A statistical method linking geological and historical eruption time series for volcanic hazard estimations: Applications to active polygenetic volcanoes

The probabilistic analysis of volcanic eruption time series is an essential step for the assessment of volcanic hazard and risk. Such series describe complex processes involving different types of eruptions over different time scales. A statistical method linking geological and historical eruption time series is proposed for calculating the probabilities of future eruptions. The first step of the analysis is to characterize the eruptions by their magnitudes. As is the case in most natural phenomena, lower magnitude events are more frequent, and the behavior of the eruption series may be biased by such events. On the other hand, eruptive series are commonly studied using conventional statistics and treated as homogeneous Poisson processes. However, time-dependent series, or sequences including rare or extreme events, represented by very few data of large eruptions require special methods of analysis, such as the extreme-value theory applied to non-homogeneous Poisson processes. Here we propose a general methodology for analyzing such processes attempting to obtain better estimates of the volcanic hazard. This is done in three steps: Firstly, the historical eruptive series is complemented with the available geological eruption data. The linking of these series is done assuming an inverse relationship between the eruption magnitudes and the occurrence rate of each magnitude class. Secondly, we perform a Weibull analysis of the distribution of repose time between successive eruptions. Thirdly, the linked eruption series are analyzed as a non-homogeneous Poisson process with a generalized Pareto distribution as intensity function. As an application, the method is tested on the eruption series of five active polygenetic Mexican volcanoes: Colima, Citlaltépetl, Nevado de Toluca, Popocatépetl and El Chichón, to obtain hazard estimates.     

The weathering and element fluxes from active volcanoes to the oceans: a Montserrat case study

The eruptions of the Soufrière Hills volcano on Montserrat (Lesser Antilles) from 1995 to present have draped parts of the island in fresh volcaniclastic deposits. Volcanic islands such as Montserrat are an important component of global weathering fluxes, due to high relief and runoff and high chemical and physical weathering rates of fresh volcaniclastic material. We examine the impact of the recent volcanism on the geochemistry of pre-existing hydrological systems and demonstrate that the initial chemical weathering yield of fresh volcanic material is higher than that from older deposits within the Lesser Antilles arc. The silicate weathering may have consumed 1.3% of the early CO₂ emissions from the Soufrière Hills volcano. In contrast, extinct volcanic edifices such as the Centre Hills in central Montserrat are a net sink for atmospheric CO₂ due to continued elevated weathering rates relative to continental silicate rock weathering. The role of an arc volcano as a source or sink for atmospheric CO₂ is therefore critically dependent on the stage it occupies in its life cycle, changing from a net source to a net sink as the eruptive activity wanes. While the onset of the eruption has had a profound effect on the groundwater around the Soufrière Hills center, the geochemistry of springs in the Centre Hills 5 km to the north appear unaffected by the recent volcanism. This has implications for the potential risk, or lack thereof, of contamination of potable water supplies for the island’s inhabitants.     

The respiratory health hazards of volcanic ash: a review for volcanic risk mitigation

Studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years, and mostly since the eruption of Mt. St. Helens in 1980. This review of all published clinical, epidemiological and toxicological studies, and other work known to the authors up to and including 2005, highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long-term health risk (silicosis, non-specific pneumoconiosis and chronic obstructive pulmonary disease) in populations from prolonged exposure to ash due to persistent eruptive activity. The acute and chronic health effects of volcanic ash depend upon particle size (particularly the proportion of respirable-sized material), mineralogical composition (including the crystalline silica content) and the physico-chemical properties of the surfaces of the ash particles, all of which vary between volcanoes and even eruptions of the same volcano, but adequate information on these key characteristics is not reported for most eruptions. The incidence of acute respiratory symptoms (e.g. asthma, bronchitis) varies greatly after ashfalls, from very few, if any, reported cases to population outbreaks of asthma. The studies are inadequate for excluding increases in acute respiratory mortality after eruptions. Individuals with pre-existing lung disease, including asthma, can be at increased risk of their symptoms being exacerbated after falls of fine ash. A comprehensive risk assessment, including toxicological studies, to determine the long-term risk of silicosis from chronic exposure to volcanic ash, has been undertaken only in the eruptions of Mt. St. Helens (1980), USA, and Soufrière Hills, Montserrat (1995 onwards). In the Soufrière Hills eruption, a long-term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population. A more systematic approach to multi-disciplinary studies in future eruptions is recommended, including establishing an archive of ash samples and a website containing health advice for the public, together with scientific and medical study guidelines for volcanologists and health-care workers.     

