Oceanic fissure eruptions,subvolcanic intrusions and volcanic magma Chambers

Rifting along the mid-Atlantic ridge seems to have been accompanied by fissure eruptions which flooded the ocean bottom. Locally these plateau lavas rose above sea level and erosion revealed plutonic bodies emplaced within them. There is also some evidence of shallow magma chambers feeding surface volcanism. All these facts can be conveniently interpreted by assuming fractional melting of the upper mantle, at depths below about 50 km, and a pulsation of the pressure, produced by a varying gravitation, which seems capable of squeezing the molten fraction and of fracturing the solid crust above. Magma chambers can then be formed, probably by subterranean cauldron subsidence of Scottish type, they can leed surface volcanoes and will eventually solidify as plutonic bodies. Phase changes of eclogite, possibly present in the oceanic upper mantle, could also explain the uplift of island platforms.     

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Réunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en échelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Réunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active.     

Thermal effect of magma intrusion on the electrical properties of magnetic rocks from Hammamat sediments, Cairo, Egypt

The thermal effects of magmatic intrusion on the conductivity and dielectric constant of magnetic rocks from Hammamat sediments, NE desert, Cairo, Egypt (latitude ∼27° and longitude ∼33°) were investigated experimentally in the laboratory using nonpolarizing electrodes. Granitic magma was intruded into the Hammamat sediments, which are a mixture of mainly magnetite with sandstone and due to the thermal effect the area around was extensively heated and altered to different degrees. Due to this magma intrusion, magnetite was transformed (by heating) to hematite to different degrees according to its location from the intrusion. Complex impedance measurements were performed in the frequency range of 10 Hz to 100 KHz at normal temperature (∼20°C) and at a relative humidity of ∼50% RH. Samples were collected at different locations perpendicular to the core of the magma intrusion. Experimental data indicate that the electrical properties vary strongly as we move away (with distance) from the magma intrusion. The conductivity of hematite is ∼10⁻² S/m and that of magnetite is ∼10⁴ S/m. As we move from magnetite to hematite (to the core of the magma intrusion) it is supposed that the conductivity will decrease but it was found that the conductivity increases (which is supposed to be abnormal). The conductivity increases with increasing frequency from ∼10⁻⁸ S/m to ∼10⁻⁵ S/m with almost power‐law dependence on frequency. The conductivity increases in the order of one decade due to the variation from magnetite to hematite. The increase of conductivity, as we move from magnetite to hematite, was argued to be due to the heating that partially or completely melts the samples, thus the porosity of the samples was decreased and accordingly the conductivity and dielectric constant increased. It was also supposed that the grains of the conductor in the samples are coated or isolated with insulator material. A percolation behaviour for the conductivity and dielectric constant, characteristic of random conductor‐insulator mixtures, was found with distance, where continuous paths of the conductive material occur accompanied by peaking of the dielectric constant. Complex impedance plots show that as we move in the direction of altered samples (towards hematite) the relation between real and imaginary impedance changes from a linear form to an arc of a depressed semicircle and increases in depression as we move in the direction of the altered samples, which is consistent with the above interpretation.     

Injection of additional batches of magma into the formed magma chamber: The case study of Lukkulaisvaara intrusion,North Karelia

The probable implications of the injection of additional batches of melt into the magma chamber and the correlation of ore formation to these processes are considered. The assumed model of hydraulic impact (stamp) explains a number of the structural features of layered basite-hyperbasite intrusions and, in particular, formation of microgranular rocks, whose structure indicates a high rate of crystallization, and, probably, platinum group element (PGE) mineralization in the Lukkulaisvaara layered intrusion, North Karelia. It is shown that intrusion of additional batches of magma can lead to thermodiffusion, which is most effective in the vicinity of the contact between the chamber and the portion of magma. This, in turn, should result in the redistribution of chemical components in this zone and, probably, to anomalous concentrations of productive components at early penetration stages.     

