Mass Eruption Rate Research Articles

The recent pumice eruptions of Mt. Pelée volcano, Martinique. Part II: Grain-size studies and modeling of the last Plinian phase P1

Abstract Some aspects of the last pumice eruption P1, recorded at Mt. Pelee volcano, Martinique, and dated at 650 years B.P., are quantified. During this event, Plinian falls were followed by pumice flows channelled by the valleys. Plinian deposits crop out over the entire Mt. Pelee massif and significant volumes fell upon the ocean. Grain-size studies of the fall deposits show a median (Md Φ) which generally ranges from −2.25 to −3.8 Φ, and a sorting coefficient (σ Φ) of between 1.33 and 2.75. The main volcanological parameters are: Tmax (thickness) up to 15 m, F (fragmentation) of 10 wt.% of clasts less than 1 mm in size, and D (dispersal) of about 900 km2. The total volume of the tephra is estimated at 0.95 km3 and the duration of the eruption at 1.5–3 h, which liberated a total thermal energy of 0.4 × 1025 ergs with a mass eruption rate of between 3.8 and 7.0 × 107 kg s−1. The column height is calculated to be 20–22 km, and the initial muzzle velocities between 230 and 310 m s−1 for blocks 0.4–0.7 m in diameter. These parameters suggest that the eruption was a relatively small Plinian eruption.

Explosive volcanic eruptions - VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties

Summary. A simple model is developed to relate the maximum down-wind and cross-wind ranges of pyroclasts forming a plinian airfall deposit to the dynamic processes in the eruption cloud from which they fall and the atmospheric wind conditions in the area. The eruption cloud dynamics are in turn related to the eruptive conditions in the vent (vent radius, exsolved magmatic volatile weight fraction, velocity with which material passes through the vent, and mass eruption rate), some or all of which can be deduced if the appropriate field measurements can be made. Some aspects of the stability of convecting volcanic eruption clouds are investigated, and the effects on eruption cloud height of the local atmospheric temperature profile and the value adopted for the entrainment constant (which relates the horizontal flow speed of atmospheric air entering the column to the vertical rise speed of the column material) are explored. It is confirmed that eruption-cloud rise height and pyroclast dispersal are mainly controlled by the mass eruption rate (per unit length of active fissure in the case of linear vents) and, hence, the heat input rate to the cloud; but a significant subsidiary dependence on the amount of exsolved magma volatiles is also found. The eruption cloud model is validated by application to observed historic eruptions, and its use in the analysis of palaeo-eruptions is discussed.

Sequence, rheological properties, and effusion rates of volcanic flows at Alba Patera, Mars

Volcanic flows with very large areas characterize the Martian volcano Alba Patera. Photogeologic analysis indicates a change of eruptive style with time, from early flood lavas to relatively short, narrow, leveed summit flows during the dying phase of activity. Estimates of mass effusion rate and rheological properties derived from dimensional parameters confirm the view that the magmas involved were mafic, and indicate extremely high effusion rates (5–1610 × 103m3 sec−1) for the earlier flows. The data substantiates the view that during the active life of Alba volcano mass eruption rates in the Tharsis region of Mars were significantly higher than in most analogous terrestrial provinces, but probably not as high as those for lunar mare basalts.

Basaltic fissure eruptions, plume heights, and atmospheric aerosols

Convective plumes that rise above Hawaiian‐style fire fountains consist of volcanic gases, aerosols, fine ash, and entrained heated air. Plume theory has been applied to observational estimates of the rate of thermal energy release from large fire fountains. The theoretically predicted heights of maintained plumes agree very well with the heights found from actual observations. Predicted plume heights for both central‐vent (point‐source) and fissure (line‐source) eruptions indicate a stratospheric penetration by plumes that form over vents with very high magma‐production rates. Flood basalt fissure eruptions that produce individual lava flows with volumes > 100 km³ at very high mass eruption rates are capable of injecting large quantities of sulfate aerosols into the lower stratosphere, with potentially drastic short‐term atmospheric consequences, like acid precipitation, darkening of the sky, and climatic cooling.

The 1982 eruptions of El Chichon volcano, Mexico (2): Observations and numerical modelling of tephra-fall distribution

Tephra fallout from the A-1 (March 29, 0532 UT), B (April 4, 0135 UT), and C (April 4, 1122 UT) 1982 explosive eruptions of El Chichon produced three tephra fall deposits over southeastern Mexico. Bidirectional spreading of eruption plumes, as documented by satellite images, was due to a combination of tropospheric and stratospheric transport, with heaviest deposition of tephra from the ENE tropospheric lobes. Maximum column heights for the eruptions of 27, 32, and 29 km, respectively, have been determined by comparing maximum lithic-clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns. These column heights suggest peak mass eruption rates of 1.1 × 108, 1.9 × 108, and 1.3 × 108 kg/s. Maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits. Sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration. New estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2.19 km3 (1.09 km3 DRE, dense rock equivalent) or roughly twice the amount of the deposit mapped on the ground. Up to one-half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra.

