Volume Of Erupted Magma Research Articles

The Astroni volcano: the only example of closely spaced eruptions in the same vent area during the recent history of the Campi Flegrei caldera (Italy)

The Astroni volcano formed during the third and most recent epoch of activity (4.8–3.8 ka) of the Campi Flegrei caldera (CFc). The activity of the volcano was dominated by explosive, mostly phreatomagmatic eruptions, with only subordinate lava effusions. We have grouped the sequence of deposits into seven distinct units, separated by erosional unconformities or very thin paleosols. The units include mostly surge beds, with subordinate strombolian deposits and lavas, and one plinian fallout layer. The total volume of erupted magma is 0.45 km 3 (DRE), while the total mass is 1.12×10 12 kg. The magma feeding the first five eruptions was alkali-trachytic and slightly zoned, while the last two eruptions tapped a magma batch resulting from mixing of the previously extruded alkali-trachytic and a less evolved trachytic magma. The volcano grew at the northwestern edge of the polygonal volcano-tectonic collapse, northwest–southeast elongated, which accompanied the Agnano–Monte Spina eruption (4.1 ka), the largest of the third epoch. Available radiometric dates and stratigraphical data constrain the age of the volcano in the final part of the 4.1–3.8 ka time span. This implies that the seven eruptions followed each other at very short time intervals. This conclusion is also supported by constancy in archaeological facies of findings within the paleosols between variable Astroni units, in the plain north of the caldera. The sequence of close eruptions in the same area, although with a slight migration of the vent from northwest to southeast, makes the Astroni volcano peculiar in the recent history of the CFc. Therefore, the definition of its history is very important in order to understand one of the past phenomenologies of the caldera, relevant elements to forecast its behavior.

Development of a volcanic hazard model for New Zealand

We commence development of a volcanic hazard model for New Zealand by applying the well- established methods of probabilistic seismic hazard analysis to volcanoes. As part of this work we use seismologically-based methods to develop eruption volume - frequency distributions for the Okataina and Taupo volcanoes of the central Taupo Volcanic Zone, New Zealand. Our procedure is to use the geologic and historical record of large eruptions (erupted magma volumes ≥ 0.01 cubic km for Taupo and ≥ 0.5 cubic km for Okataina) to construct eruption volume-frequency distributions for the two volcanoes. The two volcanoes show log-log distributions of decreasing frequency as a function of eruption volume, analogous to the shape of earthquake magnitude-frequency distributions constructed from seismicity catalogues. On the basis of these eruption volume-frequency distributions we estimate the maximum eruption volumes that Taupo and Okataina are capable of producing at probability levels of relevance to engineers and planners. We find that a maximum eruption volume of 0.1 cubic km is expected from Taupo with a 10% probability in 50 years, while Okataina may not produce a large eruption at this probability level. However, at the more conservative 2% probability in 50 years, both volcanoes are expected to produce large eruptions (0.5 cubic km for Okataina and 1 cubic km for Taupo). Our study therefore shows significant differences in eruption probabilities for volcanoes in the same physiographic region, and therefore highlights the importance of establishing unique eruption databases for all volcanoes in a hazard analysis.

The role of volatiles in magma chamber dynamics.

Many andesitic volcanoes exhibit effusive eruption activity, with magma volumes as large as 10(7)-10(9) m(3) erupted at rates of 1-10 m(3) x s(-1) over periods of years or decades. During such eruptions, many complex cycles in eruption rates have been observed, with periods ranging from hours to years. Longer-term trends have also been observed, and are thought to be associated with the continuing recharge of magma from deep in the crust and with waning of overpressure in the magma reservoir. Here we present a model which incorporates effects due to compressibility of gas in magma. We show that the eruption duration and volume of erupted magma may increase by up to two orders of magnitude if the stored internal energy associated with dissolved volatiles can be released into the magma chamber. This mechanism would be favoured in shallow chambers or volatile-rich magmas and the cooling of magma by country rock may enhance this release of energy, leading to substantial increases in eruption rate and duration.

