Magma Overpressure Research Articles

Multiscale fracturing as a key to forecasting volcanic eruptions

Among volcanoes reawakening after long repose intervals, the final approach to eruption (∼1–10 days) is usually characterised by accelerating rates of seismicity. The observed patterns are consistent with the slow extension of faults, which continue to grow until they connect a pre-existing array of subvertical fractures and so open a new pathway for magma to reach the surface. Rates of slow extension are here investigated assuming that gravitational loading and magma overpressure create a fluctuating stress field in the country rock. Fluctuations are due to the intermittent growth of small cracks that cannot be detected by monitoring instruments. The rate of detected events is then determined by the frequency with which the concentration of strain energy around the tips of macroscopic faults becomes large enough to permit fault extension. The model anticipates oscillations in seismic event rate about a mean trend that accelerates with time, and identifies the rate of increase in peak event rate as a key indicator of the approach to eruption. The results are consistent with earlier empirical analyses of seismic precursors and, applied to data before the andesitic eruptions of Mt Pinatubo, Philippines, in 1991, and of Soufriere Hills, Montserrat, in 1995, they suggest that, for such volcanoes, reliable forecasts of magmatic activity might eventually be feasible on the order of days before eruption.

A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions

The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 105 to 106 years. We address the question of why buoyant and otherwise eruptible high-silica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 1018 to 1020 Pa s–1, dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>102 km3) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The long-term magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of small-volume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits.

Magma chamber behavior beneath a volcanic edifice

The construction of a large volcanic edifice at Earth's surface generates stresses in the upper crust whose magnitude is comparable to those of tectonic stresses and overpressures within a magma chamber. We study how this affects eruption behavior. Analytical calculations are carried out in two dimensions for a cylindrical reservoir with an internal overpressure in an elastic half‐space with an edifice at the surface. Different edifice shapes are considered, from shield volcanoes with gentle slopes to stratovolcanoes with steeper flanks. Without an edifice at the top, the hoop stress at the cavity walls reaches a maximum at two symmetrical points at some distance from the axis, away from the top of the chamber. With an edifice at the top, the maximum is reached at the top of the chamber, just beneath the edifice summit. This implies preferential failure of chamber walls at the axis and hence the focussing of volcanic activity through a central vent system. Tensile failure of the cavity walls occurs for a critical value of magma overpressure which depends on the dimensions of the edifice and on the depth and size of the cavity. For a small magma chamber beneath a large stratovolcano, the magmatic overpressure at the onset of eruption increases as the edifice grows and decreases following edifice destruction. These effects may explain why pressures recorded in phenocryst assemblages at Mount St. Helens, have varied over the past 4000 years as the edifice went through successive phases of growth and destruction.

Dynamics of magma ascent and lava extrusion at Soufrière Hills Volcano, Montserrat

Abstract Growth of the andesitic lava dome during eruption of Soufrière Hills Volcano, Montserrat involved general increases in discharge rate and dome height with superposed fluctuations on timescales from hours to months. We have modelled magma conduit flow incorporating viscosity variations caused by degassing and crystallization. Gas loss is modelled by permeable flow, with variations of permeability constrained by measurements. Crystallization kinetics is modelled by an Avrami law. Observations and petrological studies constrain conduit diameter ( c . 30 m), magma chamber crystal content ( c . 60-65%), melt water content ( c . 5%), magma temperature ( c . 850°C) and chamber depth ( c . 5 km). Gas loss and rheological stiffening cause a maximum in magma overpressure (magma pressure minus lithostatic pressure) at depths of a few hundred metres below the dome. This maximum can explain ground deformation patterns, shallow seismicity, and short-lived Vulcanian explosions. Crystallization kinetics causes a strong feedback mechanism and multiple steady solutions for discharge rate. Small changes in chamber pressure, magma viscosity and conduit diameter strongly amplify discharge rate. Multiple solutions allow cyclic variation of discharge rate. Escalation of activity is attributed principally to influx of hot mafic magma into the chamber, resulting in chamber pressure increasing with time. Dome growth can be intrinsically unpredictable due to non-linear effects.

