Elena Zanella

Interaction of pyroclastic density currents with human settlements: Evidence from ancient Pompeii

Integrating field observations and rock-magnetic measurements, we report how a turbulent pyroclastic density current interacted with and moved through an urban area. The data are from the most energetic, turbulent pyroclastic density current of the A.D. 79 eruption of Vesuvius, Italy, which partially destroyed the Roman city of Pompeii. Our results show that the urban fabric was able to divide the lower portion of the current into several streams that followed the city walls and the intracity roads. Vortices, revealed by upstream particle orientations and decreases in deposit temperature, formed downflow of obstacles or inside cavities. Although these perturbations affected only the lower part of the current and were localized, they could represent, in certain cases, cooler zones within which chances of human survival are increased. Our integrated field data for pyroclastic density current temperature and flow direction, collected for the first time across an urban environment, enable verification of coupled thermodynamic numerical models and their hazard simulation abilities.

Paleomagnetic secular variation at Vulcano (Aeolian Islands) during the last 135 kyr

Abstract Paleosecular variation (PSV) of the Earth’s magnetic field during the last 135 kyr has been investigated in lavas, scoriae and pyroclastic rocks of Vulcano (Aeolian Islands). About 1000 samples have been collected at 77 sites from 25 distinct volcanic units, whose age is either known from published isotopical data or constrained on the grounds of statigraphical relationships. Magnetic mineralogy investigation shows that Ti-magnetite is the main ferromagnetic mineral. At most sites, secondary magnetization components are either absent or easily removed by stepwise thermal or alternating field demagnetization. The mean site direction of the characteristic remanent magnetization is usually well-defined, since the semi-angle of confidence is greater than 5° at only four sites. The mean paleomagnetic direction over the last 135 kyr ( D =9.4°, I =53.2°, α 95 =3.5°) differs from the geocentric axial dipole (GAD) at Vulcano ( D =0°, I =57.8°) and might be interpreted as the effect of a long-term, non-axial-dipolar component. The PSV record from Vulcano agrees well with those from the lacustrine sediments of Lago Grande di Monticchio (100 kyr BP) and Lago di Mezzano (30 kyr BP), located in the Italian peninsula [Brandt et al., Quat. Sci. Rev. 18 (1999) 961–976]. The inclination anomaly Δ I found at Vulcano corresponds to about half of the shallowing observed in the sediments of the two lakes and the declination anomaly Δ D may be used to tie the declination values, derived from azimuthally unoriented cores, to the geographical reference system. In order to find the optimum site to be used as reference for PSV studies in Italy, the angular values of the Earth’s magnetic field measured at the 113 repeat stations of the Italian Geomagnetic Network [Coticchia et al., Boll. Geod. Sci. Aff. 40 (2001) 277–291] have been analyzed with the relocation via pole method [Noel and Batt, Geophys. J. Int. 102 (1990) 753–756]. The Viterbo station (lat. 42°27′N, long. 12°02′E) proved the best, since the mean error is 0.3° for both declination and inclination, wherever the original PSV site is located in Italy. A preliminary, composite PSV curve for the last 30 kyr BP is thus proposed, merging and relocating to Viterbo the data from Vulcano and the curve from Lago di Mezzano, corrected for the GAD deviation found at Vulcano.

CHANGES OF THE GEOMAGNETIC FIELD VECTOR OBTAINED FROM LAVA SEQUENCES ON THE ISLAND OF VULCANO (AEOLIAN ISLANDS, SICILY)

Abstract A combined geochronologic ( K Ar ) and palaeomagnetic study has been conducted on five volcanic sections (70 flows) on the island of Vulcano. The sections span the interval from 15 ± 2 to 135 ± 4 ka, with large hiatuses. Rock magnetic investigations indicate that the natural remanent magnetisation (NRM) is carried by titanomagnetites in the four oldest sections, with a contribution from high coercivity magnetic minerals in the youngest section. Palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion, at around 110 ka, with virtual geomagnetic poles (VGPs) that lie over eastern Siberia. These directions may represent a volcanic record of the termination of the Blake event. Only about 15% of the samples (from 19 flows) proved suitable for palaeointensity (Thellier and Thellier) determinations. These measurements indicate that the geomagnetic field has varied at Vulcano between 16 and 53 μT in the explored time interval. When combined with previous results obtained at Mt. Etna for the period from 140 to 60 ka (Tric et al., 1994, Phys. Earth Planet. Inter. , 85: 113–129), these results document a broad low at around 115 ka. The palaeointensity values from Vulcano and Etna are consistent with global values, suggesting the presence of a dominant dipole-field contribution. These values, on the other hand, are significantly smaller than those recently obtained from La Reunion in the southern hemisphere. This suggests that large, long-lived, non-dipolar components of the geomagnetic field may be present at La Reunion in the interval from 130 to 90 ka. These observations are consistent with the fact that the VGP angular dispersion, calculated from non-excursional data, is significantly smaller at Vulcano than at La Reunion.

