David M. Pyle

The thickness, volume and grainsize of tephra fall deposits

An improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented. The widely used “area” plots of ln(thickness) against ln(isopach area) are curved, implying an exponential thinning law. Use of ln(thickness)−(area)1/2 diagrams confirm the exponential dependence of many parameters (e.g. thickness, maximum and median clast size) with distance from source, producing linear graphs and allowing volumes to be calculated without undue extrapolation of field data. The agreement between theoretical models of clast dispersion and observation is better than previously thought. Two new quantitative parameters are proposed which describe the rates of thinning of the deposit (bt the thickness half-distance) and the maximum clast size (bc the clast half-distance). Many deposits exhibit different grainsize and thickness thinning rates, with the maximum clast size diminishing 1–3 times slower than the thickness. This implies that the entrained grainsize population influences the morphologic and granulometric patterns of the resulting deposit, in addition to the effects of column height and wind-speed. The grainsize characteristics of a deposit are best described by reference to the half-distance ratio (bc/bt). A new classification scheme is proposed which plots the half-distance ratio against the thickness half-distance and may be contoured in terms of the column height.

SO2 emissions from Soufrière Hills Volcano and their relationship to conduit permeability, hydrothermal interaction and degassing regime

The time series of sulphur dioxide (SO2) emissions during the continuing eruption of Soufriere Hills Volcano, Montserrat, yields insights into conduit permeability and driving pressures, the role of the hydrothermal system and changes in magma flux both at depth and to the surface. On a time scale of years, an effectively constant supply of sulphur from a more mafic magma at depth permits evaluation of changes in the permeability of the plumbing system between 1995 and 2002 (due to magma rheology changes and hydrothermal sealing), most of which take place in the upper few hundreds of metres (dome and upper conduit). A broadly increasing SO2 emission rate from 1995 to 1997 can be attributed to a constant or increasing supply of exsolving sulphur from depth, combined with a broadly increasing magma discharge rate at the surface. Decreases in SO2 flux over three orders of magnitude, from July 1998 to November 1999, were due to a corresponding decrease in permeability of the upper conduit and dome due to cooling and ‘sealing’ by the precipitation of hydrothermal minerals and the closure of fracture and bubble networks. The second phase of dome growth, from November 1999 to the present, April 2002, has been associated with a similar range of SO2 fluxes to the first phase. Large dome collapses in 1997 and during a period of zero magma flux in 1998 were associated with instantaneous SO2 emissions of >10 kt, which indicate a capacity for significant SO2 storage in the conduit and dome prior to the collapses. SO2 data suggest that the second phase of dome building, despite a similar sulphur budget in terms of supply from depth and mean SO2 emission rate at the surface (around 500 t/d), is characterised by a higher bulk permeability at shallow depths and is a more ‘open’ system with respect to fluid through-flow than the first phase of dome building from 1995 to 1998. The lack of large SO2 emissions after large dome collapses, in 2000 and 2001, suggests limited storage of SO2 in the conduit system. The data suggest that the likelihood of a switch to explosive activity after a large collapse is more unlikely now than during the first phase of dome building. Over shorter time scales, permeability changes may be recognised from the SO2 flux data prior to the onset of dome growth and during cycles of small explosions in 1999. On time scales of minutes to hours, pulses of SO2-rich gas emissions occur after rockfalls and pyroclastic flows, due to the release of a SO2-rich fluid phase stored in closed fractures and pore spaces within the dome. Long period and hybrid seismic events may be associated with changes in SO2 emission rate at the surface at various times of the eruption, although only when the temporal resolution of SO2 monitoring is improved, will it be possible for these short-term changes to be correlated and evaluated effectively. Monitoring SO2 emission rates from Soufriere Hills Volcano is, at this stage, of primary value in the long run, on the time scale of years, where the relationships between deep supply and surface emissions can be used to evaluate whether the eruption might be waning, or has merely paused, which is of considerable value for hazard assessment.

Explosive volcanism on Santorini, Greece

Santorini volcanic field has had 12 major (1–10 km 3 or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption. Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.

Tropospheric Volcanic Aerosol

Volcanic emissions represent an important source of aerosol to the global troposphere, and have important implications for the Earth’s radiation budget at various temporal and spatial scales. Volcanogenic aerosol can also transport trace metals and other pollutants, with impacts on terrestrial ecosystems and human health. We provide here a primer on the current understanding of the origins and transformations of volcanogenic particles in the troposphere, covering their fluxes, size distribution, composition and morphology, and focusing on sulfur, halogen, and trace metal compounds. Such an understanding is essential to investigations of the atmospheric, environmental and human health impacts of volcanic volatile emissions.

