Roberto Lanza

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.

Magnetic fabric in the Biella and Traversella stocks (Periadriatic Line) : implications for the mode of emplacement

Abstract The Biella and Traversella stocks, forming part of the intrusions along the Periadriatic Line, intruded into the Sesia-Lanzo Zone (Italian western Alps) ∼ 30 Ma ago. Samples were collected from 12 localities in the Traversella massif (diorites) and 22 in the Biella massif (granites, syenites, monzonites). In most localities, the fabric is primarily planar, with a distinct foliation of intermediate to subvertical dip. The magnetic lineation is less developed. Its plunge is very variable, marking a girdle pattern. The mean foliations and lineations per locality are scattered. This is probably due to the fact that the outcropping parts of the two stocks are summital, i.e. nearer the roof, and hence the magma movements may have been less regular. In the structurally deepest part of the Biella massif, the magnetic foliation dips regularly to the northwest. Its direction could reflect the pattern of the deep fractures along which the magma rose. The classic models of Owens, to describe the process of magnetic mineral orientation, do not apply to the features of these two stocks. A new model has been constructed to take account of the differences in viscosity contrast between the magnetic mineral crystals and the matrix. Referred to as the ductile model, it offers a suitable representation of the typical situation of crystal mushes and is the most appropriate for the Biella and Traversella stocks.

Gravity modelling along the ECORS-CROP vertical seismic reflection profile through the Western Alps

Abstract New gravity data from 500 stations and results from all previous surveys on the Western Alps are used to compile a detailed profile along the ECORS-CROP vertical seismic profile. The Bouguer anomaly consists of two approximately two-dimensional regional features, namely a wide negative anomaly occurring on the central part of the belt and the pronounced positive anomaly of the geophysical “Ivrea zone”, at the border between the Alps and the Po Plain. The results of a wide-angle seismic campaign and the preliminary line drawing from the vertical seismic have been the main constraints for the modelling of the anomaly. Two models are proposed. Their common feature is a piling up of the crust-upper mantle units in the Penninic zone of maximum crustal thickness as well as in the “Ivrea Zone”. The contribution of the Apulian lower crust to the positive anomaly is greater than formerly suggested. Several hypotheses are put forward for the zones where reasonable seismic information is available, as an illustration of the uncertainties surrounding their geological interpretation. The density models are consistent with the hypothesis of lithospheric accretionary processes for the collision between the European and Apulian plates.

Palaeomagnetic and structural evidence of Neogene tectonic rotation of the Gran Sasso range (central Apennines, Italy)

Abstract Interference between discordant tectonic trends in the central Apennines (Italy) was investigated in a palaeomagnetic study of the arc-shaped E-W- to N-S-oriented overthrust units of the Lazio-Abruzzi carbonate platform (Gran Sasso), and the underlying N-S-oriented imbricates of the Marche pelagic domain (Mt. dei Fiori). Samples were taken from 26 sites in the Cretaceous-Palaeogene Scaglia formation and in the Messinian-lower Pliocene Laga flysch. Stepwise thermal demagnetization was used to remove secondary components, and a stable direction of the characteristic remanent magnetization (ChRM) was determined for 24 sites. The palaeomagnetic results point to counter-clockwise rotation of the Marche domain similar to that of the southern Umbria. Directions in both the Scaglia and Laga formations of the Lazio-Abruzzi units show counter-clockwise rotations up to 90° in the easternmost segment of the Gran Sasso belt. This setting may be framed in a structural context wherein superposition of the E-W-oriented Gran Sasso belt onto the N-S-oriented Marche imbricates derives from progressive rotation of the Lazio-Abruzzi platform, decoupled by right-lateral shearing along the N-S-oriented Rigopiano-Bussi-Rivisondoli shear zone.

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.

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.

PICTORIAL REMANENT MAGNETIZATION AS AN INDICATOR OF SECULAR VARIATION OF THE EARTH'S MAGNETIC FIELD

Abstract Magnetic measurements carried out on murals of known date painted between 1740 and 1954 showed that the haematite pigment conferring their red colours carried remanent magnetization. The mean direction was well defined and consistent with that of the Earths magnetic field at the time of painting, as deduced from the Historical Italian Geomagnetic Catalogue.

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.

