Conduit Wall Research Articles

Dynamics of magma degassing

Abstract Gas exsolution and segregation are fundamental controls on eruption dynamics and magma genesis. Basaltic magma loses gas relatively easily because of its low viscosity. However, bubbles grown by decompression and diffusion during magma ascent are too small to segregate. Coalescence, however, can create bubbles big enough for gas to escape from the rising basalt magma. In evolved magmas, such as andesite and rhyolite, high viscosity prevents bubbles rising independently through the magma. The original gas content of magma erupted as lava is commonly the same as that erupted explosively, so that a gas separation mechanism is required. A permeable magma foam can form to allow gas escape once bubbles become interconnected. Magma permeabilities can be much higher than wall-rock permeabilities, and so vertical gas loss can be an important escape path, in addition to gas loss through the conduit walls. This inference is consistent with observations from the Soufrière Hills Volcano, Montserrat, where gas escapes directly from the dome, and particularly along shear zones (faults) related to the conduit wall. Dynamical models of magma ascent have been developed which incorporate gas escape. The magma ascent rate is sensitive to gas escape, as the volume proportion of gas affects density, magma compressibility and rheology, resulting in both horizontal and vertical pressure gradients in the magma column to allow gas escape. Slight changes in gas loss can make the difference between explosive and effusive eruption, and multiple steady-state flow states can exist. In certain circumstances, there can be abrupt jumps between effusive and explosive activity. Overpressures develop in the ascending magma, caused primarily by the rheological stiffening of magma as gas exsolves and crystals grow. A maximum overpressure develops in the upper parts of volcanic conduits. The overpressure is typically several MPa and increases as permeability decreases. Thus, the possibility of reaching conditions for explosions increases as permeability decreases, both due to overpressure increase and the retention of more gas. Models of magma ascent from an elastic magma chamber, combined with concepts of permeability and overpressure linked to degassing, provide an explanation for the periodic patterns of dome growth with short-lived explosive activity, as in the 1980–1986 activity of Mount St Helens. Degassing of magma in conduits can also cause strong convective circulation between deep magma reservoirs and the Earth’s surface. Such circulation not only allows degassing to occur from deep reservoirs, but may also be a significant driving force for crystal differentiation.

Elucidating changes in the degree of tracer dispersion in a subglacial channel

AbstractTracer injections into a subglacial channel at Unteraargletscher, Switzerland, were repeated at intervals of about 2 hours over two diurnal discharge cycles in August and September 2000. Records of dye concentration reveal a pronounced hysteresis in the velocity–dispersion relationship, thereby indicating alterations in the drainage system. Theoretical considerations for Röthlisberger channels suggest an evolution of the conduit cross-section in response to a diurnally varying discharge. We studied the relation between conduit cross-section and tracer dispersion with numerical tracer experiments. The velocity field for steady flow through a given conduit geometry is calculated using a commercial flow solver. Tracer transport is represented by a scalar volume which is advected by the velocity field. Experiments were conducted for several scenarios by varying flow velocity, conduit geometry and conduit roughness. Results show only a weak dependence of dispersion on conduit size. In contrast, changes in roughness of the conduit walls reveal a strong effect on tracer dispersion. Therefore, to explain the observed hysteresis in the velocity–dispersion relationship, we suggest that the evolution of a subglacial flow path might involve changes in roughness.

The mechanism of volcanic eruptions (a steady state approach)

