Mauro Rosi

The phlegraean fields: Structural evolution, volcanic history and eruptive mechanisms

Abstract The main event within the volcanic history of the Phlegraean Fields was the eruption, about 35,000 years ago, of a huge alkali trachytic ignimbrite (80 km 3 , dre) followed by caldera collapse. The pre-caldera activity (evidence from geothermal wells and surface outcrops) changed from submarine to subaerial shortly before the ignimbrite eruption. The caldera was subsequently invaded by the sea and became progressively filled with tuffites and subordinate submarine flows in its northern half. The last submarine explosive activity dates back to 12000-10500 yr. with the eruption of mostly “hydroplinian” yellow tuffs. Recent, mostly subaerial, activity consists of a series of explosive eruptions whose volume decreases with time, until the last eruption of Mt. Nuovo in 1538 A.D. Two peaks in recent activity occur at 10000-8000 and 4700-3000 years ago. During post-caldera activity, eruptive vents migrated from the caldera rim toward the center where there are two distinct zones of recent vents. Minor collapses occurred within the caldera after the main eruptions; often they are eccentric to the vent and displaced toward the caldera center. Phlegraean explosive activity is characterized by water/magma interaction; eruptive events without a hydromagmatic component are extremely rare. In most cases, the evolved trachytic magmas interacted with surface water (either the sea or intracaldera lakes). However, there is evidence of interaction of magma with deep-seated aquifers for some Plinian events. Explosivity, i.e. the efficiency of the transformation of thermal into kinetic energy during magma/water interaction, is largely controlled by the primary fragmentation of the magmatic melt.

The 1631 Vesuvius eruption. A reconstruction based on historical and stratigraphical data

Abstract The eruption of 1631 A.D. was the most violent and destructive event in the recent history of Vesuvius. More than fifty primary documents, written in either Italian or Latin, were critically examined, with preference given to the authors who eyewitnessed volcanic phenomena. The eruption started at 7 a.m. on December 16 with the formation of an eruptive column and was followed by block and lapilli fallout east and northeast of the volcano until 6 p.m. of the same day. At 10 a.m. on December 17, several nuees ardentes were observed to issue from the central crater, rapidly descending the flanks of the cone and devastating the villages at the foot of Vesuvius. In the night between the 16th and 17th and on the afternoon of the 17th, extensive lahars and floods, resulting from rainstorms, struck the radial valleys of the volcano as well as the plain north and northeast. Deposits of the eruption were identified in about 70 localities on top of an ubiquitous paleosol formed during a long preeruptive volcanic quiescence. The main tephra unit consists of a plinian fallout composed of moderately vesicular dark green lapilli, crystals and lithics. Isopachs of the fallout are elongated eastwards and permit a conservative volume calculation of 0.07 km 3 . The peak mass flux deduced from clast dispersal models is estimated in the range 3–6 × 10 7 kg/s, corresponding to a column height of 17–21 km. East of the volcano the plinian fallout is overlain by ash-rich low-grade ignimbrite, surges, phreatomagmatic ashes and mud flows. Ash flows occur in paleovalleys around the cone of Vesuvius but are lacking on the Somma side, suggesting that pyroclastic flows had not enough energy to overpass the caldera wall of Mt. Somma. Deposits are generally unconsolidated, massive with virtually no ground layer and occasionally bearing sparse rests of charred vegetation. Past interpretations of the products emitted on the morning of December 17 as lava flows are inconsistent with both field observations and historical data. Features of the final phreatomagmatic ashes are suggestive of alternating episodes of wet ash fallout and rainfalls. Lahars interfingered with primary ash fallout confirm episodes of massive remobilization of loose tephra by heavy rainfalls during the final stage of the eruption. Chemical analyses of scoria clasts suggest tapping of magma from a compositionally zoned reservoir. Leucite-bearing, tephritic-phonolite (SiO 2 51.17%) erupted in the early plinian phase was in fact followed by darker and slightly more mafic magma richer in crystals (SiO 2 49.36%). During the nuees ardentes phase the composition returned to that of the early phase of the eruption. The reconstruction of the 1631 eruptive scenario supplies new perspectives on the hazards related to plinian eruptions of Vesuvius.