The eruption of Soufrière Hills Volcano, Montserrat (1995-1999): overview of scientific results

Abstract The eruption of Soufrière Hills Volcano, Montserrat (1995-1999) has displayed a wide range of volcanic phenomena: growth of an andesitic lava dome, generation of pyroclastic flows by lava dome collapse and by fountain collapse in explosive eruptions, Vulcanian and sub-Plinian explosivity with accompanying tephra fall, entrance of pyroclastic flows into the sea, sector collapse with formation of a debris avalanche and a high-velocity pyroclastic density current, and generation of lahars. New phenomena include: cyclic patterns of ground deformation linked with shallow seismicity and eruptive activity; pyroclastic flows formed by rapid sedimentation from pyroclastic surges; and an unprecedented slow escalation of eruption intensity. Magma pulsations with timescales of hours to years have been recognized. Transitions from extrusive to explosive activity were triggered by major dome collapses. Relationships between magma ascent dynamics and geophysical signals have been elucidated. Ascending water-rich andesitic magma becomes Theologically stiffened by degassing and groundmass crystallization. Large magma overpressures are consequently developed in the upper conduit, causing shallow seismicity, radial patterns of ground deformation, and cyclic pulsations of eruptive activity. Lava dome growth involved heterogeneous deformation with formation of spines and lobes along shear zones. Collapse of the pressurized dome resulted in substantial pyroclastic surges forming above pyroclastic flows. Influx of hydrous mafic magma remobilized hot crystalline igneous rocks at depths of 5 to 6 km to form crystal-rich andesitic magma and triggered the eruption.

Can rainfall trigger volcanic eruptions? A mechanical stress model of an active volcano: ‘Piton de la Fournaise’, Reunion Island

This paper relates the hydrology of a volcano to its eruptive activity, which seem to be correlated. Based on the groundwater pattern previously determined by hydrologic and thermal modelling, we formulate hypotheses for the mechanisms linking rainfall and volcanic eruptions. We focus on: mechanical loading by the weight of the infiltrated rain, which might favour magma overpressure or slope instability on the eastern volcanic flank; unloading of the infiltrated water, discharged toward the ocean, which could increase the tangential tensile stress above the magma chamber. We test the sensitivity of the model to the magma chamber geometry and the rainfall distribution. The mechanical unloading of the infiltrated water is the mechanism most likely to trigger eruptions. However, the presence of a sliding surface and rainfall loading could favour the development of tensile stress in the ‘shell’ surrounding the magma chamber but with a lower intensity than in the previous case.

Late Palaeozoic and Palaeocene magmatic intrusion levels in West Connacht and inferences for palaeotopography

Dolerite dykes in Connemara and Murrisk that yield c. 315 Ma K-Ar ages vesiculated in varying degrees at the present exposure level. This incipient degassing suggests that the dyke-contemporary land surface lay not more than several hundred metres above the present-day surface. The West Connacht peneplain was cut across Dalradian and Ordovician-Silurian rocks during mid-late Devonian, following which it was buried under Devonian-Carboniferous sedimentary strata since almost entirely removed. Triassic-Jurassic rifting of the central Atlantic basin produced a strong thermal imprint on the rocks of the west of Ireland, an event assumed to have been accompanied by crustal uplift and faulting to produce horst-graben terrain. Ensuing erosion throughout Jurassic-Cretaceous time stripped off much or all of the late Palaeozoic cover. If and where the stripping penetrated into the sub-Carboniferous peneplain, then to that extent the present West Connacht peneplain has an end-Mesozoic sculpture imprinted on it. Palaeocene dolerite dykes in Connemara, Murrisk and North Mayo, associated with initiation of the North Atlantic basin, degassed to a generally greater extent than their late Palaeozoic precursors. This is consistent with geologically estimated shallower levels of emplacement into the contemporary crust, assuming similar magmatic steam overpressures at given depths. Any thickness of Chalk over West Connacht cannot have been significant in terms of overburden pressure. Late Tertiary uplift of the West Connacht peneplain produced the present Connemara-Murrisk plateau which has been deeply dissected by Pleistocene glaciation.