A theory of a volcanic eruption as a steady magma flow from a magma chamber to the surface is described. Magma is considered as a two-component two-phase medium and magma flow is steady, one-dimensional and isothermal. Mass and momentum exchange between condensed and gaseous phases is assumed to be in equilibrium with nucleation of bubbles as a fast process at small oversaturations. Three basic flow structures in a conduit are recognized and three resulting types of eruption are described. (1) In the discrete gas separation (DGS) flow, melt with discrete gas bubbles erupts. This type includes lava eruptions and Strombolian activity. (2) In the dispersion regime there is continuous eruption of a gas-pyroclastic suspension. Catastrophic explosive eruptions are included in this type. (3) Here a partly destroyed foam is erupted with permeable structure that allows gas escape. Near the surface this structure evolves with an increase in permeability and a decrease in the total porosity. This type corresponds to lava dome extrusions. A criterion dividing DGS flow from the other two regimes is obtained: Di= Uηn 1/3 a/ c 0, where U is the magma ascent velocity, η is magma viscosity, n is number of bubble nuclei in a unit mass of magma, a is the solubility coefficient for volatiles in magma and c 0 is the volatile content. The boundary between the dispersion regime and the partly destroyed foam regime depends on U, c 0 and particle size d p . For the dispersion regime the basic governing parameters are: magma chamber depth H, conduit conductivity σ= b 2/ η ( b is the characteristic cross-dimension of the conduit), and excess pressure in the magma chamber. Instability of the dispersion regime is investigated numerically. The dependence of magma discharge rate on the governing parameters is described as a combination of equilibrium surfaces with cusp catastrophes. Magma chamber depth is a ‘splitting’ parameter in most cusps and controls the existence of unstable magma flow with sharp regime changes. Catastrophic rise of eruption intensity is possible if H< H cr . The critical chamber depth H cr depends approximately linearly on c 0 and is about 15 km at c 0=0.05. Some scenarios of eruption evolution are described in a steady state approximation, and the mechanism of catastrophic explosive eruptions is proposed. The theory is applied to well documented eruptions (Tolbachik, 1975–1976, and Mount St. Helens, 1980). The theory can also describe changes in a volcanic system, such as destruction of the conduit or chamber walls, destruction of the edifice, changes in magma composition, and caldera subsidence due to magma evacuation.

Minimum Depth of Soil Cover above Soil–Steel Bridges

Soil–steel bridges are built of flexible corrugated steel panels buried in well-compacted granular soil. Their design is based on the composite interaction between the soil pressures and the displacements of the conduit wall. The structure failure could be initiated by shear or tension failure in the soil cover above the steel conduit. The provisions for design given in different codes, such as the Canadian Highway Bridge Design Code, managed to avoid some of the problems associated with the failure of soil above soil–steel bridges by requiring a minimum depth of soil cover over the crown of the conduit taking into consideration the geometric shape of the conduit. However, the present code requirements for a minimum depth of cover were developed for a maximum span of 7.62 m and using nonstiffened panels of 51 mm depth of corrugation. The effect of having larger spans or using more rigid corrugated panels has not been examined before and is the subject of this paper. The present study uses the finite-eleme...

Physical analysis of the process of cavitation in xylem sap.

Recent studies have confirmed that cavitation in xylem is caused by air bubbles. We analyzed expansion of a preexistent bubble adhering to a crack in a conduit wall and a bubble formed by the passage of air through a pore of a pit membrane, a process known as air seeding. We consider that there are two equilibrium states for a very small air bubble in the xylem: one is temporarily stable with a bubble radius r1 at point s1 on the curve P(r) relating pressure within the bubble (P) with bubble radius (r); the other is unstable with a bubble radius r2 at point s2 on Pr (where r1 < r2). In each equilibrium state, the bubble collapse pressure (2sigma/r, where sigma is surface tension of water) is balanced by the pressure difference across its surface. In the case of a bubble from a crack in a conduit wall, which is initially at point s1, expansion will occur steadily as water potential decreases. The bubble will burst only if the xylem pressure drops below a threshold value. A formula giving the threshold pressure for bubble bursting is proposed. In the case of an air seed entering a xylem conduit through a pore in a pit membrane, its initial radius may be r2 (i.e., the radius of the pore by which the air seed entered the vessel) at point s2 on Pr. Because the bubble is in an unstable equilibrium when entering the conduit, it can either expand or contract to point s1. As water vaporizes into the air bubble at s2, P rises until it exceeds the gas pressure that keeps the bubble in equilibrium, at which point the bubble will burst and induce a cavitation event in accordance with the air-seeding hypothesis. However, other possible perturbations could make the air-seeded bubble contract to s1, in which case the bubble will burst at a threshold pressure proposed for a bubble expanding from a crack in a conduit wall. For this reason some cavitation events may take place at a xylem threshold pressure (Pl'*) other than that determined by the formula, Plp'* = -2sigma/rp, proposed by Sperry and Tyree (1988), which is applicable only to air-seeded bubbles at s2. The more general formula we propose for calculating the threshold pressure for bubble breaking is consistent with the results of published experiments.

