An experimental facility for investigating hydromagmatic eruptions at high-pressure and high-temperature with application to the importance of magma porosity for magma-water interaction

An experimental facility has been developed to investigate magma-water interaction (MWI). The facility operates in a high-pressure and high-temperature environment, with temperatures up to 1,200°C and pressures up to 200 MPa. Cylindrical sample-holders (20 by 180 mm in size) are heated conductively to yield a three phase (melt, crystals and gas) system, and then water (or other fluid) is injected into the sample through a capillary tube (diameter 0.5 mm, length ca. 1,000 mm) under controlled conditions. Pressure, volume and temperature changes are continuously recorded during every phase of the experiments. To test this facility, MWI is studied at subliquidus temperatures (800 and 900°C) and pressure (8 MPa), using a leucite tephrite sample with two different initial grain sizes. Because of the grain-size dependence of sintering, the two starting materials produce magmas with different textures at the same temperature: porous magma for large initial grain sizes and dense magma for small initial grain sizes. In these experiments 1.5 g of water at room temperature is injected into 6.0 g of partially molten sample at velocities ranging from 1 to 3 m/s. We find that the extent of fragmentation and transport caused by MWI are mainly controlled by the texture of the interacting sample with explosive interaction occurring only for porous magmas.     

Experimental simulations of gas-driven eruptions: kinetics of bubble growth and effect of geometry

 Simulated gas-driven eruptions using CO₂–water-polymer systems are reported. Eruptions are initiated by rapidly decompressing CO₂–saturated water containing up to 1.0 wt.% CO₂. Both cylindrical test cells and a flask test cell were used to examine the effect of magma chamber/conduit geometry on eruption dynamics. Bubble-growth kinetics are examined quantitatively in experiments using cylindrical test cells. Uninhibited bubble growth can be roughly expressed as dr/dt≈λD(β-1)/(γt ^1/3) for a CO₂–water-polymer system at 0–22  °C and with viscosities up to 5 Pa·s, where r is the radius of bubbles, λ and D are the Ostwald solubility coefficient and diffusivity of the gas in the liquid, β is the degree of saturation (decompression ratio), and γ characterizes how the boundary layer thickness increases with time and is roughly 1.0×10^–5 m/s^1/3 in this system. Unlike the radius of cylindrical test cells, which does not affect the eruption threshold and dynamics, the shape of the test cells (flask vs cylindrical) affects the dynamics but not the threshold of eruptions. For cylindrical test cells, the front motion is characterized by constant acceleration with both Δh (the height increase) and ΔV (the volume increase) being proportional to t ²; for the flask test cell, however, neither Δh nor ΔV is proportional to t ² as the conduit radius varies. Test-cell geometry also affects foam stability. In the flask test cell, as it moves from the wider base chamber into the narrower conduit, the bubbly flow becomes fragmented, affecting the eruption dynamics. The fragmentation may be caused by a sudden increase in acceleration induced by conduit-shape change, or by the presence of obstacles to the bubbly flow. This result may help explain the range in vesicularities of pumice and reticulite. Received: 16 May 1997 / Accepted: 11 October 1997     

An experimental facility for the investigation of magma fragmentation by rapid decompression

 A special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression. The facility operates in a regime similar to that of shock tubes and at temperatures up to 950  °C and pressures up to 200 bar. Cylindrical samples (diameter ca. 17 mm, length ca. 50 mm) undergo rapid decompression in a high-temperature, high-pressure section of the facility following the disruption of a diaphragm separating that section from a low-pressure, low-temperature section. Actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization. The results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature. This new fragmentation facility is presented as a tool for experimental volcanology under well-constrained conditions. Received: 19 March 1996 / Accepted: 25 August 1996     