Deformation associated with the 1997 eruption of Okmok volcano, Alaska

Okmok volcano, located on Umnak Island in the Aleutian chain, Alaska, is the most eruptive caldera system in North America in historic time. Its most recent eruption occurred in 1997. Synthetic aperture radar interferometry shows deflation of the caldera center of up to 140 cm during this time, preceded and followed by inflation of smaller magnitude. The main part of the observed deformation can be modeled using a pressure point source model. The inferred source is located between 2.5 and 5.0 km beneath the approximate center of the caldera and ∼5 km from the eruptive vent. We interpret it as a central magma reservoir. The preeruptive period features inflation accompanied by shallow localized subsidence between the caldera center and the vent. We hypothesize that this is caused by hydrothermal activity or that magma moved away from the central chamber and toward the later vent. Since all historic eruptions at Okmok have originated from the same cone, this feature may be a precursor that indicates an upcoming eruption. The erupted magma volume is ∼9 times the volume that can be accounted for by the observed preeruptive inflation. This indicates a much longer inflation interval than we were able to observe. The observation that reinflation started shortly after the eruption suggests that inflation spans the whole time interval between eruptions. Extrapolation of the average subsurface volume change rate is in good agreement with the long‐term eruption frequency and eruption volumes of Okmok.

Toba ash layers in the South China Sea: Evidence of contrasting wind directions during eruption ca. 74 ka

Two cores from the southern South China Sea contain discrete ash layers that mainly consist of rhyolithic glass shards. On the basis of the SPECMAP time scale, the ash layers were dated to ca. 74 ka, the age of the youngest Toba eruption in northern Sumatra. This link is supported by the chemical composition of the glass, which is distinct from volcanic glass supplied from the Philippines and the northern South China Sea, but is almost identical with the chemistry of the Toba ash. The youngest Toba ash layers in the South China Sea expand the previously known ash-fall zone over more than 1800 km to the east. The dispersal of ashes from Sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that (1) the Toba eruption probably happened during the Southeast Asian summer monsoon season, and (2) the volume of erupted magma was larger than previously interpreted.

Pressure evolution during explosive caldera-forming eruptions

Caldera-forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber. Field studies of caldera-forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions. Pressure evolution during explosive caldera-forming eruptions is investigated through a simple model that describes the first-order quantitative behaviour of the chamber. We consider a piston-like model that assumes a coherent block subsiding along circular, sub-vertical, ring faults into the magma chamber. This subsidence occurs after significant decompression of the chamber by an initial central vent eruption. We assume that the initial pressure distribution in the chamber is magmastatic. Once collapse has begun the chamber roof is supported by the magma, so that magma pressure at the chamber roof increases to lithostatic. We suggest that pressure variations during caldera-forming eruptions are mainly controlled by variations in magma volatile content. Regardless of what induces the formation of ring faults, the model suggests that the occurrence of explosive caldera-forming events depends on the strength of the chamber walls, and the depth, water content and aspect ratio of the magma chamber. No significant differences exist between model results for a cylindrical, or a more realistic elliptical magma chamber geometry of comparable aspect ratio. Assuming a constant strength of the host rock, the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges, for deep, gas-poor chambers, from a few percent up to 40% for shallow, gas-rich chambers. The model suggests that zoned chambers tend to collapse earlier than homogeneous chambers. Dike-shaped chambers will erupt less magma than sill-like chambers before caldera collapse initiates, although dike-like geometries are not associated with stress fields appropriate to create ring faults. The model suggests that once initiated, caldera collapse will tend to force out most or all of the volatile-rich magma from the chamber. For volatile-rich magma chambers, the total volume of erupted magma during caldera-forming event is of the same order as the chamber volume. The model also explains the variation in the erupted mass during the different phases of explosive caldera-forming eruptions, and is in good agreement with natural examples.

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

[magma volume (DRE): 24 ± 5 km3]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H2O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H2O is at least 3.7 ± 0.6 wt.% H2O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H2O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate.

The Agnano–Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy)

The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar/ 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km 3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

The Denham Caldera on Raoul Volcano: dacitic volcanism in the Tonga–Kermadec arc

Denham Caldera, on Raoul Volcano in the Tonga–Kermadec arc, is a simple collapse structure. It is 6.5×4 km 2 across, >0.3 km deep, represents an erupted magma volume of 8–16 km 3, and is considerably larger than previously estimated. Caldera formation was contemporaneous with an eruption of homogeneous dacitic magma at 2.2 ka. Tephra from this event is preserved as a pumice fall and pyroclastic flow sequence up to 120 m thick on Raoul Island. Offshore, this sequence forms a submarine deposit diminishing in thickness from 120 m near the northern caldera rim to 80 m at a distance of 3.5 km, where it rests upon an erosional planation surface of 17–20 ka age incised into older volcaniclastic rocks. Both the caldera volume and ejecta distribution of the 2.2 ka eruption are comparable to the caldera volume and ejecta distribution of the 1883 Krakatau eruption. Resurgent volcanism has built at least six submarine pyroclastic cones along a north–northeast trending lineament crossing Denham Caldera. Redistribution of pumiceous dacite from these cones, including ephemeral islands that emerged during the 1814 and 1870 eruptions, has contributed to recent shoaling in Denham Bay. A smaller eruption occurred in 1964–65, and hydrothermal activity persists at several sites along the lineament. Voluminous felsic volcanism at intra-oceanic arcs is usually associated with crustal extension, and was unexpected in the Tonga–Kermadec arc. However, because the tectonic fabric within the active back-arc Havre Trough is oblique to the Kermadec arc, rifts within the trough apparently propagate to the volcanic front and transfer extensional strain to the arc, thereby promoting the ascent of felsic magma. The orientation of Denham Caldera, as well as that of Macauley Caldera 110 km further south, thus reflect the stress field of the Havre Trough and not the predominant field of the arc.

