El Chichon: The genesis of volcanic sulfur dioxide monitoring from space

The 1982 eruption of El Chichon inspired a new technique for monitoring volcanic clouds. Data from the Total Ozone Mapping Spectrometer (TOMS) instrument on the Nimbus-7 satellite were used to measure sulfur dioxide in addition to ozone. For the first time precise data on the sulfur dioxide mass in even the largest explosive eruption plumes could be determined. The plumes could be tracked globally as they are carried by winds. Magmatic eruptions could be discriminated from phreatic eruptions. The data from El Chichon are reanalyzed in this paper using the latest version of the TOMS instrument calibration (V8). They show the shearing of the eruption cloud into a globe-circling band while still anchored over Mexico in three weeks. The measured sulfur dioxide mass in the initial March 28 eruption was 1.6 Tg; the April 3 eruption produced 0.3 Tg more, and the April 4 eruptions added 5.6 Tg, for a cumulative total of 7.5 Tg, in substantial agreement with estimates from prior data versions. TOMS Aerosol Index (absorbing aerosol) data show rapid fallout of dense ash east and south of the volcano in agreement with Advanced Very High Resolution Radiometer (AVHRR) ash cloud positions.     

Plume dynamics, thermal energy and long-distance transport of vulcanian eruption clouds from Augustine Volcano, Alaska

Augustine, an island volcano in Lower Cook Inlet, southern Alaska, erupted in January, 1976, after 12 years of dormancy. By April, when the eruptions ended, a new lava dome had been extruded into the summit crater and about 0.1 km³ of pyroclastics had been deposited on the island, mainly as pyroclastic debris avalanches and pumice flows. The ventclearing phase in January was highly explosive and we have been able to document 13 major vulcanian eruptions.The timing, thermal energy, mass loading of fine particles and the horizontal dispersion of these eruption clouds were determined from radar measurements of cloud height, reports of pilots flying in plumes, satellite photography, seismic records and infrasonic detection of air waves. A lower estimate of the mass of fine (r < 68 μm) particles injected into the troposphere from the 13 main eruptions in January is 5.5–18 × 10¹² g. The corresponding mass loading of fine particles within individual eruption clouds is 0.3–1 g m⁻³. We calculated thermal energies of 4 × 10¹⁴ to 35 × 10¹⁴ J for individual eruptions by applying convective plume rise theory to observed cloud heights and seismically determined eruption durations. This energy range compares favorably with the 4–16 × 10¹⁴ J of thermal energy, calculated from the cooling of juvenile material contained in a typical eruption cloud.The vulcanian eruption clouds stayed intact for at least 700 km downwind. Satellite images in both visible and infrared wavebands, showing the Gulf of Alaska just after sunrise on January 23, reveal a series of puffs strung out downwind from the volcano, 20–30 km in diameter and with their tops at altitudes of about 8 km, overlying a continuous plume at altitude 4 km. Each puff corresponded to a seismically and infrasonically timed eruption. A substantial portion of the material injected into the atmosphere between January 22 and 25 was rapidly transported by the subpolar jet stream through southwestern Canada and the western United States, then northeast across the States into the Atlantic. The clouds were observed passing over Tucson, Arizona, on January 25 at an elevation of 7 km.Several of the eruptions penetrated into the stratosphere. Sun photometer measurements, taken at Mauna Loa, Hawaii, six weeks after the eruption, showed an increased stratospheric optical thickness of 0.01 (wavelength 0.5 μm), which decayed in about 5 months. The maximum column mass loading of the veil was 4–10 × 10⁻⁷ g cm⁻². The mass of the veil, spread-ever a fourth of the earth's surface, is 10 to 100 times larger than can be accounted for by assuming that injected ash and converted sulfate particles from the 13 main Augustine eruptions are the only components contributing to the stratospheric turbidity observed at Mauna Loa.     

Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite

Primary igneous anhydrite was first identified in 1982 El Chichón pumices. Analysis of the sulfur budget for the eruption provided compelling evidence that the pre-eruptive magma contained a significant gas phase at ∼ 7 km depth in order to account for the “excess gas release” of ∼ 5–9 million tons of SO₂ to the stratosphere by the eruption. Primary igneous anhydrite and a larger “excess gas release” of ∼ 20 million tons of SO₂ were noted for the significantly larger eruption of Mount Pinatubo in 1991, for which a separate gas phase at ∼ 7–9 km depth was also required by the sulfur budget. Pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende, with minor biotite, and show evidence for co-nucleation and mutual inclusions of anhydrite and apatite. Both magmas were also very water-rich and highly oxidized, with oxygen fugacities $1 log unit above the synthetic Ni–NiO buffer. Furthering the similarities between these two eruptions, ion-microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from El Chichón and Mount Pinatubo both showed that individual crystals are isotopically homogeneous, but inter-crystalline variations in δ³⁴S are well beyond analytical error.     

