Volcanological Evidence Research Articles

Geochemical and Volcanological Criteria in Assessing the Links between Volcanism and VMS Deposits: A Case on the Iberian Pyrite Belt, Spain

VMS deposits in the Iberian Pyrite Belt (IPB), Spain and Portugal, constitute the largest accumulation of these deposits on Earth. Although several factors account for their genetic interpretation, a link between volcanism and mineralization is generally accepted. In many VMS districts, research is focused on the geochemical discrimination between barren and fertile volcanic rocks, these latter being a proxy of VMS mineralization. Additionally, the volcanological study of igneous successions sheds light on the environment at which volcanic rocks were emplaced, showing an emplacement depth consistent with that required for VMS formation. We describe a case on the El Almendro–Villanueva de los Castillejos (EAVC) succession, Spanish IPB, where abundant felsic volcanic rocks occur. According to the available evidence, their geochemical features, εNd signature and U–Pb dates suggest a possible link to VMS deposits. However, (paleo)volcanological evidence here indicates pyroclastic emplacement in a shallow water environment. We infer that such a shallow environment precluded VMS generation, a conclusion that is consistent with the absence of massive deposits all along this area. We also show that this interpretation lends additional support to previous models of the whole IPB, suggesting that compartmentalization of the belt had a major role in determining the sites of VMS deposition.

Volcanic geology and petrochemistry of Ashikule volcanic field in western Kunlun Mountains of the Tibetan plateau

Abstract Ashikule volcanic field, developed in the western Kunlun mountain of north Tibet, is composed of approximately 10 volcanoes, and covers a total area of about 200 km 2 at an average altitude of approximately 5000 m, one of the highest volcanic fields in the world. In this study, we conducted detailed field investigations of the geological and geomorphological features of volcanic rocks and volcanic edifices in Ashikule basin, and compiled a large-scale geological map of the study area for the first time. We also collected a series of samples for petrochemistry analysis, as well as a high-precision 40 Ar/ 39 Ar geochronology study of selected lavas from Ashikule volcanic field. Finally, we refined the eruption history for the volcanic activity in the basin, which provides some new volcanological evidence for the study of the tectonic evolution of the Northern Tibetan Plateau.

Holocene eruptions of Mt. Popa, Myanmar: Volcanological evidence of the ongoing subduction of Indian Plate along Arakan Trench

In the western part of Myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of Indian Plate under Eurasian Plate. Volcanoes of Burma arc, however, did not produce eruptions in the recorded history and are considered extinct. Such perception questions the ongoing subduction, as well as keeping local officials unaware of possibility of future volcanic eruptions in the country. We have investigated the youngest lava flows and pyroclasts of Mt. Popa, which is the best-preserved polygenetic volcanic edifice in Myanmar. The Ar/Ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately. The radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that Mt. Popa produced several eruptions in the beginning of Holocene. The youngest eruption occurred ~8000 BP and included the 1.3 km3 gravitational collapse of the volcanic cone with deposition of the 11-km-long debris avalanche, immediately followed by emplacement of 0.1 km3 of pyroclastic flow of calc-alkaline basaltic andesite composition. The collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault, which was formed during the pre-collapse asymmetric gravitational spreading of the volcanic cone. The recently erupted products are geochemically similar to products of young monogenetic volcanoes of Monywa area located 150 km north Mt. Popa, and both display patterns consistent with magma generation at an active subduction system. Low magma production rate of Mt. Popa (3 × 10−5 km3/year averaged for the 1 Ma of the volcano history) is in agreement with the highly oblique angle and slow rate of the subduction. The fact of the Holocene eruptions of calc-alkaline composition in Burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between India and Asia.

The last Vesuvius eruption in March 1944: reconstruction of the eruptive dynamic and its impact on the environment and people through witness reports and volcanological evidence

Mount Vesuvius last erupted in March 1944. The eruption began with a modest effusive activity (18–21 March, phase I) and then moved on to a lava-fountain phase (21–22 March, phase II) that quickly culminated in a 24-h paroxysmal phase (22–23 March, phase III), during which the eruptive column reached its maximum altitude and ash carried by the wind was deposited at distances of up to 500 km from the volcano. Subsequently (phase IV), the ash cloud reached altitudes not exceeding 2 km above the crater and the explosions became discontinuous, alternating with frequent tremors. From April 7th the vent remained permanently closed. Since then, the current period of quiescence may be said to have begun. In our study we present a detailed geochemical study of the volcanic products emitted throughout the stages of the eruption, including an extreme distal ash sample, collected at the time of the eruption in Albania (Devoli) by the geologist Antonio Lazzari. The results indicate that magmatic differentiation took place in two crystallization stages under different temperature and pressure. Moreover, through the use of a numerical model of volcanic ash dispersion in the atmosphere as well as the collection of new witness accounts, we inferred the maximum height reached by the eruptive column (>8–10 km above the crater) as well as its impact on the environment and people, suggesting that the damage in the area and even the number of the victims could be higher than accepted until now.

