Patrizia Landi

A review on phreatic eruptions and their precursors

Abstract An analysis of the reported information on 132 historical phreatic events indicates that most of these (115) were not followed by a magmatic or phreatomagmatic eruption. Only in 18 cases precursory phenomena are reportedly lacking, whereas they are recognized for the majority (72) of the considered cases. These precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism. Unfortunately, the quality of the available information is poor. However, data suggest that anomalous seismicity, ground deformation, changes in the output, chemistry and temperature of fumaroles and thermal springs are potential mid-term precursors, whereas seismic tremor is a common short-term precursor.

Violent explosions yield new insights into dynamics of Stromboli volcano

Crystal-poor magma may be a chief factor in the steadiness of activity at the Stromboli volcano in Italy. This was one of the findings to emerge from the documentation of a number of violent explosions there last year. The findings are giving scientists new insights into the dynamics of the volcano. Stromboli, on an island of the same name in the Aeolian Archipelago, is known as “the lighthouse of the Mediterranean” because of its persistent explosive activity, which is visible from great distances. The well-documented explosions, however, were more violent than normal “strombolian” activity, and we call them “paroxysmal” explosions.

The Pomici di Base plinian eruption of Somma-Vesuvius

Abstract The Pomici di Base eruption represents the first of a series of plinian eruptions which occurred at Somma-Vesuvius in the period 20,000 yr B.P.–79 A.D. These eruptions led to substantial demolition of the Mt. Somma stratovolcano and the formation of the 4.9×3.4 km E–W-elongated summit caldera. New 14 C datings and previous radiometric data constrain the age of the Pomici di Base eruption to between 18,000 and 19,000 yr B.P. Deposits of the Pomici di Base eruption comprise from base to top: (1) plinian fallout with minor surge deposits and (2) a succession of volcanic landslide and lithic-rich fallout, surge and flow deposits. Ballistic block distribution and thickness of tephra deposits indicate that the vent was located in a 50° wide western sector within a distance of 1–2.5 km from the present Vesuvius crater, in a fairly eccentric position with respect to the ancestral Somma cone. The plinian fallout likely blanketed an eastwards elliptical area of 2600 km2 within the 20-cm isopach. Reconstruction of isopachs yields an approximate volume calculation of 4.4 km3. Comparison of maximum thickness of the fallout deposit with other plinian deposits of Somma-Vesuvius suggests that the PB eruption was the largest explosive event of the volcano. The mass discharge rate deduced from clast dispersal models is estimated in the range of 2–2.5×107 kg/s, corresponding to a column height of 16–17 km. Part of the plinian phase was characterized by pulsatory behaviour with repeated partial column collapses (surge emplacement) and concurrent oscillation of the height of the plume (stratified fallout). The plinian phase was followed by a limited slope failure of the Somma cone and by several explosive episodes with a prominent phreatomagmatic nature. We proposed that this activity occurred in connection with a phase of substantial demolition of the Somma edifice due to caldera collapse. The plinian fallout is dominated by strong compositional zoning from white trachytic pumice (SiO2 63.0 wt.%) to black latitic scoriae (SiO2 53.7 wt.%), coupled with a marked decrease of vesicularity of juvenile clasts from 70–80% to 45–55%. The compositional variation reflects strong pre-eruptive zoning of the magma chamber probably associated with volatile zonation.

Volcanology and Magma Geochemistry of the Present‐Day Activity: Constraints on the Feeding System

Stromboli volcano is famous in the scientific literature for its persistent state of activity, which began about 1500 years ago and consists of continuous degassing and mild intermittent explosions (normal Strombolian activity). Rare lava emissions and sporadic more violent explosive episodes (paroxysms) also occur. Since its formation, the present-day activity has been dominated by the emission of two basaltic magmas, differing chiefly in their crystal and volatile contents, whose characteristics have remained constant until now. The normal Strombolian activity and lava effusions are fed by a crystal-rich, degassed magma, stored within the uppermost part of the plumbing system, whereas highly vesicular, crystal-poor light-colored pumices are produced during paroxysms testifying to the ascent of volatile-rich magma batches from deeper portions of the magmatic system. Mineralogical, geochemical, and isotopic data, together with data on the volatile contents of magmas, are presented here with the aim of discussing (1) the relationships between the different magma batches erupted at Stromboli, (2) the mechanisms of their crystallization and transfer, (3) the plumbing system and triggering mechanisms of Strombolian eruptions.