Determining magma flow in sills,dykes and laccoliths and their implications for sill emplacement mechanisms

Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills, dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Ken Thomson–deceased, April 2007     

Effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies

What controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood. I propose that for volcanoes at divergent plate boundaries, these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them. This stress concentration is mainly a function of the applied tensile stress, associated with spreading, and the aspect (height/width) ratios of the chambers. High spreading rates and/or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies. It is found that for chambers at the same depth in a volcanic zone, the one of the highest aspect ratio will normally intrude magma most frequently. Also, if the chambers are of equal aspect ratios, the one at the greatest depth will intrude magma most frequently. Because the extrusion frequency of a volcano is a fraction of its intrusion frequency, the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano. These results are applied to the divergent plate boundary in Iceland, where many of the volcanoes appear to be fed by “double chambers”, that is, shallow chambers fed by deep-seated chambers. It is found that, except when the aspect ratio of the deep-seated chamber is much less than that of the shallow chamber, the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep-seated chamber. It is concluded that whereas the short-term (i.e., ≤10³ yrs) extrusion frequencies of volcanoes at the plate boundary in Iceland may be similar to the dike intrusion frequencies of the source chambers, the long-term (i.e., ≥10⁴ yrs) extrusion frequencies may be about ten times lower than the intrusion frequencies.     

Seismological significance of the 1977–1978 eruptions and the magma intrusion process of usu volcano, Hokkaido

An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations.     

Hawaiian xenolith populations,magma supply rates,and development of magma chambers

Hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types, magma supply rates, and xenolith populations. Magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage. Magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage; magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage. These magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained. During the earliest and latest stages, no magma storage zone exists, and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava. During the tholeiitic second stage, magma storage reservoirs develop and persist both at the base of the ocean crust and 3–7 km below the caldera; only xenoliths of shallow origin are carried to the surface by differentiated lava. During the alkalic third stage, magma in the shallow subcaldera reservoir solidifies, and crustal xenoliths, including oceanic-crustal rocks, are carried to the surface in lava that fractionates in an intermediate-depth reservoir. Worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs.     

Paleomagnetic results from the Skaergaard intrusion,East Greenland

The natural remanent magnetization of 22 out of a total of 31 oriented cores from the layered series of the Skaergaard gabbroic intrusion (age: 55 m.y.) in East Greenland shows good stability in thermal and AF testing. The average direction of 22 AF and 9 thermally treated specimens isD = 170°,I = ?59°,α₉₅ = 4.2 before correction for tilt. The mean directions after rotation around strike to horizontal and after rotation to original attitudes suggested by others yields poorer population statistics. It is therefore concluded that flexuring took place between solidification and acquisition of remanent magnetization, a range in temperature of about 500°C which may represent an interval of somewhat less than 250,000 years. No evidence for secular variation is observed which may also suggest slow cooling through the blocking temperature range. The polarity is reversed and the pole position without “tilt correction” is 165°E, 61°N,d_m = 6.2,d_p = 4.6, which is similar to pole positions reported by others for the overlying slightly older basalt.     

Stability of remanent magnetization of some dykes from Mysore State,India

Summary The magnetic properties of some dykes from Mysore State, India, have been studied in detail. The rocks were found to have aQ _n ratio varying from 1.5 to 11.6, a remanent coercive force varying from 100 to 250 Oersteds, Curie temperature varying from 250 to 480°C and were found to have lamellae of ilmenite oriented in (111) plane of magnetite. The stable natural remanent magnetization of the rock seems to be of TRM origin with titanomagnetite and low grade titanomaghemite being the main carrier of remanent magnetization.N.G.R.I. Contribution No. 70-215.     

Temperatures of entering magma,formation and dimensions of magma chambers of volcanoes

A mechanism, of formation of magma chambers that feed volcanoes is discussed. Heat conditions and dimensions of magma chambers which have existed for more than several thousand years may become stable. The approximate equations of heat balance of these chambers are derived by calculating the temperatureT ₁ of the magma entering chambers and the radiia of chambers. Calculations show that the radius of the shallow « peripheral » chambers of the Avachinsky volcano is less than 3–3.5 km. Possible maximum radii of « peripheral » magma chambers were estimated for the Kamchatkan volcanoes of medial size. The temperature difference in their chambers may reach 100–200 °C. This method can be applied to the calculations of « roots » of central-type volcanoes.     

A major change in magma sources in late Mesozoic active margin of the circum-Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt,southern Primorye,Russia

More than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major- and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow-dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high-Mg and high-Al gabbro-dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)-type mantle, whereas the younger suite from an enriched mantle II (EM2)-type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic-earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long-continuing subduction-related magmatism. The plume influence reached the maximum when the unique meimechite-picrite complex formed in the region.     