Role of carbon dioxide in the dynamics of magma ascent in explosive eruptions

 The role of carbon dioxide in the dynamics of magma ascent in explosive eruptions is investigated by means of numerical modeling. The model is steady, one-dimensional, and isothermal; it calculates the separated flow of gas and a homogeneous mixture of liquid magma and crystals. The magma properties are calculated on the basis of magma composition and crystal content and are allowed to change along the conduit due to pressure decrease and gas exsolution. The effect of the presence of a two-component (water + carbon dioxide) exsolving gas phase is investigated by performing a parametric study on the CO₂/(H₂O+CO₂) ratio, which is allowed to vary from 0 to 0.5 at either constant total volatile or constant water content. The relatively insoluble carbon dioxide component plays an important role in the location of the volatile-saturation and magma-fragmentation levels and in the distribution of the flow variables in the volcanic conduit. In detail, the results show that an increase of the proportion of carbon dioxide produces a decrease of the mass flow rate, pressure, and exit mixture density, and an increase of the exit gas volume fraction and depth of the fragmentation level. A relevant result is the different role played by water and carbon dioxide in the eruption dynamics; an increasing amount of water produces an increase of the mass flow rate, and an increasing amount of carbon dioxide produces a decrease. Even small amounts of carbon dioxide have major consequences on the eruption dynamics, implying that the multicomponent nature of the volcanic gas must be taken into account in the prediction of the eruption scenario and the forecasting of volcanic hazard. Received: 6 March 1998 / Accepted: 28 October 1998     

Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano

Compositionally diverse dacitic magmas have erupted from Mount St Helens over the last 4000 years. Phase assemblages and their compositions in these dacites provide information about the composition of the pre-eruptive melt, the phases in equilibrium with that melt and the magmatic temperature. From this information pre-eruptive pressures and water fugacities of many of the dacites have been inferred. This was done by conducting hydrothermal experiments at 850°C and a range of pressures and water fugacities and combining the results with those from experiments at temperatures of 780 and 920°C, to cover the likely range in equilibration conditions of the dacites. Natural phase assemblages and compositions were compared with the experimental results to infer the most likely conditions for the magmas prior to eruption. Water contents disolved in the melts of the dacites were then estimated from the inferred conditions. Water contents in the dacites have varied greatly, from 3.7 to 6.5 wt.%, in the last 4000 years. Between 4000 and about 3000 years ago the dacites tended to be water saturated and contained 5.5 to 6.5 wt.% water. Since then, however, the dacites have been significantly water-undersaturated and contained less than 5.0 wt.% water. These dacites have tended to be hotter and more mafic, and andesitic and basaltic magmas have erupted. These changes can be explained by variable amounts of mixing between felsic dacite and basalt, to produce hotter, drier and more mafic dacites and andesites. The magma storage region of the dacitic magmas has also varied significantly during the 4000 years, with shifts to shallower levels in the crust occurring within very short time periods, possibly even two years. These shifts may be related to fracturing of overlying roof rock as a result of magma with-drawal during larger volume eruptions.     

Impacts of volcanism on pre-European inhabitants of Taveuni, Fiji

 Taveuni is a Fijian ocean-island volcano that sporadically erupted throughout the Holocene. The 437-km² island is an active monogenetic volcanic field with a constantly shifting locus of activity along a single apparent rift axis. Although the eruptions were not large ( ≤VEI 2), unexpected shifts in Taveuni volcanism had the potential to affect habitation sites. Since known human settlement of the Fiji Group (ca. 950–750 BC), there have been at least 58 eruptions on Taveuni. Up to 25 of these eruptions potentially affected pre-European inhabitants of the island and at least four former occupation sites are known to have been affected by volcanic products. Despite apparent earliest settlement of Taveuni post-dating other nearby islands by up to 600 years, volcanism probably did not hinder or stall settlement of Taveuni compared with neighbouring islands. However, a period of voluminous eruptions between 300 and 500 AD covered much of south Taveuni with lava and/or thick tephra, apparently causing abandonment of at least this portion of Taveuni until approximately 1100 AD. Most eruptions were not of catastrophic proportions and, due to their localised effects, re-settlement was rapid in marginal unaffected areas. Localised stories and a relict place name survive to describe former eruption locations and effects since approximately 120–320 AD. Knowledge of the impacts on Taveuni's past inhabitants forms the basis of volcanic disaster-mitigation strategies to minimise future effects on the current 14,500 residents. Received: 9 September 1999 / Accepted: 21 February 1999     

Magma fragmentation dynamics: experiments with analogue porous low-strength material

