Volcanological Evolution Research Articles

The Cerro Guacha Caldera complex, SW Bolivia: A long-lived, multicyclic, resurgent caldera complex in the Altiplano-Puna Volcanic Complex of the Central Andes

Measuring ∼50 × 30 km, the Cerro Guacha Caldera Complex (CGCC) is a polycyclic, nested caldera complex within the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Previous work had established that CGCC was built in three stages. Catastrophic supereruptions of the 1300 km3 (DRE - Dense Rock Equivalent) Guacha ignimbrite at ca. 5.65 Ma and the 800 km3 (DRE) Tara ignimbrite at ca. 3.49 Ma resulted in two nested caldera collapses that define the first two stages (G1C and G2C) of the CGCC. The third stage, G3C, initiated ∼1.8 Ma with the eruption of the Puripica Chico ignimbrite and associated lavas. The only previous detailed systematic study of the CGCC was of the G2C (Grocke et al., 2017a, 2017b). In this work we provide new field, petrochemical and geochronological data on the first and third cycles that now allow us to present the first comprehensive account of the eruption history, as well as the magmatic and volcanological evolution of the CGCC as a long-lived caldera complex.Erupted compositions during the G1C and G3C cycles are dominantly high-K, calc-alkaline dacites and minor andesites and these fall within the andesite to rhyolite compositional array previously defined for the G2C. In the CGCC as a whole, dacitic compositions account for over 90% of the ∼2200 km3 of magma erupted. These data reveal a consistency of magma composition, storage and evolution throughout the >4 Ma history of the CGCC.New UPb in zircon ages reveal distinct zircon age distributions that indicate that each caldera cycle represented new magmatic cycles that accumulated 10's to 100's of thousands of years prior to eruption. We find zircon antecrysts in all eruptive stages which we interpret as assimilation of non-erupted progenitors of earlier magmatic cycles. Occassional zircon xenocrysts and cores are interpreted as assimilation of upper crustal basement lithologies, supporting previous interpretations of upper crustal open system magmatic evolution for the G2C to the entire CGCC.Both the G1C and G2C calderas are west-hinged trap-door style collapses. Of these, the 5.65 Ma G1C caldera measures ∼50 km × 30 km, whereas the 3.49 Ma G2C is 30 × 20 km and is nested within the G1C. Resurgent uplift in both the calderas reveals at least a kilometer thickness of intracaldera ignimbrite. Outflow ignimbrites from the CGCC extend 100 km N-S and 40 km E-W around CGCC and were topographically limited. Intracaldera ignimbrite volume dominates outflow volumes by a ratio of ∼5:1. Repose periods between successive stages are now calculated to be 2.1 (G1C to G2C) and 1.8 (G2C to G3C) Myr respectively.The evolution of the CGCC parallels other APVC calderas and is closely linked to the history of the APVC flare-up. The two supereruptions at 5.65 Ma (Guacha Ignimbrite) and 3.49 Ma (Tara Ignimbrite) coincide with the peak of the flare-up and its eruptive and magmatic tempo correlate with the regional tempo. The eruptive and magmatic histories connote a composite granodioritic subcaldera pluton of significant extent accumulated in the upper crust beneath the CGCC.

Paleovolcanology, geochemistry, and zircon U-Pb-Hf isotopes of the volcano-sedimentary sequences from the Ediacaran Campo Alegre-Corupá Basin, southern Brazil: Linking volcanism and tectonics during Western Gondwana consolidation

The Campo Alegre-Corupá Basin (CACB) records a well-documented sequence of volcano-sedimentary episodes associated with specific periods of crustal extension during the late-convergent to post-collisional tectonic stages of the consolidation of Western Gondwana. Two main volcano-sedimentary stages were detailed through stratigraphic and lithofaciological studies, combined with whole-rock geochemistry and zircon U-Pb-Hf analysis, resulting in a more complete understanding of the tectono-magmatic cycles and volcanological evolution of the CACB. During the Basin Stage (605–590 Ma), regional collisional tectonics was responsible for initiating the basin as a syn-orogenic rift, during which the sedimentary cycle progressed from alluvial fans at the northern limit of the basin to braided rivers and into a lake to the south. U-Pb geochronology and Lu-Hf isotope signatures of detrital zircon revealed that the sediments were mainly derived from the Paleoproterozoic basement of the Luis Alves Terrane, which constitutes the direct basement of the CACB, with contributions from the nearby Piên Magmatic Arc and possibly from the Curitiba Terrane to the North. The sedimentary strata were progressively covered by transitional to mildly alkaline OIB-like basalts, occurring mostly as lava flows with structural aspects suggestive of subaqueous to subaerial environments, interbedded with trachydacites and surge-like pyroclastic deposits. Large volumes of widespread and densely-welded to rheomorphic ignimbrite sheets within the CACB attest to a second infilling period corresponding to the Caldera Stage (583–577 Ma). This stage was inaugurated by the outpouring of trachytic and very subordinated IAB-like basaltic lava flows at ∼583 Ma. The caldera eruption seems to have been initiated by the production of pumiceous fallouts during an early Plinian-like period of decompression of shallow magma chambers at ∼580 Ma, during which the rift-related structural framework of the Basin Stage probably controlled the collapse of a graben-like caldera. Compositional and isotopic signatures suggest lithospheric mantle sources for the magmas from both volcanic stages and support a connection between basic-intermediate and silicic rocks from both bimodal associations. They also suggest a cogenetic nature between the silicic rocks from the Caldera Stage and the A-type granitoids from the nearby plutonic Graciosa Province.