The Evolution of Latera Caldera (Central Italy) in the Light of Subsurface Data

Subsurface geothermal exploration has considerably added to our knowledge of the Latera volcanic complex. A syenitic body is located about 2 km below the present-day surface; K-Ar data point a 0.9 Ma age. The primary magma was a silica-saturated trachyte; undersaturated, hauyne-bearing products are found near the carbonatic wall-rocks and have been interpreted as reaction products. Subsurface data from deep drilling and geophysical surveys suggest that the Latera caldera resulted from three main successive collapse phases: (i) formation of an old caldera, now buried, related to the eruption of ignimbrites from the syenitic magma chamber; (ii) sinking of the eastern sector as a consequence of the formation of the nearby Bolsena caldera (∼0.3 Ma); (iii) multistage formation of the present Latera caldera (∼0.16 Ma).

The 1906 eruption of Vesuvius: from magmatic to phreatomagmatic activity through the flashing of a shallow depth hydrothermal system

The April 1906 eruption of Vesuvius is the type-example of the “final eruptions” that close the short cycles of semi-persistent activity that characterized the volcano in the 1631–1944 period. The eruption had a marked explosive character that accompanied the emission of lava from several vents on the southern slopes of the volcano. The observed sequence of events was characterized by repeated fluctuations of the magma level within the conduit, by large lava fountains, by conduit partial collapses, and by the final explosive decapitation of the summit cone. Contemporary chronicles, although frequently contradictory, allow reconstruction of the eruption, which can be divided into four main phases: (1) lateral lava effusions; (2) lava fountains; (3) gas-pyroclasts column; (4) low dense clouds. Pyroclastic deposits of the Monte Somma ridge and northeastern slope can be related to observed and described events and mainly refer to the 2nd and 3rd phases. The increase in the degree of fragmentation of the juvenile component together with the marked increase of the lithic component and morphologic evidence emphasize the repeated occurrence of magmawater interaction. This was most spectacular in the 3rd phase of the eruption in which, after the decapitation of the cone, a high gas-pyroclasts eruption column was formed. Because of the nature of the lithic fragments (mainly hydrothermally altered and metasomatic rocks), the huge amount of steam, and the high lithic/juvenile ratio, it is unlikely that the largest part of the energy in play was related to the contact between magma and cold phreatic water. We suggest that most of the steam involved in this phase of the eruption came from flashing of the hydrothermal system connected to the very shallow feeding system of the volcano and formed as a consequence of repeated subsurface intrusions between 1872 and 1906. Juvenile products were ejected through the eruption, and represent (at least) two different magma bodies: the first (“older”) was erupted during the initial phase of the eruption and was exhausted at the beginning of the lava fountains phase, when fresh magma was involved in the eruption.

A LiDAR survey of Stromboli volcano (Italy): Digital elevation model-based geomorphology and intensity analysis

LiDAR (Light Detection and Ranging) is a novel and very useful active remote sensing system which can be used to directly identify geomorphological features as well as the properties of materials on the ground surface. In this work, LiDAR data were applied to the study of the Stromboli volcano in Italy. LiDAR data points, collected during a survey in October 2005, were used to generate a Digital Elevation Model (DEM) and a calibrated intensity map of the ground surface. The DEM, derived maps and topographic cross-sections were used to complete a geomorphological analysis of Stromboli, which led to the identification of four main geomorphological domains linked to major volcanic cycles. Moreover, we investigated and documented the potential of LiDAR intensity data for distinguishing and characterizing different volcanic products, such as fallout deposits, epiclastic sediments and lava flows.

Novel interpretation for shift between eruptive styles in some volcanoes

The transition from effusive, low mass flow rate to explosive, high mass flow rate eruptive behavior is a common aspect of the activity of calc-alkaline volcanoes. However, the process driving the shift between the two eruptive styles is at present debatable and represents a topical theme in the volcanological literature. The main challenge is to understand the mechanism that allows a high mass flow rate when eruptions of highly porphyritic (40 vol % crystals) and viscous magmas (≥106 Pas, pascal second) occur. In this article, volcanological, compositional, and textural observations are used to demonstrate that viscous dissipation, a process that develops heating within flowing magma in a boundary layer near the conduit walls due to friction, is responsible for the long-lasting, sustained explosive phases of this eruption type, as well as for the transition from effusive to explosive behavior.