 Experiments were conducted on the fragmentation of analogue low-strength porous material (plastiprin) by rapid decompression in a shock-tube-type apparatus. The porous samples (length=365 mm, cross-section dimensions 40×40 mm) pressurized by air to pressures up to 0.9 MPa, were rapidly decompressed to 0.1 MPa. Rapid decompression of samples caused fragmentation and ejection of the fragmentation products into a large volume tank. The process of analogue material fragmentation was documented using high-speed cinematography and dynamic pressure measurements. The duration of the fragmentation event is significantly shorter than that of the ejection event. The fragmentation of material precedes the acceleration of fragments. As a result of fragmentation, sub-parallel fractures are generated. The characteristic fragment size decreases as the initial pressure differential increases. The ejected fragments obtain velocities of 60 m/s. The mechanisms of material fragmentation during unloading and fragmentation wave propagation are discussed. The experimental results provide insight into the fragmentation dynamics of highly viscous magmas in which brittle failure at high strain rate is possible. Received: 23 July 1997 / Accepted: 23 November 1997     

Fragmentation of magma during Plinian volcanic eruptions

 The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V _G /V _L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V _G /V _L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V _G /V _L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities <10⁹ Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <10⁵ Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V _G /V _L of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >10⁵ Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes. Received: 19 April 1995 / Accepted: 3 April 1996     

Mixed deposits of complex magmatic and phreatomagmatic volcanism: an example from Crater Hill,Auckland, New Zealand

 A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred. Received: July 11, 1995 / Accepted: February 13, 1996     

Holocene explosive activity of Hudson Volcano, southern Andes

 Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including that of August 1991 which produced ≥4 km³ of pyroclastic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the 1932 Quizapu eruptions, the two largest in the Andes this century. However, lack of information for either the proximal or distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic material produced by the 6700-BP event, including a  1 10-cm-thick layer of compacted tephra that constitutes a secondary thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more southerly direction than that of the 1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary lapilli, indicating a violent phreatomagmatic eruption. This eruption, which is considered to be the largest for Hudson and possibly for any volcano in the southern Andes during the Holocene, may have created Hudson's 10-km-diameter summit caldera, but the age of the caldera has not been dated independently. Received: 31 January 1997 / Accepted: 29 October 1997     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

Rhyolite magma degassing: an experimental study of melt vesiculation

 The vesiculation of a peralkaline rhyolite melt (initially containing ∼0.14 wt.% H₂O) has been investigated at temperatures above the rheological glass transition (T _g≈530  °C) by (a) in situ optical observation of individual bubble growth or dissolution and (b) dilatometric measurements of the volume expansion due to vesiculation. The activation energy of the timescale for bubble growth equals the activation energy of viscous flow at relatively low temperatures (650–790  °C), but decreases and tends towards the value for water diffusion at high temperatures (790–925  °C). The time dependence of volume expansion follows the Avrami equation ΔV (t)∼{1–exp [–(t/τ_av) ⁿ ]} with the exponent n=2–2.5. The induction time of nucleation and the characteristic timescale (τ_av) in the Avrami equation have the same activation energy, again equal to the activation energy of viscous flow, which means that in viscous melts (Peclet number <1) the vesiculation (volume expansion), the bubble growth process, and, possibly, the nucleation of vesicles, are controlled by the relaxation of viscous stresses. One of the potential volcanological consequences of such behavior is the existence of a significant time lag between the attainment of a super-saturated state in volatile-bearing rhyolitic magmas and the onset of their expansion. Received: March 20, 1995 / Accepted: October 24, 1995     

Sulfur,chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

 The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H₂SO₄ aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods. Received: 30 May 1995 / Accepted: 19 April 1996     

Shallow seismicity beneath Ruapehu Crater Lake: results of a 1994 seismometer deployment