Archaeomagnetic results from mural paintings and pyroclastic rocks in Pompeii and Herculaneum

Abstract This work investigates the magnetic remanence associated with red pigments from murals at Pompeii and compares their directions to those of the pyroclastic rocks from the Vesuvius AD 79 eruption. The remanence of the murals is shown, using X-ray analyses, to be carried by haematite. Murals in Thermae Stabianae, known to have been painted a few years before AD 79, yield an archaeomagnetic direction (D=1.2°, I=58.0°; α95=5.5°) indistinguishable from that of a nearby kiln (D=358.0°, I=59.1°; α95=1.7°) ( Evans and Mareschal, 1989 ) probably last used immediately prior to the eruption. The directions are also consistent with those of fine-grained pyroclastic rocks from the eruption (D=351.2°, I=57.9°; α95=3.4°) and lithic and tile fragments embedded within them (D=358.5°, I=60.4°; α95=8.5°). Other paintings of the 1st century AD yield similar directions, with a lower statistical definition. This study shows that murals can retain their remanent magnetization for centuries and demonstrates the viability in principle of pictorial remanence as an archaeomagnetic tool.

Magnetic fabric and remanent magnetization of pyroclastic surge deposits from Vulcano (Aeolian Islands, Italy)

Abstract Tufi di Grotte dei Rossi Inferiori are unwelded, fine-grained pyroclastic deposits of hydromagmatic origin emplaced between 21 and 11–8.6 ka at Vulcano (Aeolian Islands, Italy) by deposition through surges spreading laterally from inside the La Fossa caldera. In this study, the deposits magnetic properties were investigated and interpreted in terms of eruptive and emplacement dynamics. Rock-magnetism data were supplemented by grain size and textural characteristic analyses as well as scanning electron microscope (SEM) investigations. Curie point measurements, isothermal remanent magnetization and microprobe analyses showed that magnetization is carried by low-Ti titanomagnetite. The size of the grains ranges from about 20 to 300 micrometres, their shape from equidimensional to highly elongated. The magnetic fabric is typical of fine-grained pyroclastics. Foliation is well developed and in most sites lineation is directed towards the source area of the La Fossa caldera. The remanent magnetization consists of two components whose blocking temperature spectra partially overlap. The direction of the low-temperature component is close to that of the axial dipole, and consistent with the palaeosecular variation curve for the Aeolian Islands. The high-temperature component is systematically shallowed and close to the direction of the magnetic lineation. The overall results suggest that the high-temperature component was acquired before, and the low-temperature component after, the actual deposition of grains. Immediately after eruption, the grains cooled and moved as free particles in the turbulent cloud during the expansion of the surge flows. Those particles with high blocking temperatures acquired a thermal remanence. They were then deposited and shear at the very base of the flow oriented them and imprinted the rocks fabric and high-temperature magnetization component. Volcanological and magnetic data suggest turbulent transportation and traction deposition of particles. Finally, the rock cooled down and acquired the low-temperature magnetization. According to this model, remanence acquisition in pyroclastic rocks deposited at temperatures lower than the Curie point of their ferromagnetic grains depends on both the fluid-dynamic forces and cooling history, and must therefore be more complex than in pyroclastics deposited at higher temperatures, such as welded ignimbrites.

PALEOMAGNETISM OF PLEISTOCENE VOLCANIC ROCKS FROM PANTELLERIA ISLAND (SICILY CHANNEL), ITALY

Abstract A paleomagnetic investigation has been carried out on Pleistocene volcanic rocks from Pantelleria, in the Sicily Channel. This island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition. The radiometric data indicate that volcanic activity started around 324 ka BP. Samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka. Magnetic properties vary systematically with lithotype and indicate high-Ti titanomagnetite to magnetite as the main carriers of magnetisation. Stable characteristic remanent magnetisation (ChRM) directions isolated by alternating fields (Af) demagnetisation show normal polarity consistent with emplacement during the Brunhes chron and vary within the paleosecular variation (PSV) range. However, the inclinations are low as compared with the geocentric axial dipole (GAD) inclination at Pantelleria, especially for volcanic units younger than 50 ka. The mean ChRM direction computed from 27 sites is D=358.8°, I=46.9° (k=30, α95=5.2°), and the inclination anomaly is ΔI=−8.1°. Both tectonic movements and the presence of large magnetic anomalies around the island of Pantelleria fail to explain this low inclination, which may therefore be related to a significant long-lived, non-dipolar field contribution in the area over the past 150 kyr.