Does status of attachment influence survival time of zebra mussel, Dreissena polymorpha, exposed to chlorination?

Mussels colonize cooling water circuits of power stations by attaching themselves to the pipe or conduit walls using byssus threads. Once manually detached, they quickly try to reattach by producing new byssus threads. In many published reports on antifouling bioassays, the test specimens are exposed to the biocide in an unattached state. These mussels, while trying to reattach, are likely to expose themselves more frequently to the toxic compound when compared to firmly attached mussels. The results of the assay, therefore, could vary, depending on the status of the mussels used. In this paper, we test the hypothesis that the status of attachment could influence the toxicity response of mussels and show that byssally attached zebra mussel, Dreissena polymorpha (Pallas), is more resistant to chlorine than unattached ones. An average increase of 27% in the survival time was observed for attached mussels over unattached ones in the chlorine concentration range of 0.25 to 3 mg/L. It is conclusively shown that the increase in sensitivity of the unattached mussels was related to an increase in the byssal activity, quantified presently as the byssogenesis index. The results indicate that future laboratory toxicity experiments involving mussels should be carried out using byssally attached ones.

Temporal evolution of the Minoan eruption (Santorini, Greece), as recorded by its Plinian fall deposit and interlayered ash flow beds

The Plinian fall deposit of the Minoan eruption (Santorini, Greece) and its interlayered ash flow beds show textural, componentry and chemical trends indicative of their temporal evolution and conditions that lead to the ash-flow emplacement. The deposits have been sampled in detail at a selected locality, and the following features have been analyzed: (i) maximum clast size; (ii) grain size distribution; (iii) componentry; (iv) lithic types; (v) pumice morphology; (vi) pumice types; and (vii) pumice crystal abundance. Stratigraphic variations of these features are assumed to record changes that occurred during the eruption, including changes in eruptive plume height, nature and relative efficiency of the magma fragmentation process, conduit stability, and magma flow in the conduit. Based upon the inferred temporal changes, we propose the following eruption reconstruction. The sustained Plinian column activity reached its climax (in terms of mass eruption rate and plume altitude) soon after the beginning of the eruption, and subsequently began to gradually subside. As a result of eruption waning, the conduit walls began to collapse, causing an increase in the shear stress in the flowing vesicular magma and an increase in the efficiency of the magma fragmentation process. Eventually the conduit closed or became too narrow to support a high-standing eruption column, leading to ash flow eruption as a result of the collapse of what was left of the eruptive plume, and/or directly from emission of an overpressured jet. Subsequent conduit pressure increases led to conduit re-opening and a Plinian column with associated pumice fallout began anew. The eruptive magma fragmentation process is proposed to have been of a brittle nature and linked to the shear stress generated by conduit wall friction. Fragmentation efficiency is recorded by grain size distributions and free crystal contents of the deposits.

Ascent and decompression of viscous vesicular magma in a volcanic conduit

During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava‐filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two‐dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the “exit” boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug‐like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth‐dependent radii.

Textural heterogeneities in pumices from the climactic eruption of Mount Pinatubo, 15 June 1991, and implications for magma ascent dynamics

The climactic event of Mount Pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism. We have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits, and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions. White pumice has higher vesicularity, deformed and highly coalesced vesicles with thin walls, euhedral phenocrysts and microlite-free groundmass. Gray pumice shows lower vesicularity, wider ranges in vesicle number density, limited coalescence, vesicles with thick walls that are less deformed, phenocrysts and microphenocrysts with abundant solution pitting, and groundmass containing ubiquitous microlites and crystal fragments. The presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors. In particular, the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls, a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety.