 Virtually all the seismicity within Ruapehu Volcano recorded during a 2-month deployment in early 1994, with 14 broadband seismographs around the Tongariro National Park volcanoes in the North Island of New Zealand, was associated with the active vent and occurred within approximately 1 km of Ruapehu Crater Lake. High-frequency volcano-tectonic earthquakes and low-frequency events (similar to bursts of 2 Hz volcanic tremor) were both found to have sources in this region. The high-frequency events, which often consisted of a smaller precursor event followed approximately 2 s later by the main event, had sharp onsets and were locatable using standard techniques. The depth of these events ranged from the surface down to approximately 1500 m below Crater Lake. The low-frequency events did not have sharp onsets and were located by phase-correlation methods. Nearly all occurred under a small region on the east side of Crater Lake, at depths from 200 to 1000 m below the surface. This low-frequency earthquake source region, in which no high-frequency events occurred, may be the steam zone within the actual vent of Ruapehu Volcano. Received: 30 June 1996 / Accepted: 16 February 1998     

Physical properties of the 1980 Mount St. Helens cryptodome magma

 Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log₁₀ η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T>900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log₁₀ η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log₁₀ η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 10^10.8 Pa s. Received: 25 August 1996 / Accepted: 19 July 1997     

Erratum to Compositional variations and magma mixing in the 1991 eruptions of Hudson volcano,Chile

The August 1991 eruptions of Hudson volcano produced ~2.7 km³ (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO₂. A trend of decreasing SiO₂ is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO₂) to the end (unit D, 60–63 wt% SiO₂). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO₂ –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pre-emergent construction of a lacustrine basaltic volcano,Pahvant Butte,Utah (USA)

 The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies. Received: 10 October 1995 / Accepted: 18 April 1996     

Statistical analyses and characteristics of volcanic tremor on Stromboli Volcano (Italy)

 A study of volcanic tremor on Stromboli is carried out on the basis of data recorded daily between 1993 and 1995 by a permanent seismic station (STR) located 1.8 km away from the active craters. We also consider the signal of a second station (TF1), which operated for a shorter time span. Changes in the spectral tremor characteristics can be related to modifications in volcanic activity, particularly to lava effusions and explosive sequences. Statistical analyses were carried out on a set of spectra calculated daily from seismic signals where explosion quakes were present or excluded. Principal component analysis and cluster analysis were applied to identify different classes of spectra. Three clusters of spectra are associated with two different states of volcanic activity. One cluster corresponds to a state of low to moderate activity, whereas the two other clusters are present during phases with a high magma column as inferred from the occurrence of lava fountains or effusions. We therefore conclude that variations in volcanic activity at Stromboli are usually linked to changes in the spectral characteristics of volcanic tremor. Site effects are evident when comparing the spectra calculated from signals synchronously recorded at STR and TF1. However, some major spectral peaks at both stations may reflect source properties. Statistical considerations and polarization analysis are in favor of a prevailing presence of P-waves in the tremor signal along with a position of the source northwest of the craters and at shallow depth. Received: 15 December 1996 / Accepted: 31 March 1998     

Shallow and deep reservoirs involved in magma supply of the 1944 eruption of Vesuvius

 During the 1944 eruption of Vesuvius a sudden change occurred in the dynamics of the eruptive events, linked to variations in magma composition. K-phonotephritic magmas were erupted during the effusive phase and the first lava fountain, whereas the emission of strongly porphyritic K-tephrites took place during the more intense fountain. Melt inclusion compositions (major and volatile elements) highlight that the magmas feeding the eruption underwent differentiation at different pressures. The K-tephritic volatile-rich melts (up to 3 wt.% H₂O, 3000 ppm CO₂, and 0.55 wt.% Cl) evolved to reach K-phonotephritic compositions by crystallization of diopside and forsteritic olivine at total fluid pressure higher than 300 MPa. These magmas fed a very shallow reservoir. The low-pressure differentiation of the volatile-poor K-phonotephritic magmas (H₂O<1 wt.%) involved mixing, open-system degassing, and crystallization of leucite, salite, and plagioclase. The eruption was triggered by intrusion of a volatile-rich magma batch that rose from a depth of 11–22 km into the shallow magma chamber. The first phase of the eruption represents the partial emptying of the shallow reservoir, the top of which is within the volcanic edifice. The newly arrived magma mixed with that resident in the shallow reservoir and forced the transition from the effusive to the lava fountain phase of the eruption. Received: 14 September 1998 / Accepted: 10 January 1999