Deposit temperature of pyroclastic density currents emplaced during the El Chichón 1982 and Colima 1913 eruptions

Abstract New data on the pyroclastic density current (PDC) deposit temperature (Tdep) are provided for two prominent eruptions of Mexican volcanoes of the twentieth century: the 1982 eruption of El Chichón and the 1913 eruption of Colima. In spite of similar lithofacies, magma composition and pre-eruptive conditions, the Tdep of the PDCs from the 1982 (El Chichón) and 1913 (Colima) eruptions differ significantly, with intervals of Tdep of 360–420 °C and 250–330 °C, respectively. These new data emphasize that a full understanding of the physical mechanisms responsible for equilibrium temperature attainment within a pyroclastic deposit has not yet been realized. The Tdep measured for El Chichón PDC deposits confirm the preliminary data published elsewhere, while Colima magnetic temperatures provide different values to those published previously. supplementary-material: Tdep measurements for the different sites at El Chichon volcano and Colima volcano are available at: http://www.geolsoc.org.uk/SUP18695.

Temperatures of the pyroclastic density currents deposits emplaced in the last 22 kyr at Somma–Vesuvius (Italy)

Abstract The temperature of the deposits (Tdep) emplaced by the pyroclastic density current (PDC) generated by the seven major explosive eruptions from Somma–Vesuvius during the last 22 kyr were investigated using the thermal remanent magnetization (TRM) of lithic clasts embedded within the deposits. New data are presented for the Pomici di Base, Greenish Pumice, Mercato and 1631 AD deposits and compared to the literature data from the Avellino, 79 AD-Pompeii and 472 AD-Pollena eruptions. The Tdep mainly fall in the range 270–370 °C and no significant correlation is evidenced between sedimentological features, eruptive and depositional processes and the final Tdep. The admixture of ambient air during the run-out appears the most effective process to cool the temperature of the ash and gases of the PDC, and is therefore the main factor affecting the deposit temperature.

The anisotropy of magnetic susceptibility of uniaxial superparamagnetic particles: Consequences for its interpretation in magnetite and maghemite bearing rocks

A simple model that provides a quantitative description of the magnetic susceptibility of superparamagnetic to stable single-domain uniaxial magnetic particles can be built in the framework of the theory of stochastic resonance. This model expands that of Mullins and Tile (1973) for superparamagnetic grains by considering the dependence of superparamagnetic susceptibility on the particle orientation and thus describes the anisotropy of magnetic susceptibility (AMS) of ensembles of superparamagnetic as well as single-domain particles. The theory predicts that on the contrary of stable single domain, the maximum anisotropy of superparamagnetic particles is parallel to their easy axis and shows that the AMS of ensembles of uniaxial particle is strongly dependent on the distribution of particle grain size, coercivity, measurement temperature, and frequency. It also explains why the inverse AMS pattern expected for stable single-domain particles is rarely observed in natural samples. We use examples of well-characterized obsidian specimens to show that, as predicted by the theory, in the presence of significant superparamagnetic contributions, the maximum susceptibility axis of AMS is directed along the preferential direction of particles easy axis.

Magnetic fabric of ignimbrites: a case study from the Central Anatolian Volcanic Province

Abstract The magnetic fabric of the Pliocene Kızılkaya ignimbrite in the Central Anatolian Volcanic Province has been investigated by anisotropy of magnetic susceptibility (AMS) and isothermal remanent magnetization (AIRM). Seven sections were sampled at various stratigraphic heights within the devitrified portion of the ignimbrite. The magnetic mineralogy is complex: titanomagnetite occurs as magmatic grains, and as inclusions in other phenocryst and glass shards; an oxidized phase and hematite occur in deposit levels affected by alteration processes. The disturbance produced by lithic and pumice clasts has been reduced by discarding the specimens that deviate more than ±1σ from the site mean value of the density. The AMS fabric varies along each individual section. Neither the AMS magnetic lineation nor the magnetic foliation plunge clearly define a common area as the vent location. The AIRM fabric of low-coercivity minerals, mainly represented by free titanomagnetite grains of magmatic origin, is consistent between sites and the inferred flow directions converge on a region near Derinkuyu, in the Nevsehir plateau, previously reported as the Kızılkaya ignimbrite source area. This study shows that systematic use of the remanent fabric improves the results given by AMS and aids the identification of the primary magnetic fabric related to the ignimbrite emplacement dynamics.