Notes on Ancient Reconstruction of the Pompeii Forum
Preliminary Findings of Geologic and Structural Review
John Dobbins, Tanya Furman, Kirk Martini, Tom Baber
It is a work in progress.
Physiography and Eruptive History of Somma-Vesuvius
The modern volcano is a composite of two major cones, with the younger Vesuvius nested within the remains of the Somma caldera. Mt. Somma "blew its top" in a cataclysmic eruption roughly 17,000 years ago that far exceeded all events associated with the younger volcano. The remains of Mt. Somma are manifest in the large base of the volcano, and also in the arcuate rim preserved on the northern side of the modern volcano (Sigurdsson et al 1985).
In addition to the explosive eruption that decimated Pompeii and Herculaneum, Vesuvius had a major event in 1631 that destroyed the region between Torre del Greco and Torre Annunziata (and surrounding communities; figure 1). This event removed roughly 500 meters from the top of the volcano, leaving the modern summit crater.
Thus, while the 79AD eruption was extremely damaging, it is a minor example of the available strength of the volcanic system. Recent models of pyroclastic flows from Vesuvius indicate that modest eruptive activity (i.e., on the scale of the known modern events) could cause complete destruction within a radius of 7 km in less than 15 minutes. The actual destruction is dependent upon the prevailing winds, the direction of the blast, and the possible shielding effects of the Somma rim, but the pattern of behavior is clear (Dobran et al 1994; figure 2).
Mechanics of the 79AD Event
The eruption of 79AD was characterized by extensive pumice-fall (roughly 2.5 meters in Pompeii), followed by a series of six pyroclastic surge deposits of varying strength. The height of the eruptive column was 15-26 km during eruption of the white pumice (lower layer), and rose to a maximum of 32 km during eruption of the grey pumice (upper layer; figure 3). Pumiceous material was ejected from the volcano at speeds between 325 m/sec (white pumice) and 400 m/sec (grey pumice).
During an explosive volcanic event, most particles are too small to be ejected from the summit along ballistic trajectories. They are instead carried high into the eruptive column by convection. Individual particles ascend to a height where their convective ascent velocity is in equilibrium with their gravitational settling velocity (i.e., they reach a stable height, and spread out laterally in the atmosphere). Convecting columns of this nature result in pumice fallout, as with the first day of the 79AD eruption. Dangerous and destructive pyroclastic surges (nuee ardents, or "glowing clouds") result from the collapse of the convecting column. An individual column may collapse when the particle exit velocity decreases or when the vent radius increases. These two parameters may be causally related: in some eruptions, erosion of the vent walls during repeated blasting causes the exit velocity to decrease even though the "eruptive power" remains constant. Commonly, however, the eruptive power fluctuates during the course of the event.
Three of the surges (S-4, S-5, S-6; Sigurdsson et al 1985) reached Pompeii (figure 4); it is likely that the first of these was most damaging to people, whereas the last was most damaging to buildings. In contrast, it appears that the S-1 surge was responsible for the destruction of Herculaneum. The S-4 surge may have been ejected at temperatures up to 800°C, although values of ~400°C are perhaps more likely for the temperature upon arrival in Pompeii. Victims likely suffocated on volcanic ash in the cloud, and were also scorched by the heat of gases within the flow. The S-6 surge was likely significantly cooler, and owes its great power to the abundant presence of lithic clasts (pieces of limestone and volcanic material carried in the flow). A modern example of this activity is Mt. Pelee (1902), where a ground surge similar to the S-6 event took down masonry walls 1 meter thick, and carried a 3-ton statue for 16 meters (Sigurdsson et al 1982).
|Fig. 1: Key plan showing photo angle.||Fig. 2: Overall view of wall juncture.|
This view shows grey basaltic material on the left meeting red brick material from the right in a quoined joint. This joint occurs at the point where the wall of the Sanctuary intersects this wall on the opposite side (see figure 1 for a plan view). The sequence of construction of the basalt and brick portions of this wall is a central question. One theory holds that the two portions were built contemporarily, while another holds that the basalt portion pre-dates the earthquake of 62, while the brick portion was built afterwards. Close examination of the quoined joint provides some clues.
Figure 3 shows a detail from figure 2, focussing on the quoining pattern. The geometry and materials at the interface of the basalt and brick materials is particularly important. Figure 4 highlights the interface by overlaying it with a blue line, illustrating that the geometry of the interface is quite irregular compared with other quoined joints at pompeii [need example of regular quoining here]. This irregularity strongly suggests that the two constructions are not contemporary, but rather that one was constructed later and fitted to the first. Figure 5 highlights two areas of the juncture which indicate the sequence of construction.
|Fig. 3: Detail of juncture.||Fig. 4: Detail with interface highlighted.||Fig. 5: Detail with key areas highlighted.|
In area 2 of figure 5, the bricks at the interface are arranged in a stepping pattern, indicating that they were fit around an irregular shape, and suggesting that the basalt material was built first, and the brick constructed to fit its irregular shape. Area 1 presents a seemingly different pattern where the profile of the bricks at the interface is a regular right angle; however, the basalt material at this interface has a distincly lighter color than that of the surrounding wall, indicating that the basalt at the interface is not contemporary with the rest of the wall. This construction suggests that the light-colored basalt is contemporary with the brick and was added to the basalt wall to fill it out to a more regular profile.
The configuration of masonry at areas 1 and 2 of figure 5 point to the same conclusion: the grey basalt wall was built first, with the brick wall being added later. A possible explanation for this sequence is that the basalt wall pre-dates the earthquake, while the brick wall was built in the post-earthquake reconstruction. This raises the question of why the earthquake apparently left the basalt portion standing while damaging the walls in the location now occupied by the brick. Examination of the plan configuration of the walls provides some clues.
|Fig. 6: Plan detail of juncture.||Fig. 7: Detail with different materials noted.|
Carey, S. & H. Sigurdsson (1987) Temporal variations in column height and magma dicharge rate during the 79 A.D. eruption of Vesuvius, Geol. Soc. Am. Bull., 99, 303-314.
Dobran, F., A. Neri & M. Todesco (1994) Assessing the pyroclastic flow hazard at Vesuvius, Nature, 367, 551-554.
Sigurdsson, H., S. Cashdollar & S.R.J. Sparks (1982) The eruption of Vesuvius in A.D. 79: Reconstruction from historical and volcanological evidence, Am. Jour. Archaeol., 86, 39-51.
Sigurdsson, H., S. Carey, W. Cornell & T. Pescatore (1985) The eruption of Vesuvius in A.D. 79, National Geographic Research, 1, 332-387.