Magma Discharge Research Articles

Open-path Fourier transform spectroscopy of gas emissions from Oldoinyo Lengai volcano, Tanzania

We report here novel field spectroscopic measurements of the proportions of H 2O, CO 2, CO and SO 2 in gas emissions from Oldoinyo Lengai, the world's unique, active carbonatite volcano. We found that CO 2 constitutes <40 mol% of emissions from a lava lake, and 25 mol% from a cooler fumarole vent. These results suggest that H 2O is the predominant gas phase rather than CO 2, as reported in previous studies based on conventional sampling (Trans. Am. Geophys. Union 69 (1998) 1466; J. Geophys. Res. 101 (1996) 13819), though it is possible that water is introduced by remelting of older hydrated lava flows. We also observed rapid variations in CO 2/CO molar ratios (between 450 and 750 in 1 h) in the lava lake emissions, which could reflect mixing of gases exsolved from deep and shallow magma. Lengai's measured CO 2 flux (J. Geophys. Res. 101 (1996) 13819; Geology 23 (1995) 933) exceeds the time-averaged magma discharge rate, suggesting efficient separation of carbon and water-rich fluids from unerupted silicate magma. This may play an important role in parental magma differentiation.

Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 2. Application to the Pinatubo 1991 eruption

The granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic Plinian phase of the 1991 eruption at Pinatubo. The tephra fall deposits are composed of two units: layer C1, which corresponds to the first half of the climactic phase, and layer C2 the second half. The granulometric estimates of the expansion rate of the umbrella cloud for layers C1 and C2 are 7×1010 and 3×1010 m3/s, respectively, which agree with the observations of satellite images. These estimates indicate that the magma discharge rate decreased from 9×108 to 3×108 kg/s during the eruption. The grain‐size distribution at the top of the eruption column is characterized by depletion of coarse clasts, suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second, particularly during the second half of the climactic phase. The total amount of layers C1 and C2 is estimated to be 3×1012 kg, and the very fine particles, which did not deposit in the accessible on‐land area, may occupy up to 60% of the total ejecta. The effective duration of the eruption estimated by the present methods (a few hours or less) is substantially shorter than a previous estimate from the real‐time observation based on infrasonic data (∼10 hours). This discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and/or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous Plinian activity and generation of pyroclastic flows. It is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time‐averaged features predicted by the present methods.

Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 1. Methods

Two inversion methods are developed to reconstruct eruption column dynamics from granulometric data of Plinian deposits. They are based on the steady tephra dispersal model from umbrella clouds which spread as gravity currents. The methods provide (1) expansion rate of an umbrella cloud, (2) grain‐size distribution at the top of the eruption column, and (3) total amount of ejecta including very fine particles which are extensively dispersed in the atmosphere. The first quantity is correlated to the magma discharge rate. The second quantity provides information about sorting processes during column rise, and hence, about deceleration in the “gas thrust region” of the eruption column and the timing of column collapse. The third quantity, together with the estimated magma discharge rate, provides the effective duration of the eruption. The two methods are based on the same dispersal model but use different averaging. Combining the two methods can provide additional information about eruption dynamics, such as minor flow structures of the umbrella cloud and fluctuations of eruption rate. The methods are applicable to large‐scale eruptions where the radial expansion velocity of the umbrella cloud exceeds wind velocity. The applicability of the methods is tested from the viewpoint of comparison with real‐time observations in the companion paper (paper 2).

The Submarine Volcanic System of the Errigoiti Formation (Albian-Santonian of the Basque-Cantabrian Basin, Northern Spain): Stratigraphic Framework, Facies, and Sequences