Volcanological evolution and paleoenvironment of the Late Cretaceous Changyu volcano in the coastal SE China

火山是唯一能够直接反映地球深部存在岩浆的地球动力学现象，对于地球气候演变和宜居性具有重要影响。相比年轻火山，古老火山因其活动过程完整、且可能剥蚀出露多阶段火山喷发产物及岩浆通道、岩浆房等，从而为揭示火山岩浆系统演化和火山活动过程提供重要的研究窗口。本文选择中国东南沿海晚白垩世长屿破火山为研究对象，对其开展火山地质、岩石学、年代学及地球化学研究，以期揭示其火山活动历史及其对中国东南沿海白垩纪古环境的启示意义。长屿火山出露面积约100km²，呈近圆形分布，具有较厚的火山地层厚度(总厚度约570m)，类似于破火山内的火山堆积特点。由早至晚三个喷发阶段形成的流纹质角砾凝灰岩都显示了典型的火山碎屑流相的特点，发育典型的条纹斑状结构，但表现出不同的晶屑、玻屑及岩屑含量以及熔结程度等岩相学特征，反映火山喷发从初始到高峰、再到减弱的过程，最后岩浆沿火山通道侵出形成流纹斑岩穹隆，标志着火山活动的结束。长屿火山的火山碎屑流式喷发伴随着快速的岩浆房塌陷，以及缺少普林尼式空落堆积，反映了火山活动发生在伸展的构造背景。系统的LA-ICP-MS锆石U-Pb年代学研究获得了不同阶段火山岩一致的形成年龄(97~96Ma)，暗示它们具有较短的形成时限，是由同期火山岩浆活动先后喷发形成的。长屿火山岩高的SiO₂含量(67%~76%)以及分异的锆石微量元素地球化学特征，表明喷发岩浆来自晶粥提取的熔体，并有晶粥来源晶体的混入。此外，我们在长屿火山岩中发现了南洋杉型丝炭化木(贝壳杉型木属)，暗示晚白垩世中国东南沿海地区可能为温暖湿润的亚热带山地气候环境，表明中国东南沿海白垩纪大规模火山活动，在古武夷山脉以东地区形成了地形高耸的海岸山脉。

New insights into the recent eruptive and collapse history of Montagne Pelée (Lesser Antilles Arc) from offshore marine drilling site U1401A (IODP Expedition 340)

Marine tephrochronology provides a good alternative to study the eruptive history of volcanic islands and overcome the problem of poorly preserved on-land outcropping. Here, we provide new observations on the recent volcanological evolution (<40 kyrs) of Montagne Pelée (Martinique, Lesser Antilles) based on the tephrochonological study of a marine core from Site U1401, sampled during IODP Expedition 340 and located 28 km from the coastline, west off Martinique Island. The core (15 m recovered length) was obtained on the top of the debris avalanche deposits due to the last flank collapse that occurred on Montagne Pelée volcano. Although it was not possible to drill through the debris avalanche deposits because of the heterogeneity of the deposit and the presence of large blocks, the sediments and volcanic deposits that covered the debris avalanche deposits were sampled. A detailed multiparameter study (geophysical data acquired on-board during the IODP cruise, lithological and geochemical data and temporal constrains through 18O stratigraphy and 14C dating) of the core U1401A provides a new age for the last flank collapse leading us to update the recent volcanological history of Montagne Pelée. The last flank collapse is now dated at ~36 cal. ka BP (older than previous studies). The flank collapse, even though relatively minor in volume (2 km3), had a significant consequence on the magma plumbing system of Montagne Pelée and produced abundant explosive eruptions of basaltic-andesite magmas, during the period 36–25 cal. ka BP. This new age range obtained for this flank collapse has important implications for the post collapse activity and allows us to rethink the recent volcanological history of Montagne Pelée.