The Phlegraean Fields: magma evolution within a shallow chamber

Abstract A systematic petrological and chemical study of the volcanic products of the Phlegraean Fields has been accomplished based on the new stratigraphy described by Rosi et al. (this volume). The majority of Phlegraean rocks belong to the “potassic” series of the Roman province. The compositional spectrum ranges from trachybasalts to latites, trachytes, alkali trachytes, and peralkaline phonolitic trachytes. Trachybasalts are extremely rare and there is a sharp compositional gap between them and the latites. The series between latites and the trachytic varieties is complete. Trachytic rocks are much more voluminous than latites. The order of appearance of the main solid phases is: olivine, clinopyroxene, plagioclase, alkali feldspar, biotite, and oxides. Mineral compositions obtained by microprobe analyases are compatible with the evolution of the rock chemistry. However, primitive compositions of plagioclase (An 80 ) and clinopyroxene (diopside) persist in the cores of phenocrysts, even at the trachytic and alkali trachytic stage. Fractional crystallization within a shallow magma chamber has been the dominant process for the generation of Phlegraean rock series. The volume of the magma chamber is estimated to have been at least 240 km 3 at the moment of the eruption of the Campanian Ignimbrite, nearly 35,000 yr. ago. This event was followed by a large caldera collapse. The depth of the chamber cannot be precisely evaluated. However, its top must have been very shallow, probably at 4–5 km, as suggested by contact metamorphic rocks obtained from deep geothermal wells within the caldera. Volcanological and petrological data favor a model of upward migration of lighter liquids produced mostly by fractionation along the cool walls of the chamber, the deeper part of which is occupied by a convecting trachybasaltic magma. Progressive migration of eruptive vents toward the caldera center and the contemporaneous strong reduction in the volume of the erupted products, suggest that the chamber behaved as a closed system. The volume of magma was progressively reduced by both cooling and extraction to the surface.

Volcanic ash layers illuminate the resilience of Neanderthals and early modern humans to natural hazards

Marked changes in human dispersal and development during the Middle to Upper Paleolithic transition have been attributed to massive volcanic eruption and/or severe climatic deterioration. We test this concept using records of volcanic ash layers of the Campanian Ignimbrite eruption dated to ca. 40,000 y ago (40 ka B.P.). The distribution of the Campanian Ignimbrite has been enhanced by the discovery of cryptotephra deposits (volcanic ash layers that are not visible to the naked eye) in archaeological cave sequences. They enable us to synchronize archaeological and paleoclimatic records through the period of transition from Neanderthal to the earliest anatomically modern human populations in Europe. Our results confirm that the combined effects of a major volcanic eruption and severe climatic cooling failed to have lasting impacts on Neanderthals or early modern humans in Europe. We infer that modern humans proved a greater competitive threat to indigenous populations than natural disasters.

Magmatic and phreatomagmatic phases in explosive eruptions of Vesuvius as deduced by grain-size and component analysis of the pyroclastic deposits

Abstract Grain-size and component analyses have been carried out on pyroclastic deposits of three well-known explosive eruptions of Vesuvius: those of 79 A.D., 1631 and 1906. These eruptions cover a wide energy spectrum, from Plinian to Strombolian, and each includes a transition from a magmatic to a well-documented and clearly distinguishable phreatomagmatic phase of activity. The chronicles of the eruptions are revisited in light of the granulometric and component data obtained from the study of the deposits and interpretation of the dynamics of the main eruptive episodes is provided for each eruption. The phreatomagmatic products (originated from explosive interaction of magma with groundwater) have characteristic grain-size and component distribution patterns distinct from associated purely magmatic pyroclastic products: in any eruption phreatomagmatic products exhibit: (a) a marked increase in both the lithic/juvenile and the crystal/total juvenile ratios, and (b) a preferential fragmentation of the juvenile fraction. Moreover, eruption energy, degree of evolution of erupted magma, lithic content and depth of provenance are all clearly correlated. The abundance of lithics and crystals in phreatomagmatic deposits can be ascribed to (a) preferential fragmentation of the aquifer-hosting rocks due to explosive vaporization of ground water; (b) vent flaring and craterization possibly related to overpressure conditions following the entrance of large quantities of steam in the conduit; (c) indirect enrichment in both crystal and lithic fractions by removal of juvenile fines from the eruptive cloud. In each eruption some of the deposits with “phreatomagmatic” component distribution patterns also exhibit evidence of steam condensation (accretionary lapilli, vesiculated tuff, mud coating, soft deformations) and some do not: these are therefore to be considered sufficient but not necessary diagnostic features for a hydromagmatic origin of the deposit (“wet” in the sense of Sheridan and Wohletz, 1983). “Dry” phreatomagmatic products correspond to higher energy conditions allowing production and maintenance of overheated steam. The magma/water ratio in terms of heat exchange surface (i.e. the degree of primary fragmentation of the magma) is certainly the main factor controlling the efficiency and the energy balance of the interaction. A series of observations strongly suggest that a primarily fragmented magma is a basic requisite for the explosive magma/water interaction: (a) occurrence of short phreatomagmatic episodes interrupting lava fountains activity (1906 eruption) in connection with drawdown of the magma column in the conduit; (b) parallel increase in depth of magma fragmentation level and magma/water interaction level (as deduced by the nature of lithics), and (c) preferential occurrence of phreatomagmatic activity at the end of explosive eruptions (when juvenile gas pressure declines and water from the confining aquifer more easily enters the conduit).