Palaeomagnetism of late Precambrian dolerite dykes from Varanger peninsula,north Norway

Palaeomagnetic investigations have been carried out on 12 dykes of Late Precambrian age from the Varanger peninsula, north Norway. The dykes are separated into two groups, the Kongsfjord dykes and the Båtsfjord dykes. In the Kongsfjord dykes, titanomagnetite is almost entirely erased, as a result of an extreme degree of alteration. Pyrrhotite is the dominating magnetic mineral, and only three stable specimen directions can be defined. In the Båtsfjord dykes, however, the most important magnetic constituent is nearly pure magnetite, and a two-axis magnetization structure is revealed. The directions of the major component conform to a Fisherian distribution, and are assumed to represent the relative Late Precambrian field. Superimposed on this magnetization is a minor component which is assumed to be of Caledonian origin, probably Ordovician. This latter remanence is in accordance with other Middle Palaeozoic results obtained in Western Europe. The upper age limit of the Late Precambrian field is discussed, and it is proposed that the polar shift from the Late Precambrian position to the main Palaeozoic group may have occurred as late as Middle Ordovician.     

Paleomagnetism of Lower Devonian volcanics and Devonian dykes from northcentral New Brunswick,Canada

Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α₉₅ = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, d_p = 6.5°, d_m = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.     

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands,Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km³) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x10⁶ kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO₂=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO₂=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO₂=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).     

Boninitic geochemical characteristics of high-Mg mafic dykes from the Singhbhum Granitoid Complex,Eastern India

The high-Mg mafic dykes from the Singhbhum Granitoid Complex in East India have geochemical characteristics[e.g.,enrichment of the large ion lithophile elements and light rare earth elements(LREEs) relative to high field strength elements(HFSEs):high-MgO(8%),high-SiO_2(52%),low-TiO_2(0.5%),and high CaO/Al_2O_3(0.58)]similar to those found in boninitic/noritic rocks.Their high percentage of orthopyroxene as a mafic mineral and of plagioclase as a felsic mineral,and normative hypersthene content greater than diopside content are also indications of their boninitic/noritic affinity.On a triangular diagram of MgO-CaO-Al_2O_3 and on binary diagrams of Ti/V vs Ti/Sc and TiO_2 vs Zr,these samples show geochemical similarities with Phanerozoic boninites and Paleoproterozoic high-Mg norites.On major and trace element variation diagrams,these dykes show a normal crystallization trend and their Nb/La(0.5) and Nb/Ce(0.21) values lower than average bulk crust(0.69 and0.33,respectively) suggest no crustal contamination.Their low values of Rb/Sr(0.11-0.41) and Rb/Ba(0.10-0.27)also suggest little or no effect of post magmatic processes.Their TiO_2(0.27-0.50),Al_2O_3/TiO_2(19.30-42.48),CaO/TiO_2(12.96-32.52),and Ti/V(12-18) values indicate derivation from a depleted mantle source under oxidizing conditions such as a mantle wedge.Ni vs Zr modeling shows that the studied high-Mg dykes were generated by25-30%melting of a refractory mantle source.Enrichment of Rb,Th,U,Pb,Sr,and LREEs,and depletion of HFSEs—especially Nb,P,Ti,Zr—on primitive mantle—and chondrite-normalized spider diagrams,respectively,are clear signals that the slab-derived component played an important role in the formation of melts for these rocks in a supra-subduction zone setting.     

Ilmenite-orthopyroxene intergrowths from the moon and the Skaergaard intrusion

Myrmekitic or eutectic-like intergrowths of ilmenite and orthopyroxene, which texturally resemble intergrowths from nodules in kimberlites have been observed in lunar breccia 60016, 92 and in Lower Zone a of the Skaergaard intrusion. The lunar sample is similar in mineral chemistry to kimberlite occurrences, while the Skaergaard sample is not as Mg-rich. In all cases, however, there is a systematic distribution of Mg between orthopyroxene and ilmenite. The similar textures were formed by different processes: the lunar intergrowth is probably a eutectic-type texture resulting from crystallization of a melt, while the Skaergaard intergrowth formed by a process of sub-solidus oxidation.     