Crustal regime of deformation and ascent of granitic magma

Structural studies and gravity data demonstrate the magma flow pattern of granitic plutons and their morphology at depth. The deepest zones calculated from gravity data, provided they show vertical lineations in outcrop, are interpreted as the feeder channels which fed the pluton. Their morphology at depth, as well as the position and number of root zones, vary according to the type of deformation, suggesting that regional deformation determines the emplacement of granitic plutons. Therefore, deformation must also influence the segregation and the ascent of acid magmas. A thermal model is constructed which demonstates the structure and heat generation of the Hercynian crust. According to that model, the thermal gradient changes the conditions for the deformation of the crust. Obviously, a large amount of heat must be supplied to induce melting of the lower crust. As a result magma segregation and ascent occur entirely within the ductile field. When upwelling magma reaches the brittle/ductile transition zone, the hydrodynamic force (buoyancy) is insufficient to fracture the upper brittle crust. Parameters which control diapirism (wavelength, viscosity, thickness) do not match those related to the volume of erupted magma. As a consequence, diapiric upwelling appears to be limited under favourable conditions to the lower ductile crust and does not apply to intrusions in a brittle crust. Shearing helps to segregate magma and to feed conduits toward the surface since it creates the required zones of local extension for the magma to infiltrate.

Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril'sk area, Siberia

We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Noril'sk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive ϵ Nd values and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower ϵ Nd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equilibrium with garnet. Over time, the percentage of melting in the source region and the volume of magma produced increased, and garnet was no longer stable in the plume source. All of the plume-derived melts initially contained more than 20 wt% MgO and became less Mg rich by fractionation of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and isotopic compositions of all the erupted lavas was processing of mantle-derived magma in crustal reservoirs during periodic replenishment, periodic tapping, continuous crystal fractionation and wallrock assimilation. Rapid eruption of an extremely large volume of processed magma that varied little in chemical and isotopic composition produced the sequence of relatively monotonous tholeiitic basalts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province near Noril'sk.

A simple magma-mixing model for sulphur behaviour in calc-alkaline volcanic rocks: mineralogical evidence from Mount Pinatubo 1991 eruption

The increasing recognition of anhydrite as a primary magmatic mineral in volcanic products, including the important 1991 Pinatubo eruption, places new emphasis on the cycling of sulphur from deep sources including the upper mantle. We present a simple model for the supersaturation of sulphur during mingling and mixing in calc-alkaline magmas due to the quenching and oxidation of the mafic end member, which may have important consequences for predicting the annual flux of igneous sulphur. This model predicts the coexistence of anhydrite and sulphur dioxide gas, and explains the discrepancy between erupted magma volumes and sulphur gas emissions. Although sulphur may exist as a number of species in volcanic emissions, one of its most important phases in volcanic rocks is primary anhydrite (CaS0 4 ). A number of important historic eruptions of calc-alkaline volcanoes have now been found to involve anhydrite-bearing magmas. These include Mount Lamington in Papua New Guinea (Arculus et al. 1983), Nevado del Ruiz in Colombia (Fournelle 1990), and El Chichon in Mexico ( Rose et al. 1984 ). The Ruiz eruption involved documented magma mixing, and both the Nevado del Ruiz and El Chichon eruptions were associated with very high sulphur emissions (both approximately 3.3 x 10 6 tonnes; Krueger 1983 ; Williams et al. 1990 ). We have observed magma-mixing textures in detailed studies of two neighbouring calc-alkaline volcanoes in northern Chile, at Lascar ( Matthews 1992 ) and Tumisa ( Gardeweg 1988 ). Anhydrite has been found associated with these textures in pumice from Lascar Volcano. Both anhydrite and similar disequilibrium textures indicative of magma

Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo

Research Article| October 01, 1992 Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo H. R. Westrich; H. R. Westrich 1Geochemistry Department 6118, Sandia National Laboratories, Albuquerque, New Mexico 87185 Search for other works by this author on: GSW Google Scholar T.M. Gerlach T.M. Gerlach 2U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar Author and Article Information H. R. Westrich 1Geochemistry Department 6118, Sandia National Laboratories, Albuquerque, New Mexico 87185 T.M. Gerlach 2U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (10): 867–870. https://doi.org/10.1130/0091-7613(1992)020<0867:MGSFTS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. R. Westrich, T.M. Gerlach; Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo. Geology 1992;; 20 (10): 867–870. doi: https://doi.org/10.1130/0091-7613(1992)020<0867:MGSFTS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A water-rich magmatic gas phase escaped explosively from Mount Pinatubo on June 15, 1991, taking with it a load of crystalline and molten material sufficient to form pumice and tephra deposits with an estimated total dense-rock-equivalent volume of 3-5 km3 (Scott et al., 1991), and carrying in it enough sulfur to form a 20 Mt SO2 cloud in the stratosphere. Application of the "petrologic method" for estimating sulfur degassing during the climactic event from the sulfur content of trapped glass inclusions and matrix glasses in the pumice deposits requires an unacceptably large volume of erupted magma to account for SO2 in the stratospheric cloud. The ubiquitous presence of primary vapor bubbles in glass inclusions and unaltered anhydrite phenocrysts in the pumice suggest that sulfur was present in a separate H2O-rich gas phase of the Pinatubo magma before eruption. Thus, for this eruption, and perhaps others, the petrologic method for estimating sulfur degassing is prone to substantial underestimation of sulfur release and the potential climatic impact of past explosive eruptions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The cogenetic organ cauldron and batholith, south central New Mexico: Evolution of a large‐volume ash flow cauldron and its source magma chamber

The Organ cauldron and its underlying plutonic roots are exposed through a vertical section 5–6 km thick in the Organ Mountains of south central New Mexico. Emplaced within a 1 m.y. time span at 33.7–32.8 Ma, both cauldron fill tuffs and lavas, as well as the underlying Organ batholith, document emplacement of a chemically and mineralogically zoned magma chamber at shallow depths. The tuffs, as much as 3.0 km thick near the center of the cauldron, show rather simple normal chemical and mineralogical gradients which indicate zoning in the magma chamber from aphyric rhyolite (77% SiO2) at the top to crystal‐rich rhyolite (68% SiO2) at deeper levels. Lava flows, more than 0.3 km thick, overlie the tuffs and apparently represent still deeper, more mafic (68–61% SiO2), crystal‐rich parts of the erupted magma volume. The oldest phase of the batholith may also be compositionally zoned from quartz syenite (68% SiO2) at the top to monzodiorite (55% SiO2) at the lowest exposed level. This phase of the batholith, which forms both a concordant floor beneath the volcanic roof and a thick dikelike body along part of the cauldron margin, may be the undrained residue of the magma chamber whose silicic cap was erupted as tuffs and lavas. Small bodies of fine‐grained, porphyritic alkali feldspar granite at the top of the batholith may represent undrained remnants of the silicic cap. Diapirlike intrusions of the deeper, more mafic parts of the batholith into higher more silicic parts may be related to eruption mechanisms. Three younger plutonic phases of the batholith seemingly represent three stages of silicic core magma generation and intrusion during the progressive crystallization and differentiation of the batholith. Major normal faults which offset cauldron fill and are invaded by plutonic rocks suggest the cauldron evolved in a regional extensional stress field.

Magma supply, magma discharge and readjustment of the feeding system of mount St. Helens during 1980

The landslide and cataclysmic eruption of Mount St. Helens on May 18, 1980 triggered a sequence of explosive eruptions over the following five months. The volume of explosive products from each of these eruptions decreased uniformly over this period, and the character for each eruption progressed from a fairly continuous eruption lasting more than eight hours on May 18 to a series of short bursts, some of which were spaced 12 hours apart, on October 16–18. The transition in the character of these eruption sequences can be explained by a difference between the magma supply rate and the magma discharge rate from a shallow reservoir. The magma supply rate (MSR) is the rate with which magma is supplied to the level where disruption due to vesiculation occurs. It is determined by dividing the dense-rock-equivalent volume of eruptive products by the total duration of each eruption sequence. The magma discharge rate (MDR) is the rate with which the disrupted magma is discharged through the vent. It is determined by dividing the volume of erupted products by the duration of each explosive burst. The relative magnitude of these two quantities controls the temporal evolution of an explosive event. When MDR∼MSR the explosive phase of the eruption lasts for several hours as a single continuous event. When MDR>MSR, an eruption is characterized by a series of short explosive bursts at intervals of several minutes to several days. The MSR of the eruptions of 1980 decreased with time from 5500 m' s −1 on May 18 to 7 m 3 s −2 on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m 3 s −1. Each explosive event has been followed by an aftershock-like series of earthquakes located beneath the volcano at depth mostly between 7 and 14 km. The seismic energy released during each of these series is proportional to the corresponding volume of erupted magma. Deformation data between June and November, 1980 indicate a subsidence of the volcanic structure which can be modeled by a volume collapse of 0.25 km 3 located at 9 km depth. We propose a model in which magma is supplied from depths of 7–14 km through a narrow conduit during each eruption. It erupts to the surface at a uniform rate during each eruption. The deep seismic activity following each eruption is related to a readjustment and volume decrease in the deep feeding system. The decrease of the MSR over time is explained by an increase in the viscosity of a progressively water-depleted magma. The amount of water necessary to explain the observed decrease of the MSR is of the order of 4.6%.