The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data

The study of Paleoproterozoic rocks is crucial for understanding Earth's tectonic evolution during the time when most of the modern crust and ore deposits were formed. The rocks of the Brazilian Amazonian Craton record some of the most-complete and best-preserved Paleoproterozoic magmatic and volcanic episodes on Earth. Following previous investigations, we present new lithofaciological and stratigraphic records of the felsic rocks of the Tapajós Mineral Province (TMP) (~2–1.88 Ga) and the São Felix do Xingú region (SFX) (~1.88 Ga) which, combined with new petrological and geochronological data, help providing a more complete understanding of the tectonic, magmatic and volcanological evolution of the Amazonian Craton. This magmatism/volcanism is thought to be formed in a late-/post-orogenic to extentional regime confirmed by the new geochemical data presented here. The transition from late-convergent to extensional tectonic setting could register the beginning of the taphrogenesis that marked the Amazonian Craton throughout the Mesoproterozoic. The volcanological approach of this contribution can serve as a strategy for the modelling of the evolution of Precambrian volcano-sedimentary basins around the world. The large amount of rocks analyzed are divided into primary and secondary volcaniclastic products depending on if they resulted from a direct volcanic activity (pyroclastic) or processes that reworked pyroclastic fragments. Furthermore, the deposits are subdivided into massive and stratified, depending on their primary mechanisms of transport and emplacement. By confirming the results from previous studies, our study permits to depict a more precise paleo-environmental picture of the processes that occurred in the Amazonian Craton during the Late-Paleoproterozoic. In particular, the presence of large regional-scale fissural systems and caldera collapses produced large silicic explosive volcanic eruptions, also accompanied by the emission of large volume effusive products. Although studies on the Amazonian Craton are still scarce and controversial, the present study provides new evidence that this volcanism may have formed one of the largest Silicic Large Igneous Provinces (SLIP) on earth. Our data also confirm that at least two major Paleoproterozoic periods of formation of volcanic rocks exist in the Amazonian craton. This point is of great relevance for any future interpretation of the geological evolution of this craton.

New insights on lithofacies architecture, sedimentological characteristics and volcanological evolution of pre-caldera (> 22 ka), multi-phase, scoria- and spatter-cones at Somma-Vesuvius

Pre-caldera (>22ka) lateral activity at Somma-Vesuvius is related to scoria- and spatter-cone forming events of monogenetic or polygenetic nature. A new stratigraphic, sedimentological, textural and lithofacies investigation was performed on five parasitic cones (Pollena cones, Traianello cone, S. Maria a Castello cone and the recently found Terzigno cone) occurring below the Pomici di Base (22ka) Plinian products emplaced during the first caldera collapse at Somma-Vesuvius. A new Ar/Ar age of 23.6±0.3ka obtained for the Traianello cone as well as the absence of a paleosol or reworked material between the S. Maria a Castello cone and the Pomici di Base deposits suggest that such cone-forming eruptions occurred near the upper limit of the pre-caldera period (22–39ky). The stratigraphy of three of these eccentric cones (Pollena cones and Traianello cone) exhibits erosion surfaces, exotic tephras, volcaniclastic layers, paleosols, unconformity and paraconformity between superimposed eruptive units revealing their multi-phase, polygenetic evolution related to activation of separate vents and periods of quiescence. Such eccentric cones have been described as composed of scoria deposits and pure effusive lavas by previous authors. Lavas are here re-interpreted as welded horizons (lava-like) composed of coalesced spatter fragments whose pyroclastic nature is locally revealed by relicts of original fragments and remnants of clast outlines. These welded horizons show, locally, rheomorphic structures allowing to define them as emplaced as clastogenic lava flows. The lava-like facies is transitional, upward and downward, to less welded facies composed of agglutinated to unwelded spatter horizons in which clasts outlines are increasingly discernible. Such textural characteristics and facies variation are consistent with a continuous fall deposition of Hawaiian fire-fountains episodes alternated with Strombolian phases emplacing loose scoria deposits. High enrichment factor values, measured in the scoria deposits, imply the ejection of large proportion of ash even during Strombolian events.