Relationships among crustal structure, volcanism and strike–slip tectonics in the Lipari–Vulcano Volcanic Complex (Aeolian Islands, Southern Tyrrhenian Sea, Italy)

The Lipari–Vulcano Volcanic Complex (LVVC, Aeolian Arc, Southern Tyrrhenian Sea, Italy) develops along a strike–slip fault system that bisects the Aeolian Arc. The LVVC crustal structure is investigated by applying seismic methods to two data sets: P-wavefronts generated by local and distant events constrain the shallow velocity structure; DSS data provide shape and depth of the main crustal discontinuities. The obtained velocity model shows that the LVVC shallow structure is characterized by the presence of two sharp lateral discontinuities. The northern discontinuity bounds a low velocity basin-like structure including the La Fossa Caldera and Mt. Guardia Caldera depressions. The southern discontinuity coincides with the southern boundary of a high-velocity diffractor located beneath the La Fossa Cone. Direct modeling of DSS data define three crustal discontinuities showing a complex geometry. The intra-crustal interfaces and the Moho discontinuity show an upheaval beneath the LVVC central sector. The estimated crustal velocities are lower than those generally reported for the continental crust. This feature is due to the high heat flux and fluid circulation affecting the LVVC area. The ratio between the thickness of the upper and lower crust and the presence of the Moho upheaval are consistent with that reported for continental areas affected by extensional strain. The collected geophysical data, combined with volcanological and structural information, indicate that: (a) the Mt. Guardia and La Fossa Caldera depressions represent the surface expression of a single structure whose formation is mainly due to tectonic processes; (b) the LVVC crustal structure and the evolution of the volcanism are consistent with those recognized in zones of transition from arc-related to rift volcanism. In addition, results of a preliminary mechanical analysis based on the available structural data indicate low values of the ratio between magmatic overpressure and tectonic stress. This suggests a passive mechanism of mantle upwelling. The mantle upwelling is due the post-collisional normal strike–slip tectonics affecting the Aeolian Arc.

Mechanics of oblique spreading and ridge segmentation

Mid-ocean ridges display a variety of plan view geometries (planforms) that correlate with the tectonic setting. A mechanical analysis is proposed to rationalize the variety of planforms of mid-ocean ridges at various tectonic settings. We model spreading centers as fluid-filled cracks, and find the variation of segment orientation with ΔP/ΔS (where ΔP measures a magmatic overpressure within the crack and ΔS is the remote ‘tectonic tension’). The analogy suggests that high ΔP/ΔS tends to preserve the continuity of an oblique spreading axis, whereas low ΔP/ΔS prompts segmentation. It follows that a planform of the spreading center is an indicator for the forces driving melt injection. The results are in qualitative agreement with principal geological observations. For instance, the concordant, oblique, and continuous Reykjanes Ridge reflects pressurized magma emplacement ( ΔP/ΔS> 20), a result compatible with the proximity to the Icelandic hot spot and with low ΔS anticipated in passive margins. Away from major hot spots, the Mid-Atlantic Ridge (MAR) with its passive margins typically has a value of ΔP/ΔS of around unity. The values change widely through space and time, locally becoming negative (amagmatic spreading). This spatial and temporal variability in MAR is consistent with dynamic melt injection. Conversely, in orthogonal-discordant-segmented axes of the Pacific, magma emplacement is dominated by slab pull (high ΔS) despite high magmatic activity. The inferred value of ΔP ≈ 0 in the Pacific, stable through space and time, is consistent with passive melt injection.