ABSTRACT The Errigoiti Formation, located in the central domain of the Basque-Cantabrian basin, is an Albian to Santonian bathyal submarine volcanic system of alkali basaltic character. These volcanics occurred in an extensional geodynamic context generated by the drifting of the Iberian plate with respect to the European plate. Stratigraphic analyses reveal the facies distribution and depositional architecture of a longitudinal cross section of the Errigoiti volcanic system. Four facies groups were distinguished: sheet lavas (SL), derived from low-viscosity lava flows at high magma discharge rate; pillow lavas (PL), derived from intermediate-viscosity lava flows at moderate discharge rate; pillow breccias (PB), derived either from in situ fragmentation of lavas (PB1) or rock-falling from pillow-lava slopes (PB2); and stratified volcaniclastites (VC), derived from volcaniclast resedimentation through gravity flows, either by early remobilization (VC1) or post-volcanic reworking (VC2). The facies groups occur in a predictable repetitive succession, essentially from base to top, an SL-PL-PB-VC stacking pattern. This is interpreted to reflect an upward waning in the magma discharge rate. Five major volcanic sequences make up the Errigoiti Formation. Each comprises packages tens to hundreds of meters thick, deposited during short time spans of volcanic activity separated by long time intervals of volcanic inactivity. Consequently, boundaries between sequences are associated with marked hiatuses, reworked volcaniclasts, and/or turbidite-pelagic sedimentation. Analysis of lateral and vertical facies evolution enabled us to propose a depositional model consisting of three depositional environments. From proximal to distal settings, these are: amalgamated pillow volcanoes, scattered pillow volcanoes, and volcanic fringe, basically responding to a proximal to distal PL/PB, PL/VC, and SL/VC evolution of facies associations. Contemporaneous fossiliferous turbidite-pelagic sequences that surround the volcanic succession allow paleobathymetry to be estimated on the basis of biofacies. Sites with the highest rate of volcanic emission correspond to an average depth of 900 m; sites with thinner volcanic pile accumulated at a minimum depth of 1100 m. The facies sequence established in this work and the processes involved in its formation are comparable with the sequences and processes described in other submarine basic volcanic systems. Consequently, the depositional model for the Errigoiti Formation can perhaps serve as a reference model for this type of volcanic system.

Major and trace element composition of Skaergaard plagioclase; geochemical evidence for changes in magma dynamics during the final stage of crystallization of the Skaergaard intrusion

Plagioclase separates from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed to examine major and trace element variations. In general, plagioclase from the LS, UBS, and MBS show similar trends in major elements vs. crystallization: SiO2, Na2O, and K2O progressively increase, and CaO and MgO progressively decrease with fractionation. No abrupt changes in the trends of major components of Skaergaard plagioclase during the differentiation of the intrusion are observed. Trace elements in plagioclase reflect changes in the Skaergaard magma and changes in plagioclase distribution coefficients with differentiation. Sr, Ga, and probably Ba are included elements in Skaergaard plagioclase, but were excluded from the other cumulus phases, and as a result systematically increased in the magma and plagioclase during differentiation. Be, Cs, Hf, Rb, Ta, U, and Zr, and the transition metals Co, Cr, Cu, Ni, Sc, V, and Zn were excluded elements in Skaergaard plagioclase, and remained low in plagioclase during differentiation. Changes in the abundances of these elements in plagioclase during differentiation reflect changes in their abundance in the magma. With the exception of the lower zone, which is enriched in the light rare earth elements, rare earth elements in LS plagioclase, in general, increase with differentiation of the Skaergaard intrusion, but decrease dramatically at the UZa/UZb boundary where abundant apatite first appears. Rare earth elements in UBS plagioclase followed a similar trend to LS plagioclase, except during the initial and final stages of differentiation. UBS plagioclase is much more enriched in rare earth elements during the final 20% of crystallization, except for Eu, which is similar in plagioclase from the two series. The observed trends suggest that the floor and roof sequences became isolated from each other and that the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20% of crystallization. As the Skaergaard magma ceased convection, or convected as isolated cells, during the final stages of differentiation, volatile elements may have accumulated in the UBS magma, resulting in an increase in ƒO2, and a decrease in Eu/Sm in UBS plagioclase. The observed trends of rare earth elements in plagioclase from the LS and UBS fit well with theoretical calculations that assume closed-system crystallization, and would be difficult to reconcile with any model requiring significant discharge of magma from the chamber during the final 20% of crystallization. The enrichment of light rare earth elements in plagioclase, suggests that the lower part of the intrusion re-equilibrated with a late, light rare earth element-rich fluid or melt. The recharge model proposed by earlier workers to explain anomalous Sr and Nd isotopes appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples.

