of poor reporting in that period. Possible smaller-scale, persistent Strombolian activity from the summit crater (that may have been lying somewhat hidden in a major caldera depression left by the A.D. 79 eruption) may have escaped recording because it did not significantly affect life around the volcano. Two longer eruptive periods reminiscent of 1631-1944 eruptive sub-cycles are reported for 222-235 and 379-395, but details are lacking about the sequence of events during those periods. There is also a notable (but possibly apparent) increase in eruption frequency beginning in 1007, coinciding with better reporting on eruptions in contemporaneous sources.

	Isopach map of the A.D. 472 pumice fall and pyroclastic flow and surge deposits, from Rosi & Santacroce (1983). Click on image for larger version with legend.

According to Rosi & Santacroce (1983), the first phase of the eruption was sub-Plinian to Plinian, producing an airfall deposit of greenish pumice with maximum thicknesses of 2 m on the NNE flank of the volcano. At Avellino, 28 km to the E, this fall deposit is still 26 cm thick. The proportion of lithics increases towards the top. There are two interbedded fine-grained ash layers in this deposit, grading laterally into sandwave beds. During this phase, Vesuvio erupted about 1.6 x 10^8 m^3. The main fallout axis trends ENE from the volcano, similar to most other Plinian and sub-Plinian fall deposits from the volcano.
Like in most other large-scale eruptions of Vesuvio, the initial Plinian phase was followed by eruption column collapse and generation of pyroclastic flows and surges.The denser flows occur mostly in paleovalleys on the NW side of the volcano, decreasing in thickness at the lower flanks and towards the plain surrounding Vesuvio. Maximum thickness is >30 m at Pollena, about 5 km from the present crater. Thicknesses in excess of 10 m were also measured at the same distance on the N flank, in an area that should appear sheltered by the Somma ridge. Rosi & Santacroce (1983) distinguished four flow units, the third of which is peculiar for its abundant dark gray scoria. On top of the fourth unit, there is a horizon interpreted as deriving from a "wet" pyroclastic flow for its muddy, indurated matrix and palagonitized scoria.
Pyroclastic surges reached farther from the volcano, their deposits appearing as far as 12 km from the crater. Rosi & Santacroce (1983) mapped the pyroclastic flows and surges as a slightly bilobate deposit with its maximum extension toward the NNE. It is striking that this is the direction where the Somma ridge is highest. The total volume of pyroclastic flow deposits is 6 x 10^7 m^3. All mapped pyroclastic products of the eruption together yield a volume of 3.2 x 10^8 m^3 (0.32 km^3), equivalent to 1.65 x 10^8 m^3 of dense rock, or 2.4-2.5 x 10^8 m^3 of magma. This volume does not regard any tephra deposited on the W, SW, S and SE flanks of the volcano which, if it exists, is buried beneath the products of more recent eruptions. No lava flows have been correlated with the AD 472 eruption, but historic sources report phenomena that might be interpreted as lava flows.

The notable fact that pyroclastic flows and surges from the AD 472 eruption occur mainly (?) in the N to NE sector of the Somma-Vesuvio volcanic complex has already been mentioned. It is an indication that in the case of violent eruptions with significant collapse of the eruption column, the Somma ridge does not present that much an obstacle to pyroclastic flows and surges, even though in the case of the AD 79 eruption it appears to have diverted similar but much larger flows and surges. A possible factor influencing the capability of the Somma ridge to block and divert pyroclastic flows and surges may be the position of the eruptive vent. In the case of a vent situated at the very base of the Somma ridge, pyroclastic flows would easily spill over that ridge once collapse of the eruption column occurs. However, computer models of a future eruption of a similar size to that of the AD 472 eruption have revealed that in fact, the ridge would essentially fail to act as a protective barrier (see Volcanic Hazards of Vesuvio).

Little is known about the extent of destruction and fatalities caused by the AD 472 eruption. However, historical documents mention ash falls as far as Constantinople. Rosi & Santacroce (1983) believe that the eruption was in fact "extremely disruptive and must be considered the most violent and fatal [Vesuvian eruption] during the last nineteen centuries. A similar eruption occurring today [...] would have apocalyptic consequences."

Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"