The transition from calc-alkaline to potassic orogenic magmatism in the Aeolian Islands, Southern Italy

The Aeolian volcanic arc displays a wide range of magmatic products. Mafic lavas range from hypersthene normative calc-alkaline basalts to silica-undersaturated potassic absarokites, although the former are spatially and temporally dominant, consistent with the subduction-zone tectonic setting. In addition, intermediate and acidic members of the various fractionation series may be recognised. Large variations in trace element and isotope ratios accompany the rapid calc-alkaline to potassic transition, and it is argued that these may be largely explained in terms of subduction-zone mantle enrichment involving components derived from both basaltic ocean crust and subducted sediments. In addition, it seems that the mantle wedge itself was substantially heterogeneous prior to the onset of subduction zone processes. Not only are these subduction components similar to those proposed in a number of island arcs, but they also resemble those recognised in the ultra-potassic lavas of the Roman province, supporting recent subduction-related petrogenetic models of the Roman magmatism. Although subducted sediment plays an important role in the generation of some potassic magmatism, it is not uniquely responsible for K2O-rich lavas, which are also produced without a large sediment contribution.

The A.D. 472 “Pollena” eruption: volcanological and petrological data for this poorly-known, plinian-type event at vesuvius

Abstract The pyroclastic products of a poorly-known eruption of Vesuvius (ascribed by a combination of historic and radiocarbon data to A.D. 472) have been investigated from both volcanological and petrological points of view. The eruptive sequence starts with pumice-fall deposits (three units can be recognized) that darken upwards where there are sandwave interbeds. Surge deposits cover the pumice-fall bed and thick pyroclastic-flow deposits represent the uppermost levels of the deposit. Isopach maps of both the pumice-fall and pyroclastic-flow deposits led to an estimate of the total volume of tephra of about 0.32 km3. The eruptive sequence and the distribution of lithic ejecta are similar to those of the major Plinian eruptions of Somma-Vesuvius (although the volume involved is significantly lower) and reflect an increase in the hydromagmatic character of the eruption with time. The products range in composition from phonolites (first-erupted) to phonolitic leucitites with gradual changes upwards. Whole rock chemistry and microprobe mineralogy indicate that the Pollena sequence represents a liquid line of descent towards the phonolitic minimum of petrogenys residua system. Fractionation occurred within a shallow magma chamber (PH2O probably slightly higher than 1 kb) and was mainly controlled by leucite and clinopyroxene. The basic parental magma approached the composition of the recent period (A.D. 1631–1944), tephritic leucitites of Vesuvius. The phonolitic magma can be derived from a leucititic parent by fractionating about 50% solid phases. A two-stage fractionation model is suggested: the first stage occurred during the rise of magma from the deep source and the second within the shallow magma chamber. The rate of magma introduction during the 150 to 200 year repose time preceeding the eruption probably averaged 1.2 to 1.7 × 10−3 km3-yr−1. These conditions were probably favorable for the occurrence of magma-mixing within the convecting zone of the magma chamber.

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.

Plinian and Subplinian Eruptions

Abstract The term “plinian” encompasses powerful explosive eruptions characterized by the quasi-steady, hours-long, high-speed discharge into the atmosphere of a high-temperature, multiphase mixture (gas, solid, and liquid particles), forming a buoyant vertical column that reaches heights of tens of kilometers and often alternates with phases of column collapse. Subplinian eruptions have lower intensity but dynamics similar to plinian events, mainly distinguishing by the occurrence of high-frequency fluctuations or of temporary breaks in the discharge. In these eruptions, the repeated generation of short-lived convective plumes may alternate with phases of quiescence or of lower intensity, explosive or effusive activity.The complex dynamics of all these events is modulated by the conditions of magma withdrawal from crustal reservoirs, and ascent, fragmentation, and dispersal in the atmosphere. The accurate observation of the sedimentological and compositional features of the deposits is a powerful tool to quantify the main parameters which control these eruptions.