Calderas: A planetary perspective

Caldera‐bearing volcanoes are the largest volcanic edifices on a planet and are expressions of long‐lived, near‐surface magma chambers. On earth only three morphological classes of calderas are common: shield calderas (e.g., Mauna Loa, Hawaii), stratocone calderas (e.g., Crater Lake, Oregon), and ash flow calderas (e.g., Valles Caldera, New Mexico). Each of these classes has a characteristic caldera size, magma type, and tectonic location. No calderas have been recognized on the moon or Mercury, but Mars has large shield calderas as well as the unique highland patera calderas. The latter may represent an explosive, ultramafic style of volcanism, unlike any volcanic activity in modern earth history. Radar images suggest that Venus has at least one large caldera‐bearing shield volcano and possible calderas of other types. Jupiter's satellite Io has abundant, large shieldlike calderas but no shield massifs. The number and variety of calderas on a planet may be proportional to planet mass, consistent with a previous suggestion that volcanic complexity increases with planet size. Io is an exception, but heating in Io is generated tidally rather than by the decay of radioactive elements, so that its calderas are not related to its size. The occurrence of shield volcanoes on four solar system bodies suggests that thermal and tectonic mechanisms (i.e. hot spots) giving rise to construction of shield mountains and associated collapsed summits are basic processes in planetary geology. Hot spot volcanism is expected to be most important on planets lacking plate tectonics. On earth, large shields have lifetimes of only a few million years, but on Mars, large shields may have been active for 1.5×109 years. Shield calderas on earth are smaller than 20 km in diameter, but known or suspected shield calderas on Mars, Venus, and Io have considerably larger diameters. Assuming that caldera diameters are related to magma chamber diameters and depths, magma chambers on other planets are significantly larger and/or deeper than their counterparts on earth. Terrestrial magma chambers are small because terrestrial shield volcanoes are small in comparison to shields on Mars and Venus, and terrestrial shields are small because the earth's mobile plates move growing shields away from their deep sources, thus distributing magma along a narrow chain rather than piling it up around a central vent. Finally, the long lava flows of Mars are also explained if the volume of erupted magma has any simple relation to magma chamber volume.

Steady state volcanism: Evidence from eruption histories of polygenetic volcanoes

Some volcanoes erupt magma at average rates which are constant over periods of many years, even though this magma may appear in a complex series of eruptions. This constancy of output is tested by construction of a curve of cumulative volume of erupted magma, which is linear for steady state volcanism, and whose gradient defines the steady state rate Qss. The assumption is made that Qss is the rate at which magma is supplied to these polygenetic volcanoes. Five general types of eruptive behavior can be distinguished from the cumulative volume curves studied. These types are interpreted in terms of a simple model of batches of magma rising buoyantly through the crust and interacting with a small‐capacity subvolcanic magma reservoir. Recognition of previous steady state behavior at a volcano may enable the cumulative volume curve to be used empirically as a constraint on the timing and volume of the next eruption. The steady state model thus has a limited predictive capability. With the exception of Kilauea (Qss = 4 m3 s−1 ) all the identified steady state volcanoes have values of Qss of a few tenths of one cubic meter per second. These rates are consistent with the minimum flux rates required by theoretical cooling models of batches of magma traversing the crust. The similarity of these Qss values at a variety of volcanoes (producing basalt, andesite, and dacite magmas) in very different tectonic settings suggests that the common factors of crustal buoyancy forces and the geotherm‐controlled cooling rates control the dynamics of magma supply through the crust. Long‐term dormancy at active volcanoes may be a manifestation of the steady state accumulation of magma in large crustal reservoirs, a process that complements the intermittent periods of steady state output at the surface. This possibility has several implications, the most important of which is that it provides a constraint on the supply rate of new magma to the bases of plutons.