The morphology and formation of flow-lobe tumuli on Icelandic shield volcanoes

Three types of tumuli are recognized on monogenetic shield volcanoes in Iceland, namely (1) lava-coated tumuli, (2) upper-slope tumuli and (3) flow-lobe tumuli. Flow-lobe tumuli grade into lava rises, which are larger inflation structures, but both structures have a similar mode of emplacement. The areal distribution of flow-lobe tumuli in the flow field differs from that of lava-coated tumuli and upper-slope tumuli. Flow-lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields, whereas the other two tumulus types are more frequent in the proximal parts of the flow fields. The flow-lobe tumuli also differ from the two other tumulus types as regards their morphology: they are significantly larger, have shallower flanks, and do not have extensive outflows from the cracks. The vesicle distribution within the flow-lobe tumuli is similar to that of P-type flow lobes: a fairly dense zone is situated between the upper and lower vesicular zones. Each flow-lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust. Major lava tubes are mostly absent from the areas where flow-lobe tumuli form and, therefore, the emplacement of flow-lobe tumuli must be related to secondary, short-lived pathways of lava. The flow-lobe tumuli inside the flow fields of Icelandic shield volcanoes are morphologically identical to those flow-lobe tumuli that are found in the margins of flow fields of other Icelandic volcanoes. These observations allow us to model the flow-lobe tumuli as morphological variants of P-type flow lobes forming along terminating secondary lava paths. Flow-lobe tumuli form at low lava-supply rates of the order of 10 −2 − 10 −5 m 3/s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core. We model the tumulus as a circular elastic plate that becomes uplifted, inflated and fractured by a uniform magmatic overpressure at its bottom. We conclude that (1) the effective thickness of the lava crust that is subject to bending is 0.2–0.4 m and remains essentially constant during the formation of the tumulus, and (2) for flow-lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano, the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m. This elevation difference can give rise to a static magmatic overpressure of 2.5 MPa in the tumulus, whereas the overpressure needed to form a typical flow-lobe tumulus is only 0.2–1 MPa.

The dynamics of the Breccia Museo eruption (Campi Flegrei, Italy) and the significance of spatter clasts associated with lithic breccias

The Breccia Museo Member is a pyroclastic deposit produced during an eruptive event that occurred in the southwestern sector of Campi Flegrei about 20,000 years ago. Two depositional units divided by a co-ignimbrite ash-fall deposit have been recognized. Facies variations in the deposits resulted from the interaction between the flow and paleomorphology, from the relative abundance of the lithic and juvenile components supplied by the source, and from the variations of the flow regime. The Lower Depositional Unit is a pyroclastic flow deposit characterized by a thick, coarse valley facies laterally grading into a thin, layered and fine-grained overbank facies. These different facies are due to the interaction between a density-stratified flow and topography. The more basal, high-concentration part of the flow was deposited along the axis of the paleovalleys (valley facies), whereas the upper, low-concentration part was deposited on the slopes (overbank facies). Vertical variations of the structures observed in the deposits of the Lower Depositional Unit resulted from flow unsteadiness during emplacement and, hence, on the variations of the suspended load fallout from the low-concentration upper part of the flow to the high-concentration boundary layer. The Upper Depositional Unit, made up of the Breccia, Spatter and Upper Pumice Flow Units, consists of horizons of lithic breccias and coarse welded spatter which thicken into the valleys. They are closely related to a gas-pipe-rich ash and pumice flow deposit. The strongly fines-poor character of the breccias and spatter beds is due to a very rapid segregation of the dense and coarse clasts and to the high rates of gas ascent through the hindered-settling zone in the basal part of the flow. After deposition of the majority of the dense and coarse material, the subsequent high-density depositional system came to rest immediately, thus yielding a pyroclastic flow deposit that is closely associated with the breccia. The presence of lithic breccia and spatter beds within the stratigraphic sequence is interpreted to reflect changes in the magma chamber structure during the eruption. Due to the drainage of part of the magma during the first phase of the eruption, the roof of the magma chamber collapsed and the lithostatic pressure fell below the magmatic pressure. We suggest that this new magmatic overpressure, not related to the expansion of the gas, generated fragmentation of poorly vesiculated magma inside the magma chamber. This triggered a new eruptive phase during which a mixture of spatter, pumice and collapse-produced lithic debris were erupted.