Mafic inclusions in the silica-rich rocks of the Tolfa-Ceriti-Manziana volcanic district (Tuscan Province, Central Italy): chemistry and mineralogy

SummaryA chemical and mineralogical study of magmatic inclusions in the silica-rich rocks of the Tolfa-Ceriti-Manziana sector (Tuscan Province, Central Italy) shows that they can be grouped according to their degree of alkalinity. We distinguish: TCM latites (TCML) with an alkaline-potassic affinity; Ceriti latites (CL) with calc-alkaline affinity; and Manziana shoshonites and latites (MS-ML) with an affinity intermediate between alkaline and subalkaline. The latter are hybrid rocks originating from the mixing of two magmas with slightly different geochemical affinities.These magmas may represent liquids derived by partial melting of a heterogeneous mantle, metasomatized by a crustal source. The most alkaline magmas (TCML) are associated with the largest degree of metasomatism which, if more pronounced, could lead to the production of the potassic magmas of the Roman Province.ZusammenfassungEine chemische und mineralogische Untersuchung der magmatischen Einschlüsse in den sauren Gesteinen des Tolfa-Ceriti-Manziana Sektors (Toskanische Provinz, Zentralitalien) zeigt, daß sie nach dem Grad der Alkalinitdt unterteilt werden können. Wir unterscheiden TCM latite (TCML) mit einer Alkali-Kalium Affinität; Ceriti Latite (CL) mit einer kalkalkalischen Affinität und Manziana Shoshonite und Latite (MS-ML) mit einer Affinität die zwischen alkalisch und subalkalisch liegt. Die Letztgenannten sind Mischgesteine, die durch die Mischung zweier Magmen mit verschiedenen geochemischen Affinitäten entstanden sind.Diese Magmen, die genetisch nicht mit den Wirtsgesteinen in Beziehung stehen, dürften Schmelzen repräsentieren, die auf teilweise Aufschmelzung eines heterogenen Mantels, der durch eine krustale Quelle metasomatisiert wurde, zurückgehen. Die am meisten alkalischen Magmen (TCML) sind mit dem höchsten Grad der Metasomatose assoziiert, der wiederum, wenn noch stärker entwickelt, zur Entstehung der kalireichen Magmen der römischen Provinz führen könnte.

Mineralogical, Geochemical, and Isotopic Characteristics of the Ejecta from the 5 April 2003 Paroxysm at Stromboli, Italy: Inferences on the Preeruptive Magma Dynamics

The 5 April 2003 explosive eruption at Stromboli emplaced typical basaltic scoria, pumice, and lithic blocks. This paper reports a detailed set of mineralogical, geochemical, and isotopic data on the juvenile ejecta and fresh subvolcanic blocks, including micro-Sr isotope analyses and major and dissolved volatile element contents in olivine-hosted melt inclusions. The juvenile ejecta have compositions similar to those of their analogs from previous paroxysms; the 2003 pumice, however, does not contain stable high-MgO olivine, usually typical of large-scale paroxysms and has lower compatible element contents. Texture, composition, and Sr isotope disequilibrium of crystals in pumice indicate that most of them are inherited from the shallow crystal-rich magma and/or crystal mush. The most primitive magma is recorded as rare melt inclusion in olivine Fo 85―86 . It has a typical S/Cl (1.1) and a total volatile content of 3.1 wt % from which the total fluid pressure was evaluated >240 MPa. Hence, moderate pressure conditions can be envisaged for the mechanism triggering the April 2003 paroxysm. The subvolcanic blocks are shoshonitic basalts with 45―50 vol % of phenocrysts (plagioclase + clinopyroxene + olivine). The late-stage crystallization of the crystal-rich magma lead to the formation ofNa-sanidine with plagioclase An 60―25 + olivine Fo 68―49 + Ti-magnetite ± apatite ± phlogopite ± ilmenite assemblage. Mineralogy, chemistry, and Sr―Nd isotopic signatures of the subvolcanic blocks indicate they represent the slowly cooled equivalents of batches of crystal-rich basaltic magma stored in the uppermost subvolcanic feeding system during the last few years. Cooling might be facilitated by short breaks in the summit crater activity.