High‐precision locations of the microseismicity preceding the 2002–2003 Mt. Etna eruption

To recognize possible signals of intrusive processes leading to the last 2002–2003 flank eruption at Mt. Etna, we analyzed the spatial pattern of microseismicity between August 2001 and October 2002 and calculated 23 fault plane solutions (FPSs) for shocks with magnitude greater than 2.5. By applying the double‐difference approach of Waldhauser and Ellsworth [2000] on 3D locations, we found that most of the scattered epicentral locations further collapse in roughly linear features. High‐precision locations evidenced a distribution of earthquakes along two main alignments, oriented NE‐SW to ENE‐WSW and NW‐SE, matching well both with the known tectonic and volcanic lineaments of Etna and FPSs results. Moreover, microseismicity and swarms located along the NNW‐SSE volcano‐genetic trend suggest, together with geodetic data and volcanological evidence that progressive magma refilling has occurred since February 2002.

Evidence-based volcanology: application to eruption crises

The way in which strands of uncertain volcanological evidence can be used for decision-making, and the weight that should be given them, is a problem requiring formulation in terms of the logical principles of Evidence Science. The basic ideas are outlined using the explosion at Galeras volcano in Colombia in January 1993 as an example. Our retrospective analysis suggests that if a robust precautionary appraisal had been made of the circumstances in which distinctive tornillo signals were detected at Galeras, those events might have been construed as stronger precursory evidence for imminent explosive activity than were the indications for quiescence, given by the absence of other warning traits. However, whilst visits to the crater might have been recognised as involving elevated risk if this form of analysis had been applied to the situation in January 1993, a traditional scientific consideration of the available information was likely to have provided a neutral assessment of short-term risk levels. We use these inferences not to criticise interpretations or decisions made at the time, but to illustrate how a structured, evidence-based analysis procedure might have provided a different perspective to that derived from the conventional scientific standpoint. We advocate a formalism that may aid such decision-making in future: graphical Bayesian Belief Networks are introduced as a tool for performing the necessary numerical procedures. With this approach, Evidence Science concepts can be incorporated rationally, efficiently and reliably into decision support during volcanic crises.

The role of tephrachronology in recognizing synchronous caldera-forming events at the Quaternary volcanoes Longonot and Suswa, south Kenya Rift

Abstract The Quaternary volcanoes Longonot and Suswa, located in the south Kenya Rift, each have a caldera formed by incremental collapse accompanied by airfall, pyroclastic flow and pyroclastic surge activity. To constrain the relative ages of caldera formation at these two closely spaced centres, syn-caldera deposits are correlated using tephrachronology. Volcanic products from Longonot and Suswa display contrasting Nb/Zr ratios which provide a unique signature for each centre. Early syn-caldera pyroclastics at Suswa contain airfall pumice beds which have Longonot Nb/Zr signatures. Variation of FeO(T) and Al 2 O 3 with Zr is particularly effective at discriminating the various Longonot units in two component space, and has been combined with volcanological evidence to identify the equivalent Longonot deposits as either early syn-caldera or late syn-caldera. In both cases, the correlation indicates synchronous initiation of caldera formation. This suggests that caldera formation is a regional rather than local event in the southern part of the rift, highlighting the potential rôle of regional tectonics as a controlling mechanism. Major alignments of eruptive vents, fissures and the caldera collapse centre at each volcano are parallel to rift faults, suggesting that rift floor tension fractures or normal faults are utilized as conduits for magma transport. Synchronous caldera formation at Longonot and Suswa may, therefore, be linked to lateral magma withdrawal along rift floor fractures and decompression of the shallow-level magma chambers during a period of rift floor extension.

Stratospheric aerosol optical depths, 1850–1990

A global stratospheric aerosol database employed for climate simulations is described. For the period 1883–1990, aerosol optical depths are estimated from optical extinction data, whose quality increases with time over that period. For the period 1850–1882, aerosol optical depths are more crudely estimated from volcanological evidence for the volume of ejecta from major known volcanoes. The data set is available over Internet.