Geochemical and Volcanological Evolution of La Palma, Canary Islands

We present a comprehensive major and trace element and Sr–Nd–Pb isotope dataset from the major volcanic units exposed on La Palma and show how geochemical and volcanic evolution can be linked to asthenospheric and lithospheric processes. Lavas from the northern shield (from Basal Complex to Taburiente to Bejenado volcanism, 3–4 Ma to 400 ka) become more alkalic and SiO2-undersaturated with decreasing age, but show little change in MgO-normalized trace element compositions. Their high (Nb,Ta)/U and Ba/Th but low La/Nb ratios suggest assimilation of amphibole, probably in the lithospheric mantle that was metasomatized by earlier melts. Lavas from the Cumbre Vieja unit (<125 ka) in the southern half of La Palma are more incompatible-element enriched and probably formed through lower degrees of melting than those from the northern shield, which are nearly identical isotopically. Their Nb/U ratios are mostly within the range 47 ± 10, significantly below those of the earlier lavas. In 206Pb/204Pb versus 143Nd/144Nd, 208Pb/204Pb and 208Pb/206Pb isotope diagrams, the Basal Complex rocks and lavas from the adjacent El Hierro island form a separate trend compared with the younger subaerial La Palma lavas. Both groups share a common depleted end-member but require separate, enriched HIMU-like end-members, believed to be located within the asthenosphere. The temporal and spatial variations in the composition of La Palma and El Hierro lavas could be explained within the context of NE-directed plate motion over a zoned Canary plume. After La Palma moved away from the asthenospheric source domain of the Basal Complex, El Hierro formed above the same domain, whereas the younger units on La Palma tapped a distinct asthenospheric domain located further north. The short-lived Bejenado volcano that formed directly after the giant Cumbre Nueva sector collapse at c. 560 ka produced the isotopically most depleted lavas reported from La Palma thus far. Their compositions suggest incorporation of a depleted pyroxenitic component. The Bejenado lavas also extend to the highest Nb/U and Ba/Th and lowest La/Nb ratios of all La Palma lavas, consistent with increased melting of amphibole within the lithospheric mantle or lower crust. We propose that the collapse is related to the migration of magmatism to the south of La Palma, and led to short-term enhanced decompression melting of amphibole and pyroxenite within the lithosphere.

Interactions between Late Quaternary volcanic and sedimentary processes in the Naples Bay, Southern Tyrrhenian sea

The interactions between Late Quaternary volcanic and sedimentary processes in the Naples Bay, Southern Tyrrhenian sea, are here discussed through the results of the marine geological survey at the scale 1:25.000. The example of the geological map n. 465 “Isola di Procida”, herein presented, has put in evidence the stratigraphy of marine Quaternary deposits and related volcanic seismic units. The volcanic deposits cropping out in the Procida island have been explained as the result of eruptions of local eruptive centres. The geological survey carried out onshore indicates the occurrence of several pyroclastic units linked to the eruptive activity of the Ischian and Phlegrean volcanic complexes, interstratified with the products erupted by local volcanic centres. The occurrence in the pyroclastic sequences of Ischia and Procida islands of several marker horizons and their stratigraphic correlations have allowed to reconstruct the volcanological evolution of the two islands and the interactions with sedimentary processes at the scale of the whole Tyrrhenian margin. Four geological maps at the scale 1:25.000 covering the whole Naples Bay have been reconstructed based on the interpretation of marine geological and geophysical data. The stratigraphic relationships between the seismic units and the eruptive deposits have testified the activity of several monogenetic volcanic centers, whose products are interstratified with marine and continental deposits of the Late Quaternary depositional sequence.

A reappraisal of the eruptive history and recent (1991-2009) volcanic eruptions of the Barren Island, Andaman Sea

The Barren Island volcano in the Western Sunda Arc has displayed explosive Strombolian eruptions for more than two decades. This recent explosive event, together with the historic and prehistoric volcanic landforms, present reliable information about explosive Strombolian eruptions and the volcanological evolution of the Barren Island volcano. This study is a re-evaluation of existing knowledge and incorporates new information and interpretations of the recent and past volcanic activity on Barren Island. Direct observations of explosive eruptions since 1991 showed discrete events of bursting and ballistic transport of blocks and formation of sustained ash plumes, indicating Strombolian and violent Strombolian eruptions. Active lava flows were not seen during the observations which, instead, reveal intact preservation of the historic lava flows. A prehistoric mafic stratocone with a central depression (caldera), a central scoria cone with summit crater and abundant basaltic lava flows of historic eruptions and the scoria cones of the recent activity are the major volcanic landforms. They bear evidence of alternating effusive and explosive activity during prehistoric times accompanied by caldera forming activity; scoria cone Strombolian activity switches over to effusive events during the historic period and exclusively Strombolian activity during recent times. The results of this study differ from previous studies that interpreted several episodes of active lava flows and Hawaiian, Plinian and Vulcanian styles of the recent eruptions. It also provides new insights into the volcanological evolution of the Barren Island volcano which is crucial in understanding the future behaviour of the volcano and risk assessment.

