Violent Strombolian Eruptions Research Articles

Ash fallout scenarios at Vesuvius: Numerical simulations and implications for hazard assessment

Volcanic ash fallout subsequent to a possible renewal of the Vesuvius activity represents a serious threat to the highly urbanized area around the volcano. In order to assess the relative hazard we consider three different possible scenarios such as those following Plinian, Sub-Plinian, and violent Strombolian eruptions. Reference eruptions for each scenario are similar to the 79 AD (Pompeii), the 1631 AD (or 472 AD) and the 1944 AD Vesuvius events, respectively. Fallout deposits for the first two scenarios are modeled using HAZMAP, a model based on a semi-analytical solution of the 2D advection–diffusion–sedimentation equation. In contrast, fallout following a violent Strombolian event is modeled by means of FALL3D, a numerical model based on the solution of the full 3D advection–diffusion–sedimentation equation which is valid also within the atmospheric boundary layer. Inputs for models are total erupted mass, eruption column height, bulk grain-size, bulk component distribution, and a statistical set of wind profiles obtained by the NCEP/NCAR re-analysis. We computed ground load probability maps for different ash loadings. In the case of a Sub-Plinian scenario, the most representative tephra loading maps in 16 cardinal directions were also calculated. The probability maps obtained for the different scenarios are aimed to give support to the risk mitigation strategies.

Evaluating suitability of a tephra dispersal model as part of a risk assessment framework

In volcanic risk assessment it is necessary to determine the appropriate level of sophistication for a given predictive model within the contexts of multiple sources of uncertainty and coupling between models. A component of volcanic risk assessment for the proposed radioactive waste repository at Yucca Mountain (Nevada, USA) involves prediction of dispersal of contaminated tephra during violent Strombolian eruptions and the subsequent transport of that tephra toward a hypothetical individual via surface processes. We test the suitability of a simplified model for volcanic plume transport and fallout tephra deposition (ASHPLUME) coupled to a surface sediment-transport model (FAR) that calculates the redistribution of tephra, and in light of inherent uncertainties in the system. The study focuses on two simplifying assumptions in the ASHPLUME model: 1) constant eruptive column height and 2) constant wind speed and direction during an eruption. Variations in tephra dispersal resulting from unsteady column height and wind conditions produced variations up to a factor of two in the concentration of tephra in sediment transported to the control population. However, the effects of watershed geometry and terrain, which control local remobilization of tephra, overprint sensitivities to eruption parameters. Because the combination of models used here shows limited sensitivity to the actual details of ash fall, a simple fall model suffices to estimate tephra mass delivered to the hypothetical individual.

Vent conditions for expected eruptions at Vesuvius

Determining consistent sets of vent conditions for next expected eruptions at Vesuvius is crucial for the simulation of the sub-aerial processes originating the volcanic hazard and the eruption impact. Here we refer to the expected eruptive scales and conditions defined in the frame of the EC Exploris project, and simulate the dynamics of magma ascent along the volcanic conduit for sub-steady phases of next eruptions characterized by intensities of the Violent Strombolian (VS), Sub-Plinian 2 (SP2), and Sub-Plinian 1 (SP1) scale. Sets of conditions for the simulations are determined on the basis of the bulk of knowledge on the past history of Vesuvius [Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., Explosive activity and eruption scenarios at Somma–Vesuvius (Italy): towards a new classification scheme. Journal of Volcanology and Geothermal Research, this issue.]. Volatile contents (H 2O and CO 2) are parameterized in order to account for the uncertainty in their expected amounts for a next eruption. In all cases the flow in the conduit is found to be choked, with velocities at the conduit exit or vent corresponding to the sonic velocity in the two-phase non-equilibrium magmatic mixture. Conduit diameters and vent mixture densities are found to display minimum overlapping between the different eruptive scales, while exit gas and particle velocities, as well as vent pressures, largely overlap. Vent diameters vary from as low as about 5 m for VS eruptions, to 35–55 m for the most violent SP1 eruption scale. Vent pressures can be as low as less than 1 MPa for the lowest volatile content employed of 2 wt.% H 2O and no CO 2, to 7–8 MPa for highest volatile contents of 5 wt.% H 2O and 2 wt.% CO 2 and large eruptive scales. Gas and particle velocities at the vent range from 100–250 m/s, with a tendency to decrease, and to increase the mechanical decoupling between the phases, with increasing eruptive scale. Except for velocities, all relevant vent quantities are more sensitive to the volatile content of the discharged magma for the highest eruptive scales considered.

Explosive dynamics of violent Strombolian eruptions: The eruption of Parícutin Volcano 1943–1952 (Mexico)

Violent Strombolian is a term that was originally used by MacDonald [Macdonald, G.A., 1972. Volcanoes, Prentice-Hall inc., Englewood Cliffs, New Jersey, 510 pp.] to describe energetic Strombolian eruptions such as some of the more explosive phases of the 1943–1952 eruption of Parícutin Volcano (Michoacán, central Mexico), eruptions that disperse ‘showers of incandescent cinder and bombs…to heights of a few thousand feet’ and during which ‘a great black ash cloud rises above the volcano’. Here we re-examine accounts of the Parícutin eruption and compare them with new stratigraphic data and physical features of the tephra deposit to improve the definition of violent Strombolian activity and to better elucidate the mechanisms that can cause this distinctive eruptive style. We find characteristic violent Strombolian activity to be strongly pulsatory, with production of moderately high eruption columns (2–6 km) that eject abundant fine ash. Also characteristic is simultaneous lava effusion from lateral vents. At Parícutin, violent Strombolian activity occurred at magma eruption rates of 10 4 to 10 5 kg/s, intermediate between Strombolian and subplinian rates. A progressive decline in magma flux during the eruption led to a decrease in the relative proportion of both erupted tephra and glassy vesicular fragments in the fallout layers. Eruption characteristics can be explained by varying degrees of shallow gas segregation from water-rich basaltic magma that modulate both transitions between two-phase flow regimes in the upper conduit and effusion of degassed lava from the base of the cone.