Environmental hazards of fluoride in volcanic ash: a case study from Ruapehu volcano, New Zealand

The vent-hosted hydrothermal system of Ruapehu volcano is normally covered by a c. 10 million m³ acidic crater lake where volcanic gases accumulate. Through analysis of eruption observations, granulometry, mineralogy and chemistry of volcanic ash from the 1995–1996 Ruapehu eruptions we report on the varying influences on environmental hazards associated with the deposits. All measured parameters are more dependent on the eruptive style than on distance from the vent. Early phreatic and phreatomagmatic eruption phases from crater lakes similar to that on Ruapehu are likely to contain the greatest concentrations of environmentally significant elements, especially sulphur and fluoride. These elements are contained within altered xenolithic material extracted from the hydrothermal system by steam explosions, as well as in residue hydrothermal fluids adsorbed on to particle surfaces. In particular, total F in the ash may be enriched by a factor of 6 relative to original magmatic contents, although immediately soluble F does not show such dramatic increases. Highly soluble NaF and CaSiF₆ phases, demonstrated to be the carriers of ‘available’ F in purely magmatic eruptive systems, are probably not dominant in the products of phreatomagmatic eruptions through hydrothermal systems. Instead, slowly soluble compounds such as CaF₂, AlF₃ and Ca₅(PO₄)₃F dominate. Fluoride in these phases is released over longer periods, where only one third is leached in a single 24-h water extraction. This implies that estimation of soluble F in such ashes based on a single leach leads to underestimation of the F impact, especially of a potential longer-term environmental hazard. In addition, a large proportion of the total F in the ash is apparently soluble in the digestive system of grazing animals. In the Ruapehu case this led to several thousand sheep deaths from fluorosis.     

Silicic volcanism in Iceland: Composition and distribution within the active volcanic zones

Silicic volcanic rocks within the active volcanic zones of Iceland are mainly confined to central volcanoes. The volcanic zones of Iceland can be divided into rift zones and flank zones. Each of these zones contains several central volcanoes, most of which have produced minor amounts of silicic rocks. The silicic rocks occur as lavas and domes or as tephra layers, welded tuffs and ignimbrites, formed both in effusive and explosive eruptions. They tend to be glassy or very fine-grained, containing small amounts of phenocrysts. Plagioclase (andesine–oligoclase), anorthoclase or occasionally sanidine coexist with minerals such as augite, fayalite, pigeonite, orthopyroxene and magnetite. Quartz phenocrysts are exceedingly rare. Zoning of phenocrysts is limited and the pattern is variable. A set of 90 samples representing all active central volcanoes that have erupted silicic rocks was analysed for major- and trace-elements. The silicic rocks can be classified as dacites, trachytes, low-alkali rhyolites and alkalic rhyolites. Some of the trachytes and alkalic rhyolites are peralkaline (mostly comenditic). Trachytes and alkalic rhyolites are only found within the flank zones, while dacites and low-alkali rhyolites are mostly confined to the rift zones. The Icelandic rhyolites plot close to the thermal minimum in the “granite” system, while dacites and trachytes plot within the plagioclase field and towards the alkali feldspar temperature minimum. The silicic rocks are relatively Fe-rich and Ca-poor indicating low water pressure in the source. Trace element concentrations follow similar patterns in most central volcanoes. Exceptions are Torfajökull where silicic rocks display a negative correlation of Ba to Th and unusually high Th-contents, and the western flank zone where Ba-concentrations are highly variable. The ratios of different high field-strength elements are generally similar within each central volcano or region, which probably reflects different ratios in the source materials. Isotope systematics indicate that the silicic rocks are derived from older basaltic rocks similar to those from the same volcano, and that meteoric water has played a role in the genesis of the silicic rocks. Traditionally, the petrogenesis of silicic rocks in Iceland has been explained by various models of fractional crystallization or partial melting. The available data seems to be better explained by near-solidus differentiation than by near-liquidus differentiation. The silicic minimum melts can be extracted from the rigid framework of the near-solidus source by the process of solidification front instability or by deformation-assisted melt segregation. The source of the silicic rocks is within the intrusive complex beneath a central volcano rather than in a large, long-lived magma chamber.     