Petrology and geochemistry of volcanic rocks of the Cerro Galan Caldera, Northwest Argentina

At least 2000 km 3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km 3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO 2 ) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87 Sr/ 86 Sr = 0.7108–0.7181; 143 Nd/ 144 Nd = 0.51215–0.51225, and δ 18 O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87 Sr/ 86 Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87 Sr/ 86 Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges ( 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga). Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO 2 = 51–57%; K 2 O = 2–3% and normative plagioclase compositions of An 37–48 , and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87 Sr/ 86 Sr ranges from 0.7055 to 0.7094 and 143 Nd/ 144 Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO 2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87 Sr/ 86 Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites. The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

A model for degassing at the Soufrière Hills Volcano, Montserrat, West Indies, based on geochemical data

Abstract A model is presented to describe the degassing behaviour of sulphur and chlorine at the Soufriere Hills Volcano, Montserrat, using both geochemical analyses of glass and remote sensing data (correlation spectroscopy and open-path Fourier transform infrared spectroscopy). Constraints on total SO2 emitted at the surface (1.2 Mt up to September 2000) and petrological data indicate that the andesite was not the sulphur source. Mafic magma that intruded into the andesite magma chamber to trigger eruption at 5–7 km depth recharged the sulphur contents of the shallow volcanic system. Sulphur is removed from the mafic melt to enter a water-dominated vapour phase at depths of 5–7 km or greater. The passage of SO2 to the surface is sporadic and discontinuous in time and is governed by the permeability of the conduit and wallrock and the supply of mafic magma from depth. Chlorine is derived from the andesite magma and degasses on magma ascent, shown by the melt evolution recorded in the matrix glasses and the apparent link between extrusion rate and HCl flux at the surface. Petrological estimates of chlorine loss agree with measurements of total chlorine emission at the surface. The chlorine content of the matrix glass may be explained by fractional crystallisation combined with the partitioning of chlorine into a water-rich fluid phase with a partition coefficient of the order of 50.

Physicochemical properties of alkali carbonatite lavas:Data from the 1988 eruption of Oldoinyo Lengai, Tanzania

Alkali carbonatite lavas extruded from Oldoinyo Lengai, Tanzania, in November 1988 are similar in composition to lavas extruded in 1960. Extrusion temperatures are 585 ±10 °C. Apparent viscosities in this temperature range are between 0.3 and 120 Pa⋅s, the highest values coming from very frothy and phenocryst-rich magma. The viscosities and temperatures are the lowest known for terrestrial magmas.

Melt inclusions track pre-eruption storage and dehydration of magmas at Etna

At Mount Etna, Italy, vigorous gas-rich eruptions in A.D. 2001, 2002, and 2003 were followed by gas-poor eruptions in 2004, 2006, and 2007. Analyses of volatile (CO2, H2O, S, Cl, F), semivolatile (Cu), and involatile (Nb, La) elements trapped in olivine-hosted melt inclusions from these latest eruptions reveal the effects of the sustained interaction between a percolating gas phase and the stored magma. Melt inclusion compositions indicate that magmas erupted from 2004 to 2007 were residual from the 2001–2003 eruptions, and show significant evolution in the volatile content of the melt. These melt inclusion observations, and variations in the C/S of volcanic gases, can be accounted for if melts reequilibrated with CO2-rich gases during storage and prior to entrapment as melt inclusions. Sustained gas percolation caused loss of water and enhancement of CO2 in the evolving melt and may strongly influence the behavior of Cu, which potentially partitions into the gas phase. Vapor-melt interactions during magma storage are important controls on magma evolution at persistently degassing volcanoes.

Characterization and evolution of tropospheric plumes from Lascar and Villarrica volcanoes, Chile

Chile, reveal that both are significant and sustained emitters of SO2 (28 and 3.7 kg s � 1 , respectively), HCl (9.6 and 1.3 kg s � 1 , respectively), HF (4.5 and 0.3 kg s � 1 , respectively) and near-source sulfate aerosol (0.5 and 0.1 kg s � 1 , respectively). Aerosol plumes are characterized by particle number fluxes (0.08–4.0 mm radius) of � 10 17 s � 1 (Lascar) and � 10 16 s � 1 (Villarrica), the majority of which will act as cloud condensation nuclei at supersaturations >0.1%. Impactor studies suggest that the majority of these particles contain soluble SO4� . Most aerosol size distributions were bimodal with maxima at radii of 0.1–0.2 mm and 0.7–1.5 mm. The mean particle effective radius (Reff) ranged from 0.1 to 1.5 mm, and particle size evolution during transport appears to be controlled by particle water uptake (Villarrica) or loss (Lascar) rather than sulfate production. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 8409 Volcanology: Atmospheric effects (0370); 8494 Volcanology: Instruments and techniques; KEYWORDS: volcanoes, degassing, aerosol sulphur dioxide, sulphate, Llaima

Changes in gas composition prior to a minor explosive eruption at Masaya volcano, Nicaragua

Abstract A small explosive eruption at Masaya volcano on 23 April 2001, in which a number of people were injured, was preceded by a distinct change in plume gas compositions. Open-path Fourier transform infrared spectroscopy (FTS) measurements show that the SO 2 /HCl molar ratio increased from 1.8 to 4.6 between April 2000 and April/May 2001. The SO 2 flux decreased from 11 to 4 kg s −1 over this period. We interpret these changes to be the result of scrubbing of water-soluble magmatic gases by a rejuvenated hydrothermal system. A sequence of M 5 earthquakes with epicentres about 7 km from the volcano occurred in July 2000. These may have altered the fracture permeability close to the magmatic conduit, and caused increased magmatic–hydrothermal interaction, leading eventually to the phreatic explosion in 2001. Continuous FTS measurements at suitable volcanoes could provide useful information in support of eruption prediction and forecasting.