Cenozoic magmatism of the valencia trough (western mediterranean): Relationship between structural evolution and volcanism ∗

The Cenozoic magmatism of the Valencia trough is characterized by the existence of two volcanic cycles. The first cycle (Early Miocene-Middle Miocene) comprises calc-alkaline rocks which have been recognized in the central to eastern offshore areas and in Mallorca. The second cycle (Middle Miocene to Recent) is represented mainly by poorly differentiated alkaline rocks which are predominantly distributed along the Iberian margin of the Valencia trough. In accordance with the inferred structural evolution of the Valencia trough, petrological and geochemical data, as well as volcanological evidence, we suggest that two different geodynamic environments existed during these magmatic cycles. The Early to Middle Miocene calc-alkaline volcanism developed under compressive tectonic conditions and seems to be associated with the northwestern dipping subduction of the African plate. In contrast, the Middle Miocene to Recent alkaline volcanism is associated with extensional tectonics and can be explained within the framework of a low volcanicity rift model. Three zones—Catalan, Valencian and Columbretes—each located at different parts of the rift structure, can be distinguished in the Middle Miocene to Recent volcanism. This intraplate volcanism results from partial melting, due to extension-driven decompression, with the largest degrees of melting in the centre of the Valencia trough (Columbretes volcanic zone). In the Catalan volcanic zone a progressive increase in the extension rate from the first to the last volcanic episode has occurred, favouring the interaction of two mantle sources.

Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei caldera

Isotopic study of the origin of sulfur and carbon in the hottest (Solfatara) fumaroles of Campi Flegrei caldera, Southern Italy, was carried out on gas samples collected between 1983 and 1988, i.e. during and after the 1982–1984 seismo-volcanic crisis. The results for sulfur (H 2S), the first ever reported on these gases, indicate a mean ∂ 34 S of −0.3±0.3‰ (range: −0.7 to +0.1‰ ) versus Canyon Diablo Troilite standard, consistent with an igneous derivation of this element, from either active magma degassing or/and leaching of reduced sulfur-bearing minerals in the volcanic layers. The lack of peculiar ∂ 34S variation during and after the crisis suggests that the chemical variation of H 2S and S/C ratio in the fumaroles (increase and then decrease by a factor 3) were not due to a changing origin of sulfur. The mean ∂ 13C of carbon (CO 2) over the period of survey, −1.6±0.2‰ (range: −1.9 to −1.3‰ ) versus PDB standard, is similar to the values obtained before the crisis (since 1970). Such an isotopic constancy requires a large and stable source of carbon feeding the fumaroles. The measured ∂ 13C values are much higher than those typical of primary mantle-magmatic carbon ( −6±2‰) and plot within the ∂ 13C range for marine carbonates ( 0±2‰). Such high values may reflect either (a) 13C-fractionation during degassing of CO 2 from the underlying (⩽5 km depth) magma chamber or (b) the contribution of heavy CO 2 of sedimentary origin, derived from either thermometamorphism of Mesozoic limestone series embedding the magma chamber or, possibly, past contamination of the local mantle by subducted sediments. Various arguments, among which volcanological evidence of an isolated and cooling magma reservoir (which would have been extensively degassed and, so, depleted in 13C along with time), the low 3He/ 4He ratios and the broad 13C-enrichment of volcanic fluids in the region, and geochemical evidence of crust-magma fluid interactions, suggest that a considerable fraction (⩾60%) of CO 2 in Solfatara fumaroles derives from carbonate sediments in the basement. The contribution of magma-derived CO 2 may be higher within the central part of the caldera (including Solfatara crater) than toward its western margin, where fumarolic and geothermal well gases exhibit lower 3He/ 4He ratios and still higher ∂ 13C values. Such a geochemical pattern is consistent with the central distribution of ground deformation and seismicity during the 1982–1984 crisis and with the idea of a residual magma body, confined beneath the central part of the structure. Alternatively, higher ∂ 13C and lower 3He/ 4He ratios toward the western margin may result from dilution of Solfatara-type gas during progressively deeper water boiling. Finally, accepting that Solfatara CO 2 derives from simple crustal mixing between magmatic and sedimentary carbon, its constant isotopic composition (together with the constant He isotope ratio) would restrict the possibility of magma intrusion and/or higher magmatic gas input as mechanisms responsible for the 1982–1984 events. However, this conclusion would no more hold true if the magma itself, or even its mantle source, were previously contaminated by crustal carbon.