Ra or Ra/Ba dating of Holocene volcanic rocks: application to Mt. Etna and Merapi volcanoes

This paper shows how ²²⁶Ra–²³⁰Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes. A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method. Two examples are described here to illustrate its potential. In the case of Mt. Etna, the good correlation observed between (²²⁶Ra)₀ activities at the time of eruption and Th contents in lava flows from the last two millennia [M. Condomines, J.C. Tanguy, V. Michaud, Magma dynamics at Mt. Etna: constraints from U–Th–Ra–Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. Sci. Lett. 132 (1995) 25–41] is used to infer the ages of several newly analysed lava flows. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.C. Tanguy, I. Bucur, J.F.C. Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt. Etna, Nature 318 (1985) 453–455, J.C. Tanguy, M. Le Goff, V. Chillemi, A. Paiotti, C. Principe, S. La Delfa, G. Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C. R. Acad. Sci. Paris 329 (1999) 557–564, J.C. Tanguy, M. Le Goff, C. Principe, S. Arrighi, V. Chillemi, A. Paiotti, S. La Delfa, G. Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211 (2003) 111–124]. We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the (²²⁶Ra)/Ba ratio has probably maintained a quasi-steady state value during at least the past four millennia, and can be used to infer the (²²⁶Ra)₀/Ba ratio of old volcanics at the time of eruption, and thus their ages. Comparison with ¹⁴C ages available on three samples [R. Gertisser, J. Keller, Temporal variations in magma composition at Merapi volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity, J. Volcanol. Geotherm. Res. 123 (2003) 1–23] shows an excellent agreement. These dating methods, based on the post-eruptive decrease of ²²⁶Ra excesses can be confidently used to date young rocks on both volcanoes, an important step to infer their recent eruptive history and magmatic evolution. It also opens the possibility to extend the geomagnetic field variation curve back into the past few millennia. The promising results obtained in this work should encourage new systematic U-series studies to test the applicability of such methods to other permanently active volcanoes showing ²²⁶Ra excesses.     

Identifying and mitigating dam-induced declines in river health:Three case studies from the western United States

River health can be defined as the degree to which riverine energy source,water quality,flow regime, habitat and biota match the natural conditions.In a healthy river,physical process and form remain actively connected and able to mutually adjust,and biological communities have natural levels of diversity and are resilient to environmental stress.Both physical diversity and biodiversity influence river health.Physical diversity is governed by hydrology,hydraulics,and substrate,as reflected in the geometry of the river channel and adjacent floodplain,which create habitat for aquatic and riparian organisms.Biodiversity is governed by biological processes such as competition and predation,but biodiversity also reflects the diversity,abundance and stability of habitat,as well as connectivity. Connectivity within a river corridor includes longitudinal,lateral,and vertical dimensions.River health declines as any of these interacting components is compromised by human activities.The cumulative effect of dams and other human alterations of rivers has been primarily to directly reduce physical diversity and connectivity,which indirectly reduces biodiversity.Restoration and maintenance of physical diversity and biodiversity on rivers affected by dams requires quantifying relations between the driver variables of flow and sediment supply,and the response variables of habitat,connectivity,and biological communities.These relations can take the form of thresholds(e.g., entrainment of streambed sediment) or response curves(e.g.,fish biomass versus extent and duration of floodplain inundation).I use examples from Wyoming,Colorado,and Arizona in the western United States to illustrate how to quantify relations between driver and response variables on rivers affected by dams.     