Mecanismos de emplazamiento, deformación y transición de diques a filones manto en sedimentos jurásicos y cretácicos del norte de Chile

ABSTRACT. Mechanism of emplacement, deformation and transition of dykes to sills in Jurassic and Cretaceous sediments in northern Chile. Thick sequences of marine Jurassic and Lower Cretaceous deposits are well exposed in the Andes of northern Chile. These rocks constitute relicts of back are basin infill, that in most if not all cases, are cross-cut by andesitic dykes and sills. Dykes usually feed the sills that in turn concentrate at a certain stratigraphic level. Field studies of contact relationships and of dyke and sill internal structures lead to the conclusion that at least in the Sierra de Moreno and in El Salvador-Potrerillos areas, the intrusives were emplaced into wet and semi-consolidated deposits, during burial and basin subsidence. The analysis of the intrusive process scaled to the sill and dyke geometry let conclude that the elastic constants of the host rock at the time of intrusion were low, as those one would expect in wet, semilithified sediments. An analysis of the magmatic overpressures that caused the intrusion, indicates that the magma chamber was located at a depth varying between 3 and 5 km below the level at which the sills are concentrated. The emplacement of tabular bodies was mainly controlled by the magnitude and orientation of the principal stress field. The dyke to sill transition and the stratigraphical concentration of sills resulted from a local inversion of the stress field probably due to the effect of the interception of rocks with higher elastic constants than the rest of the sequence. The sill to dyke transition is also very sensible to the strain regime active at the time. In those places where the sediments are intruded by sills the deformation of the host rock is much less than in those places where the sills are absent. It can be inferred that the stresses required to initiate buckling of the multilayer with the sill intercalations will not be attained before the fracture strenght limit of the sill-sediment multilayer system is reached. From a different view-point it can be concluded that in those sedimentary basins with low porosity and Iow permeability rocks, the fractured sills may become effective hydrocarbon reservoirs. Contrary to what would be expected, the reservoirs will be located where the sequence is less deformed. Conclusions derived from this study also allow to predict the size and distribution of veins, thats and veinlets, provided that the hydraulic and stress constants have been adequately calibrated.

Incremental caldera collapse of Suswa volcano, Gregory Rift Valley, Kenya

Suswa volcano, located at 1°10′S, 36°20′E, is Quaternary in age (<0.4 Ma), dominantly trachytic-phonolitic in composition, and has two calderas. Regional extension was a fundamental control on caldera collapse, providing pathways for the siting, drainage and recharge of magma chambers. Caldera I collapse was associated with magmatic overpressure from volatile exsolution, magma-water interaction, influx of denser magma and magma drainage at depth. Trachybasalt ash, trachyte globular-ash ignimbrites, trachyte pumice lapilli air-fall tuffs and carbonate-trachyte ignimbrites characterize the initial subsidence. Air-fall tuffs, erupted during caldera collapse at Longonot, are interbedded, suggesting a regional collapse event. Incremental, but dominantly Valles-type, collapse continued with the eruption of trachyte agglutinate flows from concentric ring-fractures outside the caldera ring-fault (Ring-Feeder Zone) and trachyte pumice lapilli air-fall tuffs from west caldera I. Following caldera I collapse, phonolite lava flows were erupted from the caldera floor. Centrally-erupted phonolite lava flows led to the construction of Ol Doinyo Onyoke lava cone. A pit-crater on the cone was a precursor to the collapse of caldera II, both of which were generated entirely by magma withdrawal. Regional decompression caused ring-fault bounded, block-resurgence of the caldera floor