Transtensive tectonics in the Strait of Sicily: Structural and volcanological evidence from the Island of Pantelleria

A detailed structural analysis carried out on the volcanic products of the island of Pantelleria has shown that the large‐scale setting is dominated by N‐S trending normal faults and NW‐SE right‐lateral strike‐slip faults, while mesostructural data show a more complex pattern which is characterized by four systems of discontinuity. The volcanological features show a marked difference between a north western sector characterized by products of a basaltic nature and a south central one in which evolved products connected with the presence of a magmatic chamber are present. The present‐day stress field, as derived from structural analysis, is characterized by a maximum compression oriented NNW‐SSE which allowed the activation of right‐lateral strike‐slip faults bordering the Pantelleria trough giving rise to a thinning of the underlyng crust. Inside this main dextral shear zone it is possible that the development of pull‐apart basins bounded by N‐S trending normal faults could generate further thinning of the already thinned out crust zone and give rise to the magmatic phenomena known to occur mostly along the N‐S trending extension structures. Such a model would therefore help to understand the characters of the volcanism associated with a zone of continental collision as well as its distribution, which is not rigorously in accordance with a classical rifting process.

The Eruption of Vesuvius in A.D. 79: Reconstruction from Historical and Volcanological Evidence

Reinterpretation of the volcanological and historical evidence shows that the eruption of Vesuvius in A.D. 79 consisted of two main phases. initial 18 to 20hour Plinian phase caused extensive pumice-fall south of the volcano, resulting in the slow accumulation of a pumice layer up to 2.8 m. thick over Pompeii and other regions to the south. Much of the population fled the area during this non-lethal phase. On the second day of activity the Pelkan phase occurred, when a series of nudes ardentes or hot ash-avalanches swept down the south and west flanks of the volcano, affecting the region as far as Misenum, 30 km. to the west. first of two nudes ardentes which inundated Pompeii overwhelmed and asphyxiated those who remained above ground in the city and their bodies were immediately interred in the fine-grained deposit. effects of the Pelkan activity were even more severe west and northwest of Pompeii, resulting in burial of the cities of Oplontis and Herculaneum by a series of nue A. Maiuri, Herculaneum (Rome 1977) 13. 2 S.R.J. Sparks and G.P.L. Walker, The Ground Surge Deposit: a Third Type of Pyroclastic Rock, Nature 241 (1973) 62-64; S.R.J. Sparks, S. Self and G.P.L. Walker, Geology 1 (1973) 115-18; S.R.J. Sparks, Dusts of Destruction, New Scientist (1973) 134-36. 3A. Rittmann, Vulkane und ihre Tiitigkeit (Stuttgart 1960) 48. 4 Maiuri 1958 (supra n. 1) 69. 5G.A. Macdonald, Volcanoes (Englewood Cliffs 1972) 234; F.M. Bullard, Volcanoes of the Earth (Austin 1976) 200; P. Francis, Volcanoes (New York 1976) 63. 6 A. Helprin, Mont Pelke and the Tragedy of Martinique (New York 1903) 120; E.T. Merrill, Notes on the Eruption of Vesuvius in 79 A.D., AJA 22 (1918) 304-309; E.T. Merrill, Further Note on the Eruption of Vesuvius in 79 A.D., AJA 24 (1920) 262-68. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:31:16 UTC All use subject to http://about.jstor.org/terms 40 H. SIGURDSSON, S. CASHDOLLAR AND S.R.J. SPARKS [AJA 86 after the disastrous nu&e ardente eruption of Mt. Pel&e in 1902, and later revived by Sparks and Walker,7 and Lirer et al.8 VOLCANOLOGICAL EVIDENCE Products of the eruption of Vesuvius in A.D. 79 form distinct deposits on the southwestern and southern slopes of the volcano and on the Sarno plain as far south as Castellammare (ill. 1). We r. Air-fall deposits accumulate gradually during the explosive phase of a volcanic eruption and characteristically mantle the topography with a wellbedded, uniform pumice layer which shows an exponential decrease in thickness with increasing distance from the volcano. Their distribution is entirely controlled by the effect of the prevailing wind direction on the vertical eruption column, which may reach a height of tens of kilometers above the vol-

Evolution of a section of the Africa-Europe plate boundary: Paleomagnetic and volcanological evidence from Sicily

New paleomagnetic data relative to Upper Cretaceous, Neogene and Quaternary volcanic rocks from eastern Sicily definitively indicate that Sicily is a part of the African plate, which collided with the European continental plate in Middle Miocene times. These data and the tectonic evolution of Sicily as inferred from the nature, age and distribution of volcanic products, are broadly consistent with the motions of Africa relative to Europe since the Upper Trias. During the Mesozoic, eastern Sicily was affected by extensional tectonics with associated alkali basaltic volcanism, and oceanic crust was produced in the meantime between the diverging African and European plates. Near the end of Mesozoic times the two plates started to converge with consequent consumption of oceanic crust. Different times of oceanic plate consumption along the Sicily-Calabria section of the plate boundary are suggested by the occurence of andesitic volcanism of different ages. The tectonic significance of late Tertiary to present basaltic activity in eastern Sicily is also discussed.