Volcanological evolution of the Rivi–Capo Volcanic Complex at Salina, Aeolian Islands: magma storage processes and ascent dynamics

Lava flows and pyroclastic deposits from strombolian fallout related to the activity of the Rivi and Capo volcanoes, which are representative of early subaerial volcanoes on Salina (Aeolian Islands), have been investigated through a geological–petrological approach. Our geological field survey shows that Rivi and Capo volcanoes are part of a single N50°E aligned volcanic complex, here named Rivi–Capo Volcanic Complex (RCVC). Stratigraphically specific rock sampling has allowed reconstruction of the magma feeding processes through time. Whole rock major element compositions, together with core-to-rim profiles of plagioclase and clinopyroxene crystals, show a general evolution toward more basic compositions through the three formations constituting the Capo volcano and within the Rivi center. MELTS simulations and mass balance modeling suggest that the RCVC rocks are the result of fractional crystallization of plagioclase, clinopyroxene, and olivine (ca. 45 % of solid removed) from a primary magma. In addition to fractional crystallization, continuous recharge and mixing with more basic magma coming from deeper parts of the magmatic plumbing system contributed to the final volcanic rock compositions. Our textural and microanalytical data on plagioclase and clinopyroxene crystals allow the definition of a multilevel magmatic storage system with reservoirs at ~20 and ~3 km below sea level. When processes of magma differentiation, ascent, and storage are considered together with the stratigraphic position of each sample, a history of continuous modification of the RCVC plumbing system can be constructed. Volcanism may have been characterized by fissure-type eruptions during the early stages (Lower Capo, Lower Rivi, and Middle Capo Formations), gradually changing later to central-type volcanism (Upper Capo and Upper Rivi Formations).

High resolution 3D P wave velocity structure beneath Tenerife Island (Canary Islands, Spain) based on tomographic inversion of active‐source data

We present a high resolution 3 dimensional (3D) P wave velocity model for Tenerife Island, Canaries, covering the top of Teide volcano (3,718 m a.s.l.) down to around 8 km below sea level (b.s.l). The tomographic inversion is based on a large data set of travel times obtained from a 3D active seismic experiment using offshore shots (air guns) recorded at more than 100 onshore seismic stations. The obtained seismic velocity structure is strongly heterogeneous with significant (up to 40%) lateral variations. The main volcanic structure of the Las Cañadas‐Teide‐Pico Viejo Complex (CTPVC) is characterized by a high P wave velocity body, similar to many other stratovolcanoes. The presence of different high P wave velocity regions inside the CTPVC may be related to the geological and volcanological evolution of the system. The presence of high P wave velocities at the center of the island is interpreted as evidence for a single central volcanic source for the formation of Tenerife. Furthermore, reduced P wave velocities are found in a small confined region in CTPVC and are more likely related to hydrothermal alteration, as indicated by the existence of fumaroles, than to the presence of a magma chamber beneath the system. In the external regions, surrounding CTPVC a few lower P wave velocity regions can be interpreted as fractured zones, hydrothermal alterations, porous materials and thick volcaniclastic deposits.

Volcanological evolution and caldera forming eruptions of Mt. Nemrut (Eastern Turkey)

Mt. Nemrut volcano, situated at the west of Lake Van, is one of the historically active volcanoes of the Eastern Anatolia. It has an 8.5×7km diameter summit caldera. Volcanic activity of Mt. Nemrut started ~1Ma ago; the most recent eruptions were in 1441, 1597 and 1692A.D. Among the Eastern Anatolian volcanoes, Mt. Nemrut is the most hazardous volcano for its vicinity. Present day volcanic activity is represented by intra-caldera hydrothermal and fumarolic output and low-level volcano-seismic events.Geological evolution and chronostratigraphy of the volcano is subdivided in three stages: pre-caldera, syn-caldera and post-caldera stages. Pre-caldera products are dominated by felsic lava flows and domes. Trachytic Nemrut and Kantaşı pyroclastics represent the caldera forming activity, of which sequences are composed of fallout units and ignimbrite flows. Both Nemrut and Kantaşı ignimbrite units are low-aspect ratio ignimbrites, they are generally densely welded and present columnar jointed outcrops locally. Extent of Nemrut ignimbrite (volume: 32.6km3) is greater than the Kantaşı ignimbrite (volume: 3.8km3). Post-caldera activity of the volcano is marked by peralkaline rhyolitic (comendite) intra-caldera lava flows and explosive hydrovolcanic activities. Historical activity of the volcano is represented by bimodal basaltic–rhyolitic effusive activity along Nemrut rift zone.