Volcanic plumbing systems

Report of a meeting held by the Volcanic Studies Group at the Open University on 7 December 1991. This research seminar, targeted principally at postgraduates and senior undergraduates, was convened with the object of reviewing current ideas on the functioning and form of volcanic and sub-volcanic plumbing systems. The seminar was topical, with the recent activity at Mount Pinatubo not only serving to keep volcanoes and volcanology in the public eye, but also providing a timely reminder, in human terms, of the importance of understanding the mechanisms which operate within and beneath volcanoes. Increased knowledge in this area can lead only to the more accurate prediction of volcanic events, and consequently to the more effective execution of mitigation policies and reduced risk at the world’s 700 or so currently active volcanoes. The first two speakers concerned themselves primarily with the application of physical parameters measured at the surface. R. S. J. Sparks (University of Bristol) presented a simple physical model designed to interpret observations of lava eruptions in terms of changing eruption conditions. The model assumes that magma overpressure is proportional to erupted volume which is in turn directly related to the effusion rate. These relationships were used, together with field observations, to monitor factors controlling variations in lava effusion rate. Application of the model to three well documented eruptions demonstrated that variations in magma flow rate during the course of an eruption may be explained by changes in magma overpressure, modified at times by additional factors including magma solidification

The Role of Magma Overpressure in Suppressing Earthquakes and Topography: Worldwide Examples

In an extending terrane basaltic magma supplied at a pressure greater than the least principal stress (overpressure) may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting. As vertical dikes intrude, they press against their walls in the direction opposite the least principal stress and increase its magnitude. The emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses. This process tends to equalize the stresses and thus diminishes the deviatoric stress (difference between maximum and minimum stresses) that creates faults and causes earthquakes. Observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension. Recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks.

Intrusion of horizontal dikes: Tectonic significance of Middle Proterozoic diabase sheets widespread in the upper crust of the southwestern United States

Initially horizontal sheet intrusions of Middle Proterozoic diabase are abundant in a region 650 by 300 km across in Arizona and California. The diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences. Massive granite is the most common basement host for the sheets, probably because it fractured more easily than foliated hosts during sheet emplacement. Steep feeder dikes are rare compared to the sheets. The diabase in many places is exposed in fault blocks that were tilted during Tertiary tectonic extension. Structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust, to depths of at least 13 km. Sheet intrusion implies a vertical orientation of the least compressive stress, so I conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1.1 Ga. Magma overpressures, water encountered by rising magma, vertical changes in the crustal stress regime, and flotation of low‐density granite all may be important factors for sheet intrusion. The stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event. The diabase province contrasts structurally with similar‐age provinces of basaltic magmatism elsewhere in North America that show evidence of tectonic extension, such as the midcontinent rift. The consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks. Sheet intrusions in the geologic record may be seriously underreported. Their recognition is important for the interpretation of seismic reflection profiles of continental crust. Sheets in Arizona may be responsible for the Bagdad reflection sequence, which extends to depths of at least 15 km.

Basaltic melt evolution of the Hengill Volcanic System, SW Iceland, and evidence for clinopyroxene assimilation in primitive tholeiitic magmas