Development of an Ancient Back‐Arc Basin Overlying Continental Crust: The Archean Peltier Formation, Northwest Territories, Canada

The 2.67–2.69‐Ga Peltier Formation of the Point Lake volcanic belt, central Slave Province, Canada, is a mafic‐dominated succession that erupted in a back‐arc basin overlying sialic crust. The formation overlies the 3.22‐Ga Augustus Granite, is interstratified with the >2.65‐Ga turbiditic Contwoyto Formation, and is associated with 2.68‐Ga felsic and intermediate volcaniclastic rocks of the Samandre and Beauparlant Formations. Mafic rocks in the Peltier Formation range from N‐MORB (mid‐ocean ridge basalt) with flat to slightly LREE‐ (light rare earth element) enriched patterns, to tholeiitic LREE‐enriched E‐MORB with Nb and Ti depletions, and finally to strongly LREE‐enriched calc‐alkaline compositions with marked Nb and Ti depletions. The ϵNd values are between −2.94 and +3.06 and increase with decreasing (La/Yb)n ratios. Insulation of the conduit walls during ascent of early magmas may have limited the effects of crustal contamination over time. A possible tectonic model for the Point Lake belt evolution involves eruption of tholeiitic basalts between 2700 and 2680 Ma and calc‐alkaline basalts and andesites between 2680 and 2660 Ma into a back‐arc that developed following extension and rifting of an older remnant arc overlying continental crust. Construction of a new arc and subsequent mass wasting resulted in the deposition of felsic‐intermediate volcaniclastic material. Continuous erosion of the continental crust during volcanic activity is represented by the interstratification of turbidites and volcanic rocks.

Capillarity and sap ascent in a resurrection plant: does theory fit the facts?

Capillarity and sap ascent in a resurrection plant: does theory fit the facts?

Trends in wood density and structure are linked to prevention of xylem implosion by negative pressure.

Wood density (D t), an excellent predictor of mechanical properties, is typically viewed in relation to support against gravity, wind, snow, and other environmental forces. In contrast, we show the surprising extent to which variation in D t and wood structure is linked to support against implosion by negative pressure in the xylem pipeline. The more drought-tolerant the plant, the more negative the xylem pressure can become without cavitation, and the greater the internal load on the xylem conduit walls. Accordingly, D t was correlated with cavitation resistance. This trend was consistent with the maintenance of a safety factor from implosion by negative pressure: conduit wall span (b) and thickness (t) scaled so that (t/b)2 was proportional to cavitation resistance as required to avoid wall collapse. Unexpectedly, trends in D t may be as much or more related to support of the xylem pipeline as to support of the plant.

Functional and ecological xylem anatomy

Abstract Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure ( P x ), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.