Surface and subsurface facies architecture of a small hydroexplosive, rhyolitic centre in the Mesoproterozoic Gawler Range Volcanics, South Australia

At Menninnie Dam, South Australia, a drilling program has revealed a complete section through the subsurface feeder system and erupted products of a small, hydroexplosive, rhyolitic centre within the Mesoproterozoic Gawler Range Volcanics. Porphyritic rhyolite intruded near-vertical faults in the Palaeoproterozoic basement and at less than a few hundred metres depth, interacted with fault-hosted (hot?) groundwater. Hydrofracturing of the wall rock occurred in advance of and at the margins of the rhyolitic intrusions. The rhyolitic intrusions have peperitic margins and grade into discordant lithic-rich PB facies. The advancing fragmentation front intersected the palaeosurface, triggering phreatic eruptions that deposited a poorly sorted, lithic-rich explosion breccia. Rhyolite then rose to the surface through the intrusive breccias and shallow-seated magma–water interaction occurred in the conduit within <50m of the surface. As the magma discharge rate increased, “dry” explosive activity prevailed. A fall deposit, the top of which is welded, was deposited close to the vent, and in more distal locations (>800m from the inferred source), the products include muddy sandstone and pumice breccia. At the end of the eruption, rhyolitic lava was extruded in the form of a small dome. The presence of contemporaneous Pb–Zn–Ag mineralisation in the wall rocks suggests that an active hydrothermal system may have been involved in the formation of the Menninnie Dam hydroexplosive volcanic centre.

Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions

Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid‐dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvös, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic‐flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built‐in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time‐dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi‐steady lava fountains (oscillating at the frequency of annular‐flow turbulence).

Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegraean Fields, Italy)

A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km 2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km 3 for the LFU and 7 km 3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10 −4 (LARI).

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10 8 m 3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO 2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.

Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: the 1990–1995 eruption at Unzendake volcano, Kyushu, Japan

Global positioning system (GPS) measurements made around Unzendake volcano, Kyushu, Japan, since January 1991 have detected ground deformation caused by magma intrusion and lava discharge. In the intermittent phreatic and phreatomagmatic eruption stage, the ground was inflating. After growth of the lava dome and following frequent pyroclastic flows at Unzendake volcano, the ground began deflating. These ground deformations are explained by the inflation and deflation of a Mogi's source model (a point source model) located about 6 km west of the active crater at a depth of 11 km, at an aseismic region. The observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation. The horizontal displacements at the reference GPS station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other GPS baseline systems. These observations validate our estimates for the pressure source. Based on the relation between the deformation volume of the ground surface and the discharged volume of the lava, it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater. Magma is estimated to be supplied to the reservoir at an average rate of 1.1×10 5 m 3/day; magma intrusion began in December 1989 at the latest and continued for 1.9×10 3 days.

Landsat infrared analysis of fumarole activity at Unzen Volcano: time-series comparison with gas and magma fluxes

We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992. We identify two discrete regions of the dome that were heated to high temperature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships between parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters derived from the radiance data closely follow those observed in the measured rate of magma effusion. The positive correlation observed between the fumarolically driven shortwave infrared flux and the magma discharge rate ( r 2=0.64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest that gas accumulation within the edifice did not occur, this conclusion agreeing with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO 2 flux rates. A positive correlation ( r 2=0.56) was also found between the mean radiance of the pixels in the fumarolically heated region and the overall size of that region. This suggests a potential mechanism whereby, when gas pressure within the edifice increased, excess gas escaped through additional pathways to the surface as well through an increased flux at the main fumarolic vents.

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is &lt; 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.

The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 × 106 m³ (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5–5%; magma discharge rates from 2300 to 4300 m³/s; ejecta volume of 3.2 (± 0.9) × 106 m³ (DRE). Ejecta consists of pumice (ρ =1160 kg/m³), higher density vesiculated ejecta (ρ =1300 to 2000 kg/m³), dense glassy clasts (ρ =2600 kg/m³), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.