The May 18, 1980 eruption of Mount St. Helens: 2. Modeling of dynamics of the Plinian Phase

The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7–10 km depth at a rate of 1.94×107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920°–940° C), and confining pressure (190–250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95‐m‐diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model‐derived vent diameters based on 0.1‐MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.

Lionel Wilson wins VGP Award

Lionel Wilson (Department of Environmental Sciences, University of Lancaster, England) has brought physics to volcanology and transformed a largely descriptive and petrolqgical science by development of a quantitative and predictive understanding of eruption dynamics. Lionels involvement in volcanology started in 1971 when he helped George Walker determine the rates of fall of pyroclasts. This fairly simple problem led to questions of settling of ash particles onto the earth, and Lionel embarked on a series of papers that progressively traced volcanic debris back to its source crater. He described the physical processes affecting pyroclasts in strombolian and plinian eruptions, and with Steve Sparks and others modeled the formation (1976) and emplacement (1978) of ignimbrites by gravitational collapse of an eruption column. In a paper important to understanding the dispersal of tephra, Lionel and others demonstrated that eruption cloud heights are proportional to the fourth root of the mass eruption rate of magma (1978), leading ultimately to the inversion of the problem to deduce cloud height and associated eruption characteristics from mapped tephra distributions. The correctness and utility of Lionel's theoretical descriptions of explosive activity were demonstrated by a series of papers applying the models to actual eruptions at Fuego, Guatemala (1980), Ngauruhoe, New Zealand (1979), La Soufriere, St. Vincent (1982), and St. Helens, Washington (1982), as well as to tephra deposits at Askja, Iceland (1981), Toluca, Mexico (1977), and Thera, Greece (1978).

Explosive volcanism on Hecates Tholus, Mars: Investigation of eruption conditions

Evidence is presented supporting the identification of a discrete, well‐preserved air fall deposit generated by explosive volcanism on Mars. The deposit is located immediately to the west of the summit caldera of Hecates Tholus (32°N, 209°W), the northernmost of the three volcanic constructs in the Elysium Planitia region. The absence of superposed impact craters larger than the resolution limit of the Viking images (40 m per pixel) indicates a very young age for this eruption, which evidently postdates even the most recent collapse episode of the Olympus Mons caldera (∼300 m.y. B.P.; Neukum and Hiller (1981)). From the distribution of craters smaller than 2 km in diameter, it appears that the air fall material mantles an area about 50×75 km in extent to an estimated thickness of about 100 m. By adapting numerical models for terrestrial volcanic eruptions to the Martian environment, these dimensions imply an eruption cloud height of about 70 km. In order to attain such an altitude, the mass eruption rate must have been ∼107 kg/s and the volatile content either ∼1 wt % for H2O or ∼2 wt % for CO2. Although it is not possible to distinguish between silicic and basaltic volcanism on Hecates, were the volatile species CO2, the depth of most recent storage prior to eruption of the magma would have to be 50–100 km (due to the solubility of CO2 in the melt). If H2O were the driving volatile, a minimum depth of storage would be 0.2–4 km, thereby permitting the possibility of ground water assimilation by the magma.