The thick oceanic crust of Iceland is formed by tholeiitic central volcanoes arranged in en echelon patterns along the 40–50 km wide rift zones. The Hengill central volcano in the southwestern rift zone has produced 25–30 km3 of hyaloclastites and lava during the last 0.11 m.y., with maximum productivity during the isostatic rebound following the degalciations 0.13 and 0.01 m.y. ago. The eruption units cover a compositional spectrum from picrite to rhyolite, but the volume of andesite, dacite, and rhyolite is small. The petrographic relations of pillow rim and hyaloclastite glass indicate that the basaltic melts were saturated with olivine and plagioclase, except for the most primitive ones that were undersaturated with plagioclase. Saturation with clinopyroxene was reached in some of the intermediate and evolved basaltic melts. Corroded and partly resorbed crystals of clinopyroxene and partly disintegrated gabbro nodules with resorbed clinopyroxene indicate that selective assimilation contributed to the evolution of the most primitive melts. The intermediate and evolved basaltic glass compositions fall along the low‐pressure cotectic for mid‐ocean ridge basalt (MORB) compositions saturated with olivine, plagioclase, and clinopyroxene, but the primitive glasses (9–9.7 wt % MgO) fall well inside the low‐pressure olivine + plagioclase primary phase volume. The primitive Hengill glasses have significantly higher CaO and lower Al2O3 than primitive glasses from oceanic spreading centers. Their low pressure undersaturation with respect to clinopyroxene and the absence of clinopyroxene phenocrysts indicate that they are not parental to the intermediate Hengill basalts, since fractionation modelling requires a large proportion of clinopyroxene in the fractionating assemblage. The most primitive melts could be produced by fractionation of olivine and plagioclase combined with 5–30 % assimilation of clinopyroxene, and the intermediate melts could be derived by mainly olivine and plagioclase fractionation, from melts equilibrated with peridotitic residues at pressures of 1–2 GPa. The further evolution of the basaltic spectrum can be explained by fractionation of olivine, plagioclase, and clinopyroxene combined with minor contamination by anatectic crustal melts. The rate of magma supply from the mantle to the crust is controlled by the isostatic conditions and is very low in periods of glacial loading of the crust. This leads to infrequent and small eruptions of dominantly evolved magmas. The dense picritic magmas (9–9.7 wt % MgO in the glass phase) were driven to the surface by magmatic overpressure in the mantle at an early deglaciation stage characterized by the absence of large, trapping magma chambers in the lower crust. The assimilation of clinopyroxene in these melts could proceed by direct contact with the solidified cumulate sequences and gabbro intrusions. Clinopyroxene assimilation in combination with olivine fractionation may also contribute to the chemical evolution of some of the most primitive MORB magmas.

A model for the 1975–1976 Tolbachik eruption, Kamchatka, as deduced from geodetic data

Abstract Geodetic measurements made between 1971 and 1978 in the region of the large Tolbachik fissure eruption of 1975–1976 in Kamchatka revealed two general types of crustal deformations: 1. (1) uplift and extension of a large area occurring during the eruption. 2. (2) subsidence and compression of the zone near the newly-formed volcanoes occurring after the eruption. Furthermore, rapid horizontal and vertical displacements of the area adjacent to the eruption region occurred during the eruption. Displacements both preceding and occurring after the magma issue to the surface have been measured. Their duration was found to be from a few minutes to several days. Crustal deformations associated with volcanic activity may be caused by pressure sources located at certain depths below volcanoes. The configuration and the origin of pressure sources may be different (feeding channel, peripheral chamber, intruding column or dyke), but the deformation source mechanism in all cases is the same. Based on these simulations, the depth and radius of a simple spherical pressure source were calculated. This source creates a field of elastic displacements of the earth's surface best coinciding with geodetic data. The magma overpressure was calculated from data on the width and length of a fissure during the magma issue to the surface. It proved to be not more than 300 kg/cm2. the location of pressure sources was deduced from data on vertical displacements. Based on these data, the existence and depths of two shallow magma chambers which fed the North vents eruption have been calculated. They are located at depths of about 6 and 1.5 km, one over another. Verification of the calculated deformation sources was supported by deep seismic sounding data. Two seismic shadows apparently caused by the melt occurrence in the magma chambers have been detected at just these same depths.

Anatomy of a granophyre intrusion

Various structural and textural features of a drusy granophyric ring-dyke intrusion can be explained in terms of the behaviour of magmatic water. Initial crystallization of water-rich magma close to ternary eutectic composition resulted in textures grading from granophyric to microgranitic. Eventual entrapment of water-rich pockets resulted in crystallization from a silicate liquid and a hydrothermal liquid simultaneously, probably at magmatic overpressure, to form isolated bodies of pegmatite containing giant euhedral quartz crystals. Later aplite, rhyolite breccia, and tuffisite veins reflect the escape of magma and magmatic gas through the congealed parts of the intrusion, at first permissively, but later with increasingly explosive effects.