Estimates of heat and pyroclast discharge by volcanic eruptions based upon the eruption cloud and steady plume observations

Fumarolic steam plumes and eruption clouds rise like convetive turbulent columns into the atmosphere. Formulae are presented here for estimating the heat power of plumes, the production rate of juvenile pyroclasts ejected during eruptions and the heat output of fumaroles. Their accuracy is tested using the well-studied examples of eruptions of Kamchatkan volcanoes.The Briggs (1969) formula may be used in observing the ascending part of a plume in crosswinds. The best results have been obtained using the CONCAWE formula which permits estimation of the heat power in crosswinds based on the axis height of a horizontal part of a maintained plume. Three connected equations have been suggested for a stable atmosphere and calm weather conditions. The first one, which is applicable for heights ranging from 100 m to 1 km, is the formula proposed by Morton et al. (1956). This equation changes for higher layers of the troposphere (1–10 km) and stratosphere (10–55 km).A classification scale was constructed allowing us to compare volcanic eruptions and fumarolic activity in terms of the intensity of their plumes.The described method is useful for volcano surveillance; it helps in the study of the energetics and mechanics of volcanic and magmatic processes.     

Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows

The eruption of Unzen Volcano commenced on 17 November 1990. Phreatic and phreatomagmatic eruptions occurred by early May 1991. No large-scale explosive eruptions preceded the extrusion of lava domes. Lava domes appeared in a summit crater on 20 May 1991, and they grew on the steep slope of Mt. Fugen at Unzen Volcano. Rockfalls from the margins of the domes frequently generated pyroclastic flows. Major pyroclastic flows occurred on 3 June, 8 June, and 15 September 1991. The 3 June pyroclastic flow killed forty-three persons. Many of the pyroclastic flows seem to have resulted from the simple rockfalls, except one flow on 8 June, which was accompanied by an explosion from the crater. Many of the rockfalls that generated pyroclastic flows were witnessed. As of November 1991. Unzen Volcano was still active with a nearly constant magma-supply rate of about 0.3 × 10⁶ m³/d. The total magma output exceeded 45 × 10⁶ m³ by the beginning of November 1991. The volume of the lava domes is more than 23 × 10⁶ m³.     

The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens

The initial eruption column of May 18th, 1980 reached nearly 30 km altitude and released 10¹⁷ joules of thermal energy into the atmosphere in only a few minutes. Ascent of the cloud resulted in forced intrusion of a giant umbrella-shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m/s. The mushroom cloud expanded 15 km upwind, forming a stagnation point where the radial expansion velocity and wind velocity were equal. The cloud was initiated when the pyroclastic blast flow became buoyant. The flow reduced its density as it moved away from the volcano by decompression, by sedimentation, and by mixing with and heating the surrounding air. Observations indicate that much of the flow, covering an area of 600 km², became buoyant within 1.5 minutes and abruptly ascended to form the giant cloud. Calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant. Assuming an initial temperature of 450°C and a magmatic origin for the explosion, these calculations indicate that the flow became buoyant when its temperature was approximately 150°C and the flow consisted of a mixture of 3.25 × 10¹¹ kg of pyroclasts and 5.0 × 10¹¹ kg of air. If sedimentation is considered, these figures reduce to 1.1 × 10¹¹ kg of pyroclasts and 1.0 × 10¹¹ kg of air.     

Coda Q before and after the eruptions of 13 November 1985, and 1 September 1989, at Nevado del Ruiz Volcano, Colombia

 Coda Q^–1 was calculated at Nevado del Ruiz Volcano (NRV) before and after two phreatomagmatic eruptions (November 1985, September 1989) and for a period of stability (May 1987) using a functional form for coda derived from a single scattering model (Sato 1977). Substantial changes were found before and after the eruptions. The highest value of Q^–1 was found during the November 1985 eruption, an intermediate value for the September 1989 eruption, and the lowest value for May 1987. It seems that the changes in coda Q^–1 at NRV have a still-unknown relationship with the volume or magnitude of the eruption. A relatively strong frequency dependence was found for all periods. Also Q^–1 clearly changed with time, suggesting that the scattering was strong for the eruption of November 1985 and decreased for the eruption of September 1989, and that the intrinsic absorption probably increased. This suggests the possibility that crystallization is taking place in the NRV magma. The clear change of coda Q^–1 before and after the eruptions at NRV also suggests the possibility that coda Q^–1 is a premonitory tool of activity at this volcano. Received: 25 October 1996 / Accepted: 21 January 1998     

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 MDRMSR 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 m³ s⁻² on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m³ 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 km³ 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%.     