Distinguishing primary and resedimented vitric volcaniclastic layers in the Burdigalian carbonate shelf deposits in Monferrato (NW Italy)

Abstract A multidisciplinary study, including stratigraphic, sedimentological, biostratigraphic, petrographic, magnetic fabric and SEM analyses, has been performed on six volcaniclastic layers (VLs) interbedded in the Burdigalian shelf succession of the Monferrato (NW Italy). The aim was to distinguish between the volcanic and sedimentary processes that produced these deposits, to suggest a depositional model for volcaniclastic sedimentation in a shelf environment and to discuss the use of VLs for stratigraphic correlations. Two kinds of VLs have been distinguished: single volcaniclastic layers (SVLs) and multiple volcaniclastic layers (MVLs). SVLs are single beds of well sorted vitric siltites, mainly consisting of volcanic components and minor terrigenous and intrabasinal grains; the vitric fraction mainly consists of blocky fragments. They show a very low magnetic anisotropy degree and a prominent magnetic lineation. These VLs are interbedded in outer shelf marls and are interpreted as primary pyroclastic fall deposits. MVLs, which can be up to 10 m thick, show limited lateral continuity and are made up of several-decimetre-thick graded beds, separated by erosional surfaces and consisting of vitric arenites and siltites with about 15% non-volcanic components. Two kinds of MVLs have been distinguished: (1) Type 1 MVLs, interbedded in storm-dominated glaucony-rich calcarenites and showing rough, low-angle cross-stratification (hummocky cross stratification), water escape and load structures. These deposits are characterized by a slightly foliated magnetic fabric and are interpreted as storm layers, deposited between fairweather and storm wave base. (2) Type 2 MVLs are interbedded in outer shelf marls, and are characterized by parallel lamination and by a well developed magnetic foliation. They are interpreted as storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf. The Monferrato VLs resulted from explosive eruptions of volcanic edifices, located outside of the basin, that produced an extensive tuff blanket that was uniformly distributed on a carbonate-dominated shelf. Above storm wave base the VLs were immediately reworked by storm activity, and the resulting deposits are type 1 MVLs. Below storm wave base, primary pyroclastic fall deposits were preserved, corresponding to SVLs. Storm-induced turbidity currents gave rise to type 2 MVLs, that were deposited below storm wave base. Preservation of VLs in a shelf environment is hampered by the high-energy conditions of the shelf. Consequently, these deposits are characterized by a restricted lateral continuity and their use as correlation tools may be misleading.

New data on the rock magnetism in the Ivrea-Verbano Zone (Northern Italy) and its relationships to the magnetic anomalies

Abstract The Ivrea-Verbano Zone (Southern Alps, Northern Italy) is regarded as a typical example of a section of the lower continental crust. A considerable magnetic anomaly runs along its outcrop area. A study has been made of 153 samples collected along three transverse sections (the Cannobina, Strona and Sessera valleys) and from the southernmost outcrops near Ivrea to determine the relationships between this anomaly and the magnetic properties of the rocks, and to gain an insight into the nature of magnetism of the lower crust. Most rocks displayed magnetic susceptibility values ranging from 100 to 10,000 × 10 −6 SI units. Serpentinized peridotites from the Finero and Baldissero massifs, the main gabbro and kindred units, and a few amphibolites and mafic granulites of the Kinzigite Series are the only rocks with values (29,000−200,000 × 10 −6 SI) comparable with those required to produce the observed anomaly. In addition, the intensity of their remanent magnetization is of the same order of magnitude as the induced magnetization, and the directions of remanence are mostly scattered around the direction of the present field. Their total magnetization is enough to explain the anomaly of the Ivrea-Verbano Zone. The results of these measurements and the surface geology suggest that a set of distinct slab-like bodies formed of serpentinized peridotite and main gabbro can be proposed as a model for interpretation of the Ivrea-Verbano magnetic anomaly. Amphibolites and mafic granulites can be regarded as responsible for the short-wavelength anomalies only. The main magnetic minerals are pyrrhotite, which is widely distributed but always in low concentrations, and magnetite, which is confined to a few lithotypes but can reach high concentrations. Magnetite must thus be held responsible for the anomaly, as in other sections of the lower crust investigated so far. The reason for its presence in individual lithotypes, however, is not always the same and no systematic regional correlation with mineralogical and metamorphic characteristics can be established. In conclusion, the phenomena observed in the Ivrea-Verbano Zone suggest that the magnetism of rocks regarded as typical of the lower crust is highly variable, and that there is no ready explanation for the high magnetization values postulated in the light of the satellite data.