Abundances,distribution and sizes of volcanoes in the Pacific Ocean and implications for the origin of non-hotspot volcanoes

In this paper I present data on the abundances, sizes and crustal age for all volcanoes (volcanic islands and seamounts) which appear on published bathymetric charts of the Pacific Ocean. These new data shed light on the origin of non-hotspot volcanoes and are important, in combination with data on the chemical compositions of seamounts and volcanic islands, for estimates of the bulk composition of ocean crust. These data also provide firm constraints on off-ridge oceanic volcanism models. Results of this study show that the size-frequency distribution of Pacific volcanoes is Poisson-like and that the smallest volcanoes are much more abundant than large ones. This study shows clearly that the most abundant volcanoes on the Earth are the submerged oceanic volcanoes which comprise 5–25% of the oceanic volcanic layer. On Pacific crust of Eocene age and younger, the abundance of volcanoes (number of volcanoes per unit area) increases monotonically with increasing age. Assuming steady state, the production rate of new off-ridge volcanoes (number of volcanoes per unit area per unit time) is inversely proportional to the square root of the lithosphere age [1]. On crust older than Eocene, the number of volcanoes per unit area of crust decreases monotonically with increasing age, however the total volume of lava represented by these edifices increases with increasing age. Size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old Pacific crust as well as the effect of increased lithosphere thickness on seamount size. These general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes [2] and volcanoes built at fossil constructional plate margins [3].     

Distal record of multi-sourced tephra in Onepoto Basin,Auckland, New Zealand: implications for volcanic chronology,frequency and hazards

We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5-9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K₂O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K₂O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.     

Slope instability induced by volcano-tectonics as an additional source of hazard in active volcanic areas: the case of Ischia island (Italy)

Ischia is an active volcanic island in the Gulf of Naples whose history has been dominated by a caldera-forming eruption (ca. 55 ka) and resurgence phenomena that have affected the caldera floor and generated a net uplift of about 900 m since 33 ka. The results of new geomorphological, stratigraphical and textural investigations of the products of gravitational movements triggered by volcano-tectonic events have been combined with the information arising from a reinterpretation of historical chronicles on natural phenomena such as earthquakes, ground deformation, gravitational movements and volcanic eruptions. The combined interpretation of all these data shows that gravitational movements, coeval to volcanic activity and uplift events related to the long-lasting resurgence, have affected the highly fractured marginal portions of the most uplifted Mt. Epomeo blocks. Such movements, mostly occurring since 3 ka, include debris avalanches; large debris flows (lahars); smaller mass movements (rock falls, slumps, debris and rock slides, and small debris flows); and deep-seated gravitational slope deformation. The occurrence of submarine deposits linked with subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches impacted on the sea. The obtained results corroborate the hypothesis that the behaviour of the Ischia volcano is based on an intimate interplay among magmatism, resurgence dynamics, fault generation, seismicity, slope oversteepening and instability, and eruptions. They also highlight that volcano-tectonically triggered mass movements are a potentially hazardous phenomena that have to be taken into account in any attempt to assess volcanic and related hazards at Ischia. Furthermore, the largest mass movements could also flow into the sea, generating tsunami waves that could impact on the island’s coast as well as on the neighbouring and densely inhabited coast of the Neapolitan area.     

The dynamics of the velocity field beneath the North group of volcanoes,Kamchatka, and its relationship to the evolution of volcanic activity

P- and S-wave travel times from local volcanic earthquakes recorded in the North group of volcanoes area during the 2005–2009 period were treated by the “reverse wave” method to calculate the V _P velocity field and the TAU parameter, which is an analogue of the P- to S-wave velocity ratio. We constructed 3D velocity distributions along the line traversing the volcanic group along the direction from Ploskii Tolbachik Volcano in the southwest toward Shiveluch Volcano in the northeast. Dynamic changes in the velocity field were identified, both over time and depth. We examine the relationships of these dynamic changes to the evolution of volcanic activity during the period indicated.     