Precursors of eruptions at Vesuvius (Italy)

The historical record of activity of Mount Vesuvius is uncommonly long and may serve as a guide to understand precursors before the outbreak of new activity. Reposes of different lengths have been observed in the past, with long ones preceding violent explosive eruptions. Eruptions occurring during periods of permanent activity have been preceded by possible deformation of the volcanic edifice and by short duration, earthquake swarms. Otherwise they have occurred without any reported precursors. The renewal of activity after long periods, like the current one, has been preceded by unrest lasting years to weeks, as a new eruption would require connection to the surface of a reservoir at depth ranging between 6 and 4 km. Since 1944, episodic seismic swarms, have occurred with a frequency similar to that of the violent strombolian eruptions during the last period of permanent activity; they are interpreted as intrusions and arrest of magma batches into a reservoir at the same depth of that feeding past sub-plinian eruptions.

Persistent activity and violent strombolian eruptions at Vesuvius between 1631 and 1944

During the period 1631–1944, Vesuvius was in persistent activity with alternating mild strombolian explosions, quiet effusive eruptions, and violent strombolian eruptions. The major difference between the predominant style of activity and the violent strombolian stages is the effusion rate. The lava effusion rate during major eruptions was in the range 20–100 m 3/s, higher than during mild activity and quiet effusion (0.1–1 m 3/s). The products erupted during the mild activity and major paroxysms have different degree of crystallization. Highly porphyritic lava flows are slowly erupted during years-long period of mild activity. This activity is fed by a magma accumulating at shallow depth within the volcanic edifice. Conversely, during the major paroxysms, a fast lava flow precedes the eruption of a volatile-rich, crystal-poor magma. We show that the more energetic eruptions are fed by episodic, multiple arrival of discrete batches of magma rising faster and not degassing during the ascent. The rapidly ascending magma pushes up the liquid residing in the shallow reservoir and eventually reaches the surface with its full complement of volatiles, producing kilometer-high lava fountains. Rapid drainage of the shallow reservoir occasionally caused small caldera collapses. The major eruptions act to unplug the upper part of the feeding system, erupting the cooling and crystallizing magma. This pattern of activity lasted for 313 y, but with a progressive decrease in the number of more energetic eruptions. As a consequence, a cooling plug blocked the volcano until it eventually prevented the eruption of new magma. The yearly probability of having at least one violent strombolian eruption has decreased from 0.12 to 0.10 from 1944 to 2007, but episodic seismic crises since 1979 may be indicative of new episodic intrusions of magma batches.

The deep feeding system of Vesuvius involved in recent violent strombolian eruptions

Mt. Vesuvius has experienced a period of semipersistent volcanic activity over the last three centuries, producing mainly lava effusions and some mixed effusive‐explosive eruptions of higher magnitude. We present a systematic study on major and volatile elements of melt inclusions trapped in olivine crystals peculiar to lapilli fallout emplaced during the more intense episodes of explosive activity. Based on these data, we propose that the deep feeding system active during this period was formed by a vertically extended volume of crust containing interconnected cracks filled by magma, at pressures >200 MPa. The rise of deep CO2‐H2O‐rich magma blobs from this system triggered the high Volcanic Explosivity Index (VEI 2–3) eruptions, provoking the emission of intense lava effusions of degassed magmas and abrupt transition to intense lava fountains and sustained columns.

Violent strombolian and subplinian eruptions at Vesuvius during post-1631 activity

On the basis of historical chronicles and field investigations the tephrostratigraphic sequence of post-1631 activity of Vesuvius is reconstructed. It has been established that, during this period, in addition to numerous totally effusive eruptions and/or normal strombolian activity, 16 explosive events produced well-traceable tephra deposits in the area outside the Mount Somma caldera. Ages of tephra beds were established on the basis of stratigraphic relationships with historical lava flows and comparison with chroniclers information. The dispersal and lithological characteristics of tephra deposits combined with description of explosive activity lead to the identification of three styles: (a) periods of violent strombolian activity; (b) violent strombolian eruptions; and (c) subplinian eruptions. Violent strombolian eruptions and periods of discrete activity are characterized by the formation of lapilli falls from eruptive columns only some kilometers high. Subplinian eruptions are defined on the basis of their lapilli fall volumes which is of the order of 107 m3, on eruptive column heights of approximately 10 km, bt higher than 1.5, and mass discharged rate values not lower than 106 kg/s. During the first century of activity after the 1631 eruption, two periods of violent strombolian activity occurred at Vesuvius (1682–1707 and 1707–1719) preceded, and followed, by a series of violent strombolian eruptions (1660, 1682, 1707, 1723, 1730, 1790, 1872). Between 1730 and 1779 a relevant change in the eruptive style of Vesuvius occurred by an increase in the explosivity of the eruptions. During the past two centuries of activity, only a few eruptions reached subplinian magnitude and only five eruptions had a phreatomagmatic phase (1779, 1794, 1822, 1906, 1944). Therefore, the previously accepted model of cyclic activity, in which each cycle is closed by an important explosive eruption with phreatomagmatic characteristics, is unfounded. The tephrostratigraphy of the 1906 eruption proposed in this work differs substantially from some previous reconstructions, on which the basis for the modeling of Vesuvius’ behavior in this time span was formed.