The October 1902 plinian eruption of Santa Maria volcano, Guatemala

The October, 1902, eruption of Santa Maria Volcano, Guatemala, was one of the largest this century. It was preceded by a great earthquake on April 19 centered at the volcano, as well as numerous other major earthquakes. The 18–20 hour-long plinian eruption on October 25 produced a column at least 28 km high, reaching well into the stratosphere.The airfall pumice deposit covered more than 1.2 million km² with a trace of ash and was only two meters thick at the vent. White dacitic pumice, dark gray scoriaceous basalt (with physically and chemically mixed intermediate pumice) and loose crystals of plagioclase, hornblende, hypersthene, biotite and magnetite make up the juvenile components of the deposit. Lithic fragments are of volcanic, plutonic, and metamorphic origin. The plinian deposit is a fine-grained, crystal-rich, single pumice fall unit and shows inverse grading. Mapping of the deposit gives a volume of 8.3 km³ within the one mm isopach. Crystal concentration studies show that the true volume erupted was at least 20 km³ (equivalent to 8.5 km³ of dense dacite) and that 90% of the ejecta was less than 2 mm in diameter.The plinian volume eruption rate averaged 1.2 × 10⁵ m³s⁻¹ and the average gas muzzle velocity of the column exceeded 270 ms⁻¹. A total of 8.3 × 10¹⁸ J of energy were released by the eruption. A knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms.This eruption was the major stratospheric aerosol injection in the 1902–1903 period. However, mid- to low- latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease. This may be explained by the sulfur-poor nature of dacite magmas, suggesting that volatile composition, rather than mass of volatiles, is the controlling parameter in climatic response to explosive eruptions.     

The prediction of large explosions of andesitic volcanoes

During the large explosions of the Bezymianny (1956), Shiveluch (1964) and Mount St. Helens (1980) volcanoes, 4.8·10¹², 3.0·10¹² and 8.2·10¹² kg of resurgent and magmatic material were ejected respectively. The eruptions were preceded and accompanied by significant crustal deformations and by a great number of volcanic earthquakes. In all three cases, earthquakes with an energy of E = 10⁹ J occurred 8–11 days before the eruption; their foci were at a distance of less than 5 km from the floor of the active crater and the power of earthquake swarms increased continuously and monotonously until the beginning of the eruption. The data obtained on deformations, earthquakes and volcanic activity may be used for the prediction of the place, time, energy and hazards of large explosions of andesitic volcanoes.     

Airborne measurements of particle and gas emissions from the December 1994–January 1995 eruption of Popocatépetl volcano (Mexico)

Airborne and ground-based (correlation spectrometer, cascade impactor, and photoelectric counter together with intake filter probes) measurements are described for the volcanic emissions from Popocatépetl volcano (Mexico) from December 23, 1994 to January 28, 1995. Measurements of SO₂ restarted 48 h after the eruption onset of December 21, 1994. Maximum sulfur dioxide (4560 t d⁻¹) plus 3.8×10⁴ t d⁻¹ of particulate matter were ejected on December 24, 1994. The maximum rate of ejection occurred coincidentally with the maximum amplitude of harmonic tremor and the maximum number of seismic type B events. Sulfur dioxide emission rates ranged from 1790 to 2070 t d⁻¹ (December 23–24, 1994). Afterwards, sulfur dioxide emission rates clearly indicated a consistent decline. However, frequent gas and ash emission puffs exhibited SO₂ fluxes reaching values as high as 3060 t d⁻¹. The emission SO₂ baseline for the period of study (February 1994–January 1995) was about 1000 t d⁻¹. Ejection velocity of particulate matter was approximately 270 m s⁻¹ reaching a height of about 2.5 km over the summit. The immediate aerosol dispersion area was estimated at 6.0×10⁴ km² maximum. The microscopic structure of particles (aerosol and tephra) showed a fragile material, probably coming from weathered crustal layers. X-ray fluorescence and neutron-activation analysis from the impactor samples found the following elements: Si, Al, Ca, S, P, Cl, K, Ni, Fe, Ti, Sc, Cu, Zn, Mn, Sr, Cr, Co, Y, Br, Se, Ga, Rb, Hg and Pb. Morphological analysis shows that ash samples might be from pulverized basaltic rock indicating that the Popocatépetl eruption of December 21, 1994 was at low temperature. The microscopic structure of puff material showed substance aggregates consisted of fragile rock, water and adsorbed SO₂. These aggregates were observed within water droplets of approximately 1 mm and even larger. Sulfur transformations in the droplets occurred intensively. Volcanic ash contained 5–6% of sulfur during the first expulsion hours. Elemental relative concentrations with respect to Al show that both Si and S have relative concentrations >1, i.e., 13.73 and 2.17, respectively in agreement with the photoelectric counter and COSPEC measurements.     