ADAP software for automatic detection of ash emission at active volcanoes and calculations of ash plume height using seismological data

ADAP (Automatic Detection of Ash Plume) software intended for automatic detection of the seismic signals accompanying ash emission at the active volcanoes is designed and introduced. This software also calculates the height of the ash plume in real time and automatically sends information on the detected hazardous ash emissions by e-mail and SMS to scientists on duty. The reliability of the program is estimated at 70%. The program has no current analogues.     

Dike propagation within active central volcanic edifices: constraints from Somma-Vesuvius,Etna and analogue models

Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of the volcano.     

Major and trace element distributions around active volcanic vents determined by analyses of grasses: implications for element cycling and bio-monitoring

Samples of grass were collected at Masaya Volcano (Nicaragua; Rhynchelytrum repens and Andropogon angustatus) and the Piton de La Fournaise (around the April 2007 eruptive vent, La Réunion; Vetiveria zizanioides) to investigate the controls on major and trace element concentrations in plants around active volcanic vents. Samples were analysed using inductively coupled plasma mass spectrometry for a wide range of elements, and atomic absorption spectroscopy for Hg. At Masaya, As, Cu, Mo, Tl and K concentrations in both grass species showed a simple pattern of variability consistent with exposure to the volcanic plume. Similar variability was found in A. angustatus for Al, Co, Cs, Hg and Mg. At the Piton de La Fournaise, the patterns of variability in V. zizanioides were more complex and related to variable exposures to emissions from both the active vent and lava flow. These results suggest that exposure to volcanic emissions is, for many elements, the main control on compositional variability in vegetation growing on active volcanoes. Thus, vegetation may be an important environmental reservoir for elements emitted by volcanoes and should be considered as part of the global biogeochemical cycles.     

Chaos and stochasticity in volcanic eruptions the case of Mount Etna and Vesuvius

The series of historical eruptions of Mount Etna and Vesuvius volcanoes are analyzed to verify the presence of low-dimensional chaos in the mechanism driving eruptive activity. A recently developed optimal methodology which is efficient on relatively small sets of data is used. The results indicate that there is no evidence of low-dimensional chaos in the sequences considered, and the mechanism appears better described by a classical stationary stochastic process.     

The identification and diagnostics of active and potentially active volcanic features in the Kuril-Kamchatka island arc and the Commander Islands link of the Aleutian arc

We consider the identification and diagnostics of active and potentially active volcanic features (regional zones of cinder cones, fields sheet volcanism, fields of concentrated multivent extrusive volcanism, calderas, and underwater eruption centers in the sea) in the Kuril-Kamchatka island arc and in the Commander Islands link of the Aleutian island arc, as well as the condition of this region as of late 2007. We have identified and examined three periods in the research of active and potentially active volcanic features in the region: the early (1697–1934), the new (1935–1962), and the most recent, still in progress (1963 until today). We provide a new definition of the term “active volcano,” which is scientifically well-grounded, for the first time here. We present modified (compared with those available until now) catalogs of active and potentially active volcanic forms in Kamchatka and the Kuril Islands. For typical multieruption volcanoes now in phase I (the active) and II (the passive) of their evolution, we provide long-term forecasts of the character and parameters of future eruptions and the associated volcanic hazard.     

Methana,the westernmost active volcanic system of the south Aegean arc (Greece): Insight from fluids geochemistry

An extensive geochemical survey of the fluids released by the volcanic/geothermal system of Methana was undertaken. Gases were characterized based on the chemical and isotopic [helium (He) and carbon (C)] analysis of 27 samples. Carbon dioxide soil gas concentration and fluxes were measured at 179 sampling sites throughout the peninsula. Forty samples of thermal and cold groundwaters were also sampled and analysed to characterize the geochemistry of the aquifers.