Ascent and eruption of basaltic magma on the Earth and Moon

Geological and physical observations and constraints are applied to the development of a model of the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2 km); thus the ascent of bubble‐free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2–0.6 m are needed to allow eruptions from between depths of 0.5–20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian‐style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5–1 m/s if strombolian activity is to be avoided and relatively steady fire fountaining is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/gas dispersion emerging through the vent, increasing with increasing magma gas content and mass eruption rate, and decreasing with increasing magma viscosity. Terrestrial fire fountain heights up to 500 m imply the release of up to 0.4 wt % water from the magma, corresponding to initial water contents up to 0.6 wt %. The presence of extremely long lava flows and sinuous rules on the moon has often been cited as evidence for very high extrusion rates and thus a basic difference between terrestrial and lunar magmas and crustal environments. However, the differences between terrestrial and lunar magma rheologies and crustal environments do not lead to gross differences between the effusion rates expected on the two planetary bodies, for similar‐sized conduits or fissures. Thus the presence of these features implies only that tectonic and other forces associated with the onset of some lunar eruptions were such as to allow wide fissures or conduits to form. The surface widths of elongate fissure vents need be no wider than 10 m to allow mass eruption rates up to 10 times larger than those proposed for terrestrial flood basalt eruptions; 25‐m widths would allow rates 100 times larger. It therefore appears unlikely that source vents on the moon with widths greater than a few tens of meters represent the true size of the unmodified vent. The main volatile released from lunar magmas was probably carbon monoxide, released in amounts proportionally less than terrestrial magmas by more than an order of magnitude. However, decompression to the near‐zero ambient lunar atmospheric pressure causes much greater energy release per unit mass and this, coupled with vertical and horizontal expansion of the gas, suggests a much more efficient use of the available gas on the moon than on the earth. Some amount of magma disruption must always have taken place in lunar eruptions unless the gas content was truly zero or the magma possessed an appreciable yield strength. Pyroclastic deposits, such as the extensive lunar dark mantling material, could be produced from a single source vent and extend to diameters of up to 200 km. Such deposits could result either from steady eruptions at high effusion rates (with less than 1% of the magma disrupted into submillimeter droplets) or low effusion rate eruptions in which strombolian activity occurred. The wide dispersal of pyroclastic debris is a result of small particles being locked into the expanding gas cloud. Finally, we consider the details of basaltic eruption processes on the moon and predict the nature and geometry of several types of volcanic landforms (flows, pyroclastic blankets and cones, cinder and spatter ridges, etc.) that should result from specific eruption conditions.

Explosive volcanic eruptions -- IV. The control of magma properties and conduit geometry on eruption column behaviour

Summary Plinian air-fall deposits and ignimbrites are the principal products of explosive eruptions of high viscosity magma. In this paper, the flow of gas/pyroclast dispersions and high viscosity magma through various magma chamber/conduit/vent geometries is considered. It is argued that after the first few minutes of an eruption magma fragmentation occurs at a shallow depth within the conduit system. Gas pressures at the fragmentation level are related to exsolved gas contents by consideration of the exsolution mechanism. The sizes of blocks found near vents imply that gas velocities of 200 to 600 m s−1 commonly occur. These velocities are greater than the effective speed of sound in an erupting mixture (90-200 m s−1) and the transition from subsonic to supersonic flow is identified as occurring at the depth at which the conduit has its minimum diameter. The range of values of this minimum diameter (∼ 5 to ∼ 100 m) is estimated from observed and theoretically deduced mass-eruption rates. The energy and continuity equations are solved, taking account of friction effects, for numerous geometries during the evolution, by wall erosion, of a conduit. Conduit erosion ceases, near the surface, when an exit pressure of one atmosphere is reached. Eruption velocities are found to depend strongly on exsolved magma gas content and weakly on radius of conduit and friction effects. Assuming water as the main volatile phase, velocities of 400-600 m s−1 for plinian events imply magma water contents of 3-6 per cent by weight. Three scenarios are presented of eruptions in which: (1) conduit radius increases but gas content remains constant; (2) conduit radius increases and gas content decreases with time; and (3) conduit radius remains fixed and gas content decreases. These models demonstrate that the reverse grading commonly observed in plinian air-fall deposits is primarily a consequence of conduit erosion, which always results in increasing eruption intensity and eruption column height with time. The models also show that a decrease in gas content as deeper levels in a magma chamber are tapped or an increasing vent radius as conduit walls are eroded leads to the prediction of a progression from air-fall activity through ignimbrite formation to cessation of eruption and caldera collapse.

Changes in blood pressure produced by prostatic massage: Hammer, H. J., and Schulte, T. L.: J. A. M. A. 111: 308, 1938

During operational ash-cloud forecasting, prediction of ash concentration and total erupted mass directly depends on the determination of mass eruption rate (MER), which is typically inferred from plume height. Uncertainties for plume heights are large, especially for bent-over plumes in which the ascent dynamics are strongly affected by the surrounding wind field. Here we show how uncertainties can be reduced if MER is derived directly from geophysical observations of source dynamics. The combination of infrasound measurements and thermal camera imagery allows for the infrasonic type of source to be constrained (a dipole in this case) and for the plume exit velocity to be calculated (54–142 m/s) based on the acoustic signal recorded during the 2010 Eyjafjallajökull eruption from 4 to 21 May. Exit velocities are converted into MER using additional information on vent diameter (50±10 m) and mixture density (5.4±1.1 kg/m3), resulting in an average ∼9×105 kg/s MER during the considered period of the eruption. We validate our acoustic-derived MER by using independent measurements of plume heights (Icelandic Meteorological Office radar observations). Acoustically derived MER are converted into plume heights using field-based relationships and a 1D radially averaged buoyant plume theory model using a reconstructed total grain size distribution. We conclude that the use of infrasonic monitoring may lead to important understanding of the plume dynamics and allows for real-time determination of eruption source parameters. This could improve substantially the forecasting of volcano-related hazards, with important implications for civil aviation safety.