Eruption and mass-wasting-induced processes during the late holocene destructive phase of Nevado del Ruiz volcano, Colombia

The November 13, 1985, eruption was characteristic of the Arenas eruptive stage of Nevado del Ruiz, the most recent of a series of twelve eruptive stages that have occurred in the past 11,000 years. Eruptive sequences, deposits and processes similar to that of 1985 have characterized the behavior of Nevado del Ruiz during three major prehistorical and historical eruptive stages: the approximately 3300-3100 yr. B.P. Hedionda, the 16th century Azufrado, and the mid-1800's Lagunillas eruptive stages, that partly destroyed the present Ruiz summit.According to the interpretation of the stratigraphic record of prehistorical eruptions and historical accounts, almost every recent magmatic event was small or short-lived. Nevertheless, rockslide-debris avalanches and catastrophic debris flows were triggered in all the eruptions owing to slope failures related to specific tectonic features of Ruiz volcano and/or to significant interactions between pyroclastic debris and the ice cap. Evidence for headward retreat of avalanche scarps during multiple eruptions reinforce the case that large slope failures can occur repeatedly at a large-volume volcano like Ruiz without reconstruction of the edifice. The latest Ruiz eruptions that involved rockslide-debris avalanches resemble in part the Shiveluch 1964 event for which evidence of lateral blast deposits are lacking, but differ in part from this type because non-eruptive and mass-wasting processes also triggered rockslide-debris avalanches.Many factors render the cluster of domes of the Ruiz summit unstable, including: (1) deeply dissected troughs opened toward the north-northeast (Azufrado), east (Lagunillas), and south (Recio) flanks; (2) strongly hydrothermally altered north and east flanks of the summit; (3) currently glaciated or recently deglaciated, high cliffs; (4) steep unstable margins of the ice cap on the north and east.Thus, in light of its past behavior, a small eruption or an earthquake might trigger catastrophic rockslide-debris avalanches. Furthermore, such avalanches as well as glacial outburst-floods and ice avalanches could induce debris flows by mobilizing weathered, water-saturated, and unconsolidated rocks or deposits.     

The December 4, 1983 to February 18, 1984 eruption of Piton de la Fournaise (La Reunion, Indian Ocean): Description and interpretation

On December 4, 1983 an eruption started at vents located 1.5 km southwest of the summit of Piton de la Fournaise at the base of the central cone. After 31 months of quiescence this was one of the longest repose period in the last fifty years. The eruption had two phases: December 4 to January 18 and January 18 to February 18. Phase 1 produced about 8 × 10⁶ m³ of lava and Phase II about 9 × 10⁶ m³. The erupted lava is an aphyric basalt whose mineralogical and geochemical composition is close to that of other lavas emitted since 1977.The precursors of the December 4 outbreak were limited to two-week shallow (1.5–3 km) seismic crisis of fewer than 50 events. No long-term increase was noted in the local seismicity which is very quiet during repose periods and no long-term ground inflation preceded the eruption. Outbreaks of Phases I and II were preceded by short (2.5 hours and 1.5 hours) seismic swarms corresponding to the rise of magma toward the surface from a shallow reservoir. Large ground deformation explained by the emplacement of the shallow intrusions, was recorded during the seismic swarms. A summit inflation was observed in early January, before the phase II outbreak, while the phase I eruption was still continuing.Piton de la Fournaise volcanological observatory was installed in 1980. Seismic and ground deformation data now available for a period of 4 years including the 1981 and the 1983–1984 eruptions, allow us to describe the physical behavior of the volcano during this period. These observations lead us to propose that the magma transfer from deep levels to the shallow magma reservoir is not a continuous process but a periodic one and that the shallow magma reservoir was not resupplied before the 1981 and 1983–1984 eruptions. Considerations on the eruptive history and the composition of recent lavas indicate that the reservoir was refilled in 1977.     