Soufrière Hills Eruption, Montserrat, 1995–1997: Volcanic earthquake locations and fault plane solutions

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel‐time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE‐SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano‐tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late‐1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth ‐ the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.

The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan

This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.

Thermodynamics and fluid dynamics of effusive subglacial eruptions

We consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions. Subglacial eruptions typically generate catastrophic floods (jokulhlaups) due to melting of ice by lava and generation of a large water cavity. We consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in Iceland. Our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater. In subglacial eruptions the temperature difference driving convection is in the range 10–100 °C. Average temperature of the meltwater must exceed 4 °C and is usually substantially greater. We calculate melting rates of the walls of the ice cavity in the range 1–40 m/day, indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of Katla and the 1996 eruption of Gjalp fissure in Vatnajokull. The volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity. These pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography. Previous models of water-cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions. Energy released by cooling of basaltic lava at the temperature of 1200 °C results in a volume deficiency due to volume difference between ice and water, provided that heat exchange efficiency is greater than approximately 80%. A negative pressure change inhibits escape of water, allowing large cavities to build up. Rhyolitic eruptions and basaltic eruptions, with less than approximately 80% heat exchange efficiency, cause positive pressure changes promoting continual escape of meltwater. The pressure changes in the water cavity can cause surface deformation of the ice. Laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base. The results confirm that the water cavity develops by convective heat transfer.

P12 珪長質噴火におけるマグマ上昇過程と噴出率の関係 : 十和田火山におけるケーススタディ

P12 珪長質噴火におけるマグマ上昇過程と噴出率の関係 : 十和田火山におけるケーススタディ

Pumice fall deposits of the western Vulsini Volcanoes (central Italy)

Widely dispersed pyroclastic fall deposits are prominent among the volcanic products of the western part of the Vulsini Volcanic District (central Italy). In this paper we describe major pumice fall deposits in the area, that correlate to the early activity of the Latera Volcanic Complex (0.3-0.2 Ma). They represent excellent stratigraphie markers for a wide area mostly extending west and south of the present Latera caldera. Pumice fall deposits lower B and upper B are closely associated with major pyroclastic flow units of the Canino Eruptive Unit; pumice fall C caps the Canino products without relevant time hyatus. Pyroclastic falls D and E, the latter part of the minor Stenzano explosive eruption, belong to the stratigraphic interval between Canino and Farnese eruptive units. Pumice fall F is part of the initial products of the Farnese eruptive sequence. We present detailed stratigraphy, depositional features, vertical and lateral variations of grain size, componentry and chemical composition in individual fallout beds. Source areas, as inferred from thickness and maximum pumice and lithic clast size distributions, are located in the area of the present-day Latera caldera. Tentative estimates of volume, column height and magma discharge rate indicate moderate-large to large Plinian eruptive events.

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.

The intensity and magnitude of Holocene plinian eruptions from Mount St. Helens volcano

Dispersal characteristics of the T, We, Wn, Pu, Ps, Ye, Yn and Yb plinian fall deposits of Mount St. Helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport. Isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities (magma discharge) based on a theoretical model of tephra dispersal. New proximal thickness measurements are combined with an empirical distal extrapolation, based on studies of 53 plinian deposits, to calculate the magnitude (erupted mass) of each eruption. Layer Yn (3510 y r B.P.) represents the highest intensity and largest magnitude eruption at Mount St. Helens in post-glacial times. Modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase (~ 26 hours). Several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of Mount St. Helens. Peak intensity of the plinian phase was 10 8 kg/s and the total erupted volume was 4 km 3 (DRE of magma). Small plinian-style eruptions are represented by layers such as Ps and Pu of the Pine Creek eruptive period (3000-2500 yr B.P.) and have intensities of only ~ 10 6 kg/s. When compared with plinian eruptions from other volcanoes, the Holocene eruptions of Mount St. Helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate-sized stratovolcanoes. There is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle. This relationship may be related to a time-dependent process for the accumulation of differentiated and volatile-rich magma within the chamber beneath Mount St. Helens.