Geochemical evidence of the renewal of volcanic activity inferred from CO2 soil and SO2 plume fluxes: the 2007 Stromboli eruption (Italy)

On 27 February 2007, a new eruption occurred on Stromboli which lasted until 2 April. It was characterized by effusive activity on the Sciara del Fuoco and by a paroxysmal event (15 March). This crisis represented an opportunity for us to refine the model that had been developed previously (2002–2003 eruption) and to improve our understanding of the relationship between the magmatic dynamics of the volcano and the geochemical variations in the fluids. In particular, the evaluation of the dynamic equilibrium between the volatiles (CO2 and SO2) released from the magma and the corresponding fluids discharged from the summit area allowed us to evaluate the level of criticality of the volcanic activity. One of the major accomplishments of this study is a 4-year database of summit soil CO2 flux on the basis of which we define the thresholds (low–medium–high) for this parameter that are empirically based on the natural volcanological evolution of Stromboli. The SO2 fluxes of the degassing plume and the CO2 fluxes emitted from the soil at Pizzo Sopra la Fossa are also presented. It is noteworthy that geochemical signals of volcanic unrest have been clearly identified before, during and after the effusive activity. These signals were found almost simultaneously in the degassing plume (SO2 flux) and in soil degassing (CO2 flux) at the summit, although the two degassing processes are shown to be clearly different. The interpretation of the results will be useful for future volcanic surveillance at Stromboli.

Age and whole rock–glass compositions of proximal pyroclastics from the major explosive eruptions of Somma-Vesuvius: A review as a tool for distal tephrostratigraphy

A review of compositional data of the major explosive eruptions of Vesuvius is presented, comparing compositions (major elements) of whole rock with glass shards from the proximal deposits, hopefully useful for long-distance correlation. A critical review of published and new geochronological data is also provided. All available 14C ages are calibrated to give calendar ages useful for the reconstruction of the volcanological evolution of the volcanic complex. The pyroclastic deposits of the four major Plinian eruptions (22,000 yr cal BP “Pomici di Base”, 8900 yr cal BP “Mercato Pumice”, 4300 yr cal BP “Avellino Pumice”, and A.D. 79 “Pompeii Pumice”) are widely dispersed and allow a four-folded, Plinian to Plinian, stratigraphic division: 1. B–M (between Pomici di Base and Mercato); 2. M–A (between Mercato and Avellino); 3. A–P (between Avellino and Pompeii); 4. P–XX (from the Pompeii Pumice to the last erupted products of the XXth century). Within each interval, the age, lithologic and compositional features of pyroclastic deposits of major eruptions, potentially useful for tephrostratigraphic purposes on distal areas, are briefly discussed. The Vesuvius rocks are mostly high Potassic products, widely variable in terms of their silica saturation. They form three groups, different for both composition and age: 1. slightly undersaturated, older than Mercato eruption; 2. mildly undersaturated, from Mercato to Pompeii eruptions; 3. highly undersaturated, younger than Pompeii eruption. For whole rock analyses, the peculiar variations in contents of some major and trace elements as well as different trends in element/element ratios, allow a clear, unequivocal, easy diagnosis of the group they belong. Glass analyses show large compositional overlap between different groups, but selected element vs. element plots are distinctive for the three groups. The comparative analysis of glass and whole rock major element compositions provides reliable geochemical criteria helping in the recognition, frequently not obvious, of distal products from the different single eruptions.

Nature of mantle heterogeneity and its role in the short-term geochemical and volcanological evolution of Mt. Etna (Italy)

Clues for investigating on the origin and short-term geochemical changes of erupted magmas at Mt. Etna have arisen from an extensive sampling of historic (pre-1971) and recent (post-1971) volcanic rocks, mostly carried out on the southern flank of the volcano. Major and trace element data, obtained by WD-XRF and ICP-MS respectively, display rather similar abundances in both the historic and recent products; only K 2O (mean 1.43 and 1.93 wt.% respectively) and Rb (mean 32 and 50 ppm respectively) exhibit a significant time-correlated increase, whereas Th (mean 10.1 and 9.4 ppm respectively) gradually decreases. Radiogenic isotope ratios, obtained by TIMS, increase through time for 87Sr/ 86Sr (from 0.703343 to 0.703624), whereas for Pb (i.e., 206Pb/ 204Pb from 19.93 to 19.73) and 143Nd/ 144Nd (from 0.512908 to 0.512852) the ratios gradually decrease. Finally, δ 18O values, obtained by dual inlet mass spectrometry, slightly decrease from the historic to recent products (mean + 6.6‰). In order to interpret the observed trends, the role of magma evolution processes in the shallow feeding system was first tested by numerical modeling. Results show that both fractional crystallization and crustal assimilation cannot fully account for the observed variations of some trace elements (particularly Rb and Th) as well as of 87Sr/ 86Sr and δ 18O. On the other hand, partial melting modeling, performed with pre-1971 lava compositions equilibrated to those of mantle melts, supports the occurrence of accessory hydrous phases, such as amphibole and phlogopite, in the source of Etnean magmas. Partial melting of a source characterized by increased amounts of hydrous phases appears consistent with the peculiar geochemical features of the post-1971 magmas. Our findings imply that metasomatic influxes could have heterogeneously affected the pristine mantle, and that the degree of partial melting of the volatile-enriched portions increased. In accordance with trace element and isotope ratios, the model proposed for the genesis and evolution of magmas takes into account melting of a depleted mantle re-fertilized (hundreds of Ma ago) by plume-related silicate melts/fluids, inheriting a signature likely derived from ancient hydrothermally altered oceanic crust stored for long times in the mantle. A limited mantle upwelling, due to the complex recent geodynamics of Southern Italy, could have easily triggered partial melting of this heterogeneously metasomatized mantle, which produced magmas differing in their composition depending on the involved mantle domain. Since the increased extent of partial melting results in a larger melt production, our data are also consistent with the observed volcanic behavior, and could account for the increased frequency of eruptive events and emission rates of volatile-rich magmas observed since 1971, and especially after the 2001 eruption.