The possible effects of large 19th and 20th century volcanic eruptions on zonal and hemispheric surface temperatures

The contribution of volcanic material to the stratosphere from major eruptions within the last two centuries has been estimated using volcanological criteria, including eruption type, eruption column height, volume and duration of eruption, and composition and degree of fragmentation of magma. The chronology of major explosive volcanic eruptions is compared with a record of mean surface-temperature deviation (ΔT) for the same interval constructed from all available temperature data. The temperature records are divided into 6 latitudinal zones, allowing analysis for individual zones where temperature changes induced by aerosol perturbation might be intensified.We focus on the explosive volcanic events which by our estimates injected the most material into the stratosphere. These include Tambora 1815, Krakatau 1883, Santa Maria 1902, Katmai 1912 and Quizapu 1932. Such eruptions appear to have produced a consistent but small temperature decrease on the order of 0.2° to 0.5°C on a hemispheric scale for periods ranging from one to five years, although these changes are similar to background temperature variations. The maximum change in ΔT after some of these explosions appears to lag by up to three years in going from equatorial to polar latitudes.Somewhat smaller eruptions, e.g. Agung 1963 and possibly Cosiguina 1835, seem to have produced about the same perturbation in ΔT as the larger eruptions. This suggests either a limiting mechanism on loading of the aerosol layer after a volcanic eruption or, that the composition of injected material (i.e., the ratio of silicate “dust” to volatiles, and composition of the volatiles) may significantly effect stratospheric optical depth perturbation. Temperatures do not remain depressed for a longer period after a series of closely timed eruptions (e.g., the 1881–1889 or the 1902–1903 sequences) than after single events.     

The effect of the Mt St Helens eruption on tropospheric and stratospheric ions

Balloon-borne observations of electrical conductivity in the troposphere and stratosphere were performed using conductivity sondes at Garmisch-Partenkirchen, West Germany, from June to November, 1980, after the Mt St Helens eruption. A significant decrease of atmospheric ions in the altitudes from the troposphere to lower stratosphere has been detected until several months after the eruption in comparison with the observational results obtained before the eruption. Simulteneous ruby lidar observation a month after the eruption indicates an increased amount of aerosol at nearly the same altitude as that of conductivity decrease. Several months after the eruption it appears that aerosols detected by lidar and those effective in reducing ion concentration have different profiles.     

UHF radar observation of strato-tropospheric transfers on the anticyclonic side of a jet streak

An observation by UHF ST radar of a subsidence pattern on the right side of the exit region of a jet streak is reported. The onset of the subsidence pattern occurred at 23:30 UTC on the 29 November 1991, when a downward motion was initiated above 14 km. The injections of stratospheric air in this region seem to have an intermittent nature; they occur during at least three intervals during the lifetime of the subsidence pattern. Comparison of these results with an ECMWF analysis suggests that it is an unfolding case. However, observation of turbulent intensities w’ greater than 60 cm s⁻¹ at the tropopause level also suggests the existence of a turbulent flux between the stratosphere and the troposphere. From the turbulence characteristics measured by the radar and the potential temperature profile obtained by radiosonde data, the eddy diffusivity at the tropopause level has been calculated. An eddy diffusion coefficient ranging between 5 and 7 m² s⁻¹ is found. From these values, and with the assumption of a climatological gradient of the volume mixing ratio of ozone in the lower stratosphere, it is possible to deduce a rough estimate of the amount of ozone injected from the stratosphere into the troposphere during this event. A rate of transfer of 1.5×10²⁰ molecules of ozone per day and per square meter is found.     

Simulation of the 1980 eruption of Mount St. Helens using the ash-tracking model PUFF

