Deposition temperature of the AD 472 Pollena pyroclastic density current deposits, Somma-Vesuvius, Italy

Abstract

The deposition temperature of the pyroclastic density current (PDC) deposits emplaced during the AD 472 Pollena eruption (Somma-Vesuvius, Italy) has been investigated using the thermal analysis of the magnetic remanence carried by lithic clasts embedded within the deposits. A total of 310 lithic clasts were collected from all the PDC units in the Pollena stratigraphic succession, at different distances from the inferred vent and at different stratigraphic levels. The temperature reached by each individual clast during residence in the PDC was estimated through stepwise thermal demagnetization, with the values from all clasts collected at each site being used to infer the deposition temperature (T dep ). Although the sedimentological features of these PDC deposits show some variation, the deposition temperature typically falls in the range 300 to 320°C, with a maximum range of 260 to 360°C. The fairly uniform temperature observed in both the dune bedded and massive deposits points to homogeneity in attainment of T dep for the different deposits and suggests similarity in the depositional regime of the different PDCs and/or in heat transfer to lithic fragments. Similarity in depositional regime was also favoured by the limited control exerted by topography on the distribution of these PDCs, with the northern wall of the Somma caldera that did not act as a morphological barrier. As a result the currents were capable of moving away from the vent, without topographic disturbances and, thus, significant variations in the cooling regime. Because the Pollena eruption is considered similar to the maximum expected event at Somma-Vesuvius, the characteristics of its deposits best simulate the likely maximum hazard for the Vesuvius region. In this regard, Pollena produced hot, dilute PDCs which were able to travel up to 12 km from the vent maintaining high temperatures across this distance.

Paleomagnetic determination of emplacement temperatures of pyroclastic deposits: an under-utilized tool

“…A mean paleomagnetic direction was obtained by grouping sites CP2-6 (Dec. = 352.6 • , Inc. = 57.1 • , α 95 = 8.5 • , N = 95, R = 70.9, k = 3.9, 3R 2 /N = 158.6). This direction is consistent with paleomagnetic directions recorded in previous studies (e.g., Tanguy et al 2003;Zanella et al 2008)). To isolate clasts that record a consistent direction, data from sites CP2-6 were excluded if the paleomagnetic direction was >30 • away from the mean paleomagnetic direction.…”

“…Instead, they proposed a temperature estimate based on the overlap of the emplacement temperature of each clast at one locality. They exclude outliers of this overlapping range on the basis of adverse cooling or heating prior to deposition (Cioni et al 2004;Zanella et al 2007Zanella et al , 2008. In the case of a thin deposit, the approach of Bardot and McClelland (2000) should underestimate the true temperature of the deposit.…”

“…This variation of a few hundred degrees over short distances illustrates the effect that urban areas can have on the temperature of pyroclastics and might explain the temperature variations documented at Herculaneum. Zanella et al (2008) recently investigated the 472 A.D. deposits from Vesuvius. These deposits were uniformly hot with T dep ∼260-360 • C irrespective of locality and the facies sampled.…”

“…At our sampled locality, Pollena quarry (Fig. 12), Cioni et al (2004) estimated the emplacement temperature of the 79 A.D. pyroclastics to be 250-310 • C, while Zanella et al (2008) estimated the 472 A.D. deposits to have been emplaced at 280-320 • C. We sampled 124 lithic clasts from the 472 A.D. deposits at the Pollena quarry, on the western flank of Vesuvius (Fig. 12).…”

The historic magmatic-hydrothermal eruption of the Breccia di Commenda, Vulcano, Italy

“…The temperature of the deposit (T dep, Cioni et al 2004) was estimated according to the procedure of Cioni et al (2004) and Zanella et al (2007Zanella et al ( , 2008. Measurements were carried out at the University of Torino's Alpine Laboratory of Paleomagnetism (Peveragno-Italy) using a JR-6 spinner magnetometer and Schonstedt furnace.…”