Volatile-induced magma differentiation in the plumbing system of Mt. Etna volcano (Italy): evidence from glass in tephra of the 2001 eruption

Mount Etna volcano was shaken during the summer 2001 by one of the most singular eruptive episodes of the last centuries. For about 3 weeks, several eruptive fractures developed, emitting lava flows and tephra that significantly modified the landscape of the southern flank of the volcano. This event stimulated the attention of the scientific community especially for the simultaneous emission of petrologically distinct magmas, recognized as coming from different segments of the plumbing system. A stratigraphically controlled sampling of tephra layers was performed at the most active vents of the eruption, in particular at the 2,100 m (CAL) and at the 2,550 m (LAG) scoria cones. Detailed scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS) analyses performed on glasses found in tephra and comparison with lava whole rock compositions indicate an anomalous increase in Ti, Fe, P, and particularly of K and Cl in the upper layers of the LAG sequence. Mass balance and thermodynamic calculations have shown that this enrichment cannot be accounted for by “classical” differentiation processes, such as crystal fractionation and magma mixing. The analysis of petrological features of the magmas involved in the event, integrated with the volcanological evolution, has evidenced the role played by volatiles in controlling the magmatic evolution within the crustal portion of the plumbing system. Volatiles, constituted of H2O, CO2, and Cl-complexes, originated from a deeply seated magma body (DBM). Their upward migration occurred through a fracture network possibly developed by the seismic swarms during the period preceding the event. In the upper portion of the plumbing system, a shallower residing magma body (ABT) had chemical and physical conditions to receive migrating volatiles, which hence dissolved the mobilized elements producing the observed selective enrichment. This volatile-induced differentiation involved exclusively the lowest erupted portion of the ABT magma due to the low velocity of volatiles diffusion within a crystallizing magma body and/or to the short time between volatiles migration and the onset of the eruption. Furthermore, the increased amount of volatiles in this level of the chamber strongly affected the eruptive behavior. In fact, the emission of these products at the LAG vent, towards the end of the eruption, modified the eruptive style from classical strombolian to strongly explosive.

Hawaiian and Strombolian style monogenetic volcanism in the extra-Andean domain of central-west Argentina

In the provinces of Mendoza and La Pampa (central-western Argentina), retroarc volcanism was active from the Miocene through historic times. There are approximately 40 basaltic eruptive centers between 36°S and 37°30′S to the East of 68°40′W. These centers were studied in order to define their volcanological evolution. The vents are constituted of seven lithofacies: i.e., (A-1) weakly welded lapilli and bomb beds; (A-2) agglutinated spatter and bomb beds; (A-3) agglutinated spatter beds; (B) ash beds; (C) lava flows; (D) conduit fill; and (E) dikes. Three eruptive episodes (phases) were identified. The first one generated agglutinated spatter and bomb beds (lithofacies A-2), which constitute the internal part of the edifices. These are followed by weakly welded lapilli and bomb beds (lithofacies A-1) and agglutinated spatter beds (lithofacies A-3). The final episode includes lava flows (lithofacies C), minor agglutinated spatter deposits (lithofacies A-3) and isolated lapilli and bomb clasts. We postulate Hawaiian style fountaining activity as responsible for the generation of agglutinated spatter and bombs beds, and variations between Strombolian and Hawaiian activity for the formation of successions of weakly welded lapilli and bomb beds and agglutinated spatter beds. The vent features and the homogeneity of the erupted magma suggest that the studied volcanoes were monogenetic.