On the heat and mass transfer from an ascending magma

The maximum heat transfer possible from a sphere of magma ascending through a viscous lithosphere is estimated using a Nusselt number formulation. An upper bound is found for the Nusselt number by using the characteristics of a potential flow which, it is argued, is similar in the limit to a non-isothermal Stokes-flow in which the fluid (wall rock) viscosity is sensitive to temperature. A set of cooling curves are calculated for a magma ascending at a constant velocity beneath an island arc. If the magma is to arrive at the surface without solidifying its ascent velocity must be greater than about 5.8 × 10^?3 cm s^?1, for a magma radius of 1 km, and greater than about 2.7 × 10^?5 cm s^?1, for a magma radius of 6 km. If the magma begins its ascent crystal free it will generally become superheated over most of its ascent. Using essentially the same formulation as for heat transfer the mass transfer to or from a spherical body of magma ascending at these velocities is given approximately by ΔC ? ΔW/10, where ΔC is the change in weight percent of a component in the magma during ascent and ΔW is the compositional contrast of that component between the magma and its wall rock.     

Role of precursory less-viscous mixed magma in the eruption of phenocryst-rich magma: evidence from the Hokkaido-Komagatake 1929 eruption

During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing. A similar eruption sequence has been reported for some other eruptions (Pallister et al. 1996; Venezky and Rutherford 1997), suggesting that eruption of a mixed magma is a precursor of phenocryst-rich magmas. For the purpose of understanding the tapping processes of the phenocryst-rich magma chamber, we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography. For the precursory mixed magma we estimate 33dž vol.% phenocrysts, andesitic-dacitic melt composition, 3 wt.% H₂O content, and temperature of 1040°C. In comparison, for the climactic, silicic end-member magma we estimate 48Dž vol.% phenocryst, high-silica rhyolitic melt, 3 wt.% H₂O, and temperature of 950°C, respectively. The mafic end-member magma, which was not erupted, is thought to be an almost aphyric basaltic-andesitic magma, based on mass balance calculation of the phenocryst content. The proportion of the mafic end-member magma component in the mixed magma was calculated to be 20-40 wt.%. On the basis of these data, we estimate magma viscosities of 10^3.9, 10^6.9, and 10^2.0 Pa s for the mixed, silicic end-member, and mafic end-member magmas, respectively. The calculated density differences among these magmas are inconsequential when possible errors are considered. We calculate the minimum excess pressure required for dike propagation to be 31 MPa for the silicic end-member magma and 8 MPa for the mixed magma, using the estimated viscosity and dike propagation model of Rubin (1995). If we assume that excess pressure is limited by the wall rock strength of the magma chamber, excess pressure retainable in the magma chamber is less than ca. 20 MPa. This suggests that the mixed magma was able to ascend to the surface without freezing, whereas the viscous silicic end-member magma could not. The formation and precursory eruption of the mixed magma are, therefore, effective and necessary initiation processes for the phenocryst-rich, viscous magma eruption.     

Petrological characteristics and volatile content of magma from the 2000 eruption of Miyakejima Volcano, Japan

Among the series of eruptions at Miyakejima volcano in 2000, the largest summit explosion occurred on 18 August 2000. During this explosion, vesiculated bombs and lapilli having cauliflower-like shapes were ejected as essential products. Petrological observation and chemical analyses of the essential ejecta and melt inclusions were carried out in order to investigate magma ascent and eruption processes. SEM images indicate that the essential bombs and lapilli have similar textures, which have many tiny bubbles, crystal-rich and glass-poor groundmass and microphenocrysts of plagioclase, augite and olivine. Black ash particles, which compose 40% of the air-fall ash from the explosion, also have similar textures to the essential bombs. Whole-rock analyses show that the chemical composition of all essential ejecta is basaltic (SiO₂=51–52 wt%). Chemical analyses of melt inclusions in plagioclase and olivine phenocrysts indicate that melt in the magma had 0.9–1.9 wt% H₂O, <0.011 wt% CO₂, 0.04–0.17 wt% S and 0.06–0.1 wt% Cl. The variation in volatile content suggests degassing of the magma during ascent up to a depth of about 1 km. The ratio of H₂O and S content of melt inclusions is similar to that of volcanic gas, which has been intensely and continuously emitted from the summit since the end of August 2000, indicating that the 18 August magma is the source of the gas emission. Based on the volatile content of the melt inclusions and the volcanic gas composition, the initial bulk volatile content of the magma was estimated to be 1.6–1.9 wt% H₂O, 0.08–0.1 wt% CO₂, 0.11–0.17 wt% S and 0.06–0.07 wt% Cl. The basaltic magma ascended from a deeper chamber (10 km) due to decrease in magma density caused by volatile exsolution with pressure decrease. The highly vesiculated magma, which had at least 30 vol% bubbles, may have come into contact with ground water at sea level causing the large explosion of 18 August 2000.Editorial responsibility: S. Nakada, T. DuittAn erratum to this article can be found at