The dispersal of volcanic ash from the May 18, 1980 eruption of Mount St. Helens (MSH) has been simulated using the Lagrangian ash-tracking model PUFF. Previous applications of the model were limited to smaller, short-lived eruptions with ash dispersal occurring mainly within the troposphere. Two high-resolution atmospheric reanalysis datasets (ERA-40 and NCEP/NCAR-40) allowed MSH ash cloud dispersal to be simulated up to 30 km elevation. The 1980 eruption was divided into two distinct eruptive phases, (1) an initial, relatively short-lived blast/surge phase that injected ash up to 30 km and (2) a subsequent nine-hour plinian phase that maintained an average eruption column height of 16 km. Using PUFF, the two phases of the MSH eruption were modeled separately based on a range of individual input parameters and then combined to produce an integrated simulation of the entire eruption. The trajectory and areal extent of the modeled atmospheric ash cloud best match the actual distribution of MSH ash when input parameters are set to values inferred from satellite and radar data collected on May 18, 1980. The prevailing wind field exerts the strongest control on the advection and ultimate position of the modeled ash cloud, making the maximum column height and the vertical distribution of ash the most sensitive of the PUFF input parameters for this event. The results indicate that the PUFF model works well at simulating the dispersal of ash injected well into the lower stratosphere from a moderate, relatively long-lived eruption, such as MSH. However, attempts to use PUFF to recreate some granulometric aspects of the MSH fallout deposit, such as the maximum particle size as a function of distance from source, were not successful. PUFF consistently predicts much greater fallout distances for small ash particles (< 500 µm) than actually observed in the MSH deposit. The effective settling velocities used by the PUFF model appear to be too slow to accurately predict fallout distances of small ash particles. As a consequence the PUFF model may overestimate the duration of ash loading in the atmosphere associated with the distal fine ash component of explosive eruptions.     

Storage, migration, and eruption of magma at Kilauea volcano, Hawaii, 1971–1972

The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 10⁷ m³ and 8 × 10⁶ m³, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 10⁶ m³/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971.The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September.The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones.     

Eruption characteristics and cycles at Pavlof Volcano, Alaska, and their relation to regional earthquake activity

Pavlof Volcano (55° 25′N, 161° 54′W) exhibits two eruption styles: magmatic eruptions of one-to-two-days duration, and phreatic-phreatomagmatic activity lasting several days to two months. Thirty-four eruptions have occurred in historic times; of these the largest are Volcano Explosivity Index=3. Nine magmatic and 13 phreatomagmatic eruptions occurred between 1973–1983. All the magmatic eruptions occurred in the fall, between Sept. 9–Nov. 20. Four magmatic eruptions occurred during November 11–15, but in four different years. A 3-year-long period of eruptive activity between 1973–1976 bears striking resemblance to a period of activity between 1980–1983. No locatable shallow earthquakes (<50 km) have occurred within 30 km of Pavlof since 1973, which is quite unusual for an active island-arc volcano. Shallow events in the adjacent are segments have focal mechanisms with P-axes perpendicular to the arc (and parallel to plate convergence). Deep earthquakes (> 100 km) are clustered beneath Pavlof and several other volcanoes. Their T-axes show downdip tension within the slab. Deep teleseisms (> 160 km) mostly occurred between 1977–1979 when the volcano was not erupting. Catalogued volcanic activity throughout the Alaska/Aleutian arc shows a weak tendency to increase around the time of great (M > 7.8) earthquakes.     

Plinian activity during the early eruptive history of the Sabatini Volcanic District, Central Italy

The early activity of the Sabatini Volcanic District (SVD; central Italy) was characterised by highly explosive eruptions that produced widespread subplinian and plinian fall deposits. In this study, four major eruptive units—informally named as units A, B, C and D—were recognised in the 514–449 ka age interval. In particular, unit D was emplaced during the early phase of the 449 ka Tufo Rosso a Scorie Nere pyroclastic flow-forming eruption, the most important event in the whole SVD activity history. Estimates of relevant eruptive parameters indicate tephra fall volumes up to 4 km³ for individual units, peak eruption column heights in the range of 14–29 km and corresponding mass eruption rates of 7.8×10⁶–1.3×10⁸ kg/s. Isopach and isopleth maps of fallout deposits—as well as the distribution of the proximal lag-breccia of the Tufo Rosso a Scorie Nere—consistently indicate a common vent area, which does not correspond to any volcanic centre identified up to now in the SVD. This was located along NE–SW-trending tectonic lineaments that also controlled the location of the other major volcanic centres of the SVD. The characterisation by means of field aspects, grain size, componentry and density and chemical composition of juvenile clasts, renders the studied fall deposits as valuable stratigraphic markers for the SVD and well beyond it. In fact, their wide areal dispersals toward the E and SE may allow correlations on a regional scale for the Quaternary successions of intermountain basins of the Central Apennine and the Adriatic Sea basin successions. Finally, the correct identification of distal tephra from plinian and co-ignimbrite plumes and their attribution to specific explosive eruptions of the SVD and the other volcanic districts of the Roman Province—rather than to local intra-Apennine centres—provides crucial implications for geodynamic reconstructions.