Volcanic geomorphology and tectonics of the Aeolian archipelago (Southern Italy) based on integrated DEM data

The geomorphological and morphometric analysis of the sea floor topography surrounding the Aeolian Islands, South Tyrrhenian Sea, Italy, provides insights into the relationships between the volcanological evolution of the islands and their tectonic features. We constructed geomorphological maps of the submarine portions of the seven large edifices constituting the islands on the basis of a DEM with a 5 m resolution step. These maps include constructional and destructional landforms such as submarine volcanic vents located west of Lipari and north of Alicudi, and hummocky surfaces recognised north of Lipari and Salina. The latter landforms, together with the occurrence of large scars affecting the main edifices on land, suggest that sector collapses affected some islands. Geomorphological data indicate that the location of subaerial and submarine vents is strongly controlled by local tectonic structures striking WNW-ESE (Alicudi-Filicudi sector), NNW-SSE (Salina-Lipari-Vulcano sector) and NE-SW (Panarea-Stromboli sector). The islands can be divided into two groups on the basis of some morphometric parameters: a first group with a pancake-like shape, Dp/D (abrasion platform diameter/basal diameter) higher than 0.40 and H/D (total height/basal diameter) lower than 0.13, and a second group with a conical shape, characterised by Dp/D lower than 0.34 and H/D higher than 0.14. These ratios and other morphometric parameters reflect the different volcanological and structural evolution of the Aeolian Islands. The pancake-like shaped complexes have been created, in addition to their submarine stage, by extrusive and highly explosive activity, whereas the cone-shaped edifices have been characterised by effusive or moderate explosive activity.

Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad

Tibesti volcanoes (Chad) are poorly documented, as much for volcanological evolution as for mineralogy and geochemistry. Emi Koussi volcano is a wide ignimbritic shield-like volcano that results from the succession of three eruptive sequences, each one related to a caldera collapse. Activity occurred from 2.4 Ma to the recent Quaternary. The continental alkaline volcanics of Emi Koussi exhibit two bimodal lava series, silica-saturated (up to 11% normative quartz) and silica-undersaturated (up to 29% normative nepheline). Basanite and tephrite, trachyte and phonolite are dominant while trachyandesite is very scarce. The basanites and tephrites contain olivine, diopside, plagioclase, nepheline, kaersutite and titanomagnetite. The trachytes contain alkali feldspar, plagioclase, Na–Ca amphibole (kataphorite, richterite), mica, augite, aegirine augite, titanomagnetite and ilmenite. In phonolites, alkali feldspar, andesine, nepheline or sodalite, hedenbergite, aegirine augite, amphibole (ferropargasite and kaersutite), titanite and titanomagnetite occur. Trachyandesite contains olivine, augite, edenite, phlogopite, andesine, alkali feldspar, titanomagnetite and ilmenite. The two suites exhibit a wide compositional gap of about 10% SiO 2. Trachytes are peralkaline and comenditic. Major and trace element data are compatible with differentiation dominated by fractional crystallisation. The fractional crystallisation has been modelled using major-element least-squares mass balance models. Crystal fractionation involving olivine, clinopyroxene, amphibole and plagioclase is the best way to explain the petrogenesis of the undersaturated suite. The silica-saturated suite recorded removal of olivine, pyroxene, amphibole, mica, plagioclase, alkali feldspar, magnetite and apatite. Trace element data, including rare earth elements, calculated using the Rayleigh equation, confirm the models for the undersaturated suite, because there is a close agreement between the observed and calculated data. But the petrogenesis of the silica-saturated suite is probably related to other processes, such as crustal contamination.

Structure and evolution of the Bay of Pozzuoli (Italy) using marine seismic reflection data: implications for collapse of the Campi Flegrei caldera

Digital marine seismic reflection data acquired in 1973 in the Bay of Pozzuoli, and recently reprocessed, were used to study the volcanological evolution of the marine sector of Campi Flegrei Caldera during the last 37 ka. In order to gain more information, interpretation also involved estimation of the "pseudo-velocity" and the "pseudo-density" from the resistivity logs of two onshore deep exploration wells. The main results are: (1) discovery of ancient pre-18 ka and post-37 ka submarine and mainly effusive volcanic activity, along coeval emission centers located at the edges of Campi Flegrei Caldera; (2) confirmation that the caldera collapse in the marine sector of Campi Flegrei seems strongly controlled by regional NE–SW and NW–SE structural discontinuities; (3) the finding of at least two episodes of collapse in the bay; and (4) identification of a post-18 ka volcanic deflation phase that has caused about 150–200 m of subsidence in the central sector of the Bay of Pozzuoli in the last 18 ka.