Adiabatic temperature changes of magma–gas mixtures during ascent and eruption

Most quantitative studies of flow dynamics in eruptive conduits during volcanic eruptions use a simplified energy equation that ignores either temperature changes, or the thermal effects of gas exsolution. In this paper we assess the effects of those simplifications by analyzing the influence of equilibrium gas exsolution and expansion on final temperatures, velocities, and liquid viscosities of magma–gas mixtures during adiabatic decompression. For a given initial pressure (p 1 ), temperature (T 1 ) and melt composition, the final temperature (T f ) and velocity (u max ) will vary depending on the degree to which friction and other irreversible processes reduce mechanical energy within the conduit. The final conditions range between two thermodynamic end members: (1) constant enthalpy (dh=0), in which T f is maximal and no energy goes into lifting or acceleration; and (2) constant entropy (ds=0), in which T f is minimal and maximum energy goes into lifting and acceleration. For ds=0, T 1 =900 °C and p 1 =200 MPa, a water-saturated albitic melt cools by ~200 °C during decompression, but only about 250 °C of this temperature decrease can be attributed to the energy of gas exsolution per se: the remainder results from expansion of gas that has already exsolved. For the same T 1 and p 1 , and dh=0, T f is 10–15 °C hotter than T 1 but is about 10–25 °C cooler than T f in similar calculations that ignore the energy of gas exsolution. For ds=0, p 1 =200 MPa and T 1 =9,000 °C, assuming that all the enthalpy change of decompression goes into kinetic energy, a water-saturated albitic mixture can theoretically accelerate to ~800 m/s. Similar calculations that ignore gas exsolution (but take into account gas expansion) give velocities about 10–15% higher. For the same T 1 , p I =200 MPa, and ds=0, the cooling associated with gas expansion and exsolution increases final melt viscosity more than 2.5 orders of magnitude. For dh=0, isenthalpic heating decreases final melt viscosity by about 0.7 orders of magnitude. Thermal effects of gas exsolution are responsible for less than 10% of these viscosity changes. Isenthalpic heating could significantly reduce flow resistance in eruptive conduits if heat generation were concentrated along conduit walls, where shearing is greatest. Isentropic cooling could enhance clast fragmentation in near-surface vents in cases where extremely rapid pressure drops reduce gas temperatures and chill the margins of expanding pyroclasts.

A Study on Installation Process of Telecommunication Wire into Conduit with Bends.

Modeling of phenomena and derivation of basic equations have been described about the installation of parachute attaching telecommunication cable in the conduit with bends using spiral airflow. It was clarified that in the bends relatively high friction force generated by contacting and/or attaching the cable to the conduit (pipe) wall in a limited distance yield to a jump in the tensile force exerting the cable. At the same time, the evaluation of the friction coefficient between the cable (and/or parachute) and the pipe wall was based upon the hypothesis that an efficient installation of the telecommunication cable using the spiral airflow owed to its reducing effect of the friction force. Through the analysis conducted, the availability of the numerical simulation was demonstrated.

Theory of acoustic propagation in a multi-phase stratified liquid flowing within an elastic-walled conduit of varying cross-sectional area

A theoretical solution is derived for the sound field in an arbitrarily layered viscous fluid moving with the non-negligible Mach number within a duct with elastic walls and varying cross-sectional area. The solution is applied to the interpretation of infrasonic and seismic signals preceding and accompanying volcanic eruptions, and can be used to study the acoustic response of various types of volcanic fluids, including magma–gas mixtures, ash–gas mixtures, and bubble-rich liquids. The acoustic field in a magma conduit can be propagated into the atmosphere through an open vent, and coupled into the ground through the displacement of the magma conduit walls. The theoretical solutions predict that fluids moving with the non-negligible Mach number will exhibit significant attenuation in the upstream direction, thereby reducing the quality factor of the conduit resonance. Thus, acoustic energy generated during an eruption may be preferentially radiated downstream, exacerbating the acoustic decoupling between the upper and lower parts of a stratified magma column. A source model for a repeated cavitation process is introduced as a possible excitation mechanism for tremor signals.

Processes in High-Mg, High-T Magmas: Evidence from Olivine, Chromite and Glass in Palaeogene Picrites from West Greenland

Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation, West Greenland, contain 6·5–30 wt % MgO, averaging 15·5 wt % MgO. Olivine (mg-number 77·4–93·3) forms diverse assemblages of zoned phenocrysts and xenocrysts showing evidence for equilibrium and fractional crystallization, oxidation, partial to complete re-equilibration, as well as magma mixing. The olivine crystals contain glass inclusions and have high contents of Ca and Cr, indicating that all olivines with up to mg-number 93·0 crystallized from melts. Associated chromites (mg-number 45·4–77·2) are essentially unzoned and in equilibrium with the olivines. Matrix glasses from pillow breccias have 6·7–8·8 wt % MgO and quenched close to 1200°C with oxidation states one log-unit above the NNO (nickel–nickel oxide) buffer. Compositional differences between the glasses from different volcanic members are inherited from the primary melts. The magmas erupted as crystal-charged melts, and liquids with more than ∼14 wt % MgO were not erupted. The compositions of the unerupted parental melts were calculated by stepwise addition of equilibrium olivine to the matrix glasses, and these melts had 20–21 wt % MgO and liquidus temperatures of 1515–1560°C. They had lower FeO* than the erupted rocks with 20–21 wt % MgO because the rocks contain more iron-rich olivine than the melts would have had. The accumulated primary melts ascended through a lithospheric lid of ∼100 km thickness, and we envisage that major crystallization took place from ∼45 km depth and to the surface. Magma batches ascended in narrow dyke-like conduits and fractionated high-Mg olivine and chromite at deep levels and less magnesian crystals at shallower levels. At high ascent velocities numerous olivine crystals were carried in suspension to the surface, even some from deep levels, whereas during slower ascent most olivines settled out before eruption. Pulsating ascent rates led to mixing of magma batches in various stages of fractionation. On average, 13 wt % olivine was left within the crust, presumably as olivine-plated conduit walls. The conduit systems are similar to the crystal-rich narrow magma chambers suggested for mid-ocean ridges but are of much greater vertical extent.

Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results. MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.

Re–Os fractionation in eclogites and blueschists and the implications for recycling of oceanic crust into the mantle

Metabasalts (eclogites, blueschists and mafic granulites) metamorphosed in paleosubduction zones show a range in rhenium (Re) abundances between 3 and 1689 parts per trillion (ppt), with a median value of 331 ppt Re. The median Re abundance of the metabasites corresponds to only ∼40% of the Re abundances expected for likely mafic protoliths. Osmium (Os) abundances in the metabasites (2–42 ppt, with one sample at 909 ppt) are comparable to abundances in mid-ocean ridge basalt (MORB). Re was lost from the protoliths either during dehydration, the most likely explanation, or during alteration of the protoliths near the ocean ridges. Radiogenic 187Os/ 188Os and 206Pb/ 204Pb of HIMU ocean island basalts (OIB) that are believed to contain a component of recycled altered MORB would require excessive amounts (>80–90%) of ∼0.5–1 Gyr old recycled altered MORB in the mantle source, if constraints on Re/Os and U/Pb from the metabasites are applied. The necessary amount of recycled component is only marginally lower (70%) if equal amounts of altered and unaltered 2 Gyr old MORB are present in the mantle source. Considering the major element abundances in alkaline OIB, these estimates appear unreasonably high. One possibility is that the simple bulk mixing models commonly employed are not applicable. Rapid melting during the formation of alkaline basalts and sluggish kinetics may prevent complete equilibration with Os-rich phases such as sulfides and alloys. Other possible explanations include the shielding of Os-rich phases in peridotite by silicates from reaction with partial melts of eclogite and the isolation of melt from peridotite by means of pyroxene-rich, Os-poor reaction zones along conduit walls. In the case of disequilibrium, the actual fraction of recycled component in OIB sources could be much lower than in equilibrium mixing models.

Giuseppe Imbò and his contribution to volcano seismology

Together with Fusakichi Omori and Kenzo Sassa, Giuseppe Imbò is one of the founders of volcano seismology. Imbò's novel idea in volcanic tremor analysis was the introduction of what he called "numeri indici dell'intensità eruttiva" (numerical indices of the eruptive intensity). It was one of the first attempts to quantify eruptive activity on the basis of seismic data. The indices permitted him to establish correlations at Mt. Vesuvius between seismic observations and eruptive phenomena. He found significant differences in the character of the recorded seismic signals between pre- and post-eruptive phases of effusive and explosive activity of the March 1944 eruption of Vesuvius. According to Imbò, volcanic tremors are the product of a rhythmic hammering caused by the degassing magma normal to the vertically-oriented walls of the feeding conduits.