Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour
Michael J. HeapValentín R. TrollAlexandra KushnirH. Albert GilgAmy CollinsonFrances M. DeeganHerlan DarmawanNadhirah SeraphineJürgen NeubergThomas R. Walter
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Abstract:
Abstract Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments.Keywords:
Lava dome
Outgassing
Volcanic hazards
Cabin pressurization
Each of the three phases of the 2006 eruption at Augustine Volcano had a distinctive eruptive style and flowage deposits. From January 11 to 28, the explosive phase comprised short vulcanian eruptions that punctuated dome growth and produced volcanowide pyroclastic flows and more energetic hot currents whose mobility was influenced by efficient mixing with and vaporization of snow. Initially, hot flows moved across winter snowpack, eroding it to generate snow, water, and pyroclastic slurries that formed mixed avalanches and lahars, first eastward, then northward, and finally southward, but subsequent flows produced no lahars or mixed avalanches. During a large explosive event on January 27, disruption of a lava dome terminated the explosive phase and emplaced the largest pyroclastic flow of the 2006 eruption northward toward Rocky Point. From January 28 to February 10, activity during the continuous phase comprised rapid dome growth and frequent dome-collapse pyroclastic flows and a lava flow restricted to the north sector of the volcano. Then, after three weeks of inactivity, during the effusive phase of March 3 to 16, the volcano continued to extrude the lava flow, whose steep sides collapsed infrequently to produce block-and-ash flows. The three eruptive phases were each unique not only in terms of eruptive style, but also in terms of the types and morphologies of deposits that were produced, and, in particular, of their lithologic components. Thus, during the explosive phase, low-silica andesite scoria predominated, and intermediate- and high-silica andesite were subordinate. During the continuous phase, the eruption shifted predominantly to high-silica andesite and, during the effusive phase, shifted again to dense low-silica andesite. Each rock type is present in the deposits of each eruptive phase and each flow type, and lithologic proportions are unique and consistent within the deposits that correspond to each eruptive phase. The chief factors that influenced pyroclastic currents and the characteristics of their deposits were genesis, grain size, and flow surface. Column collapse from short-lived vulcanian blasts, dome collapses, and collapses of viscous lavas on steep slopes caused the pyroclastic currents documented in this study. Column-collapse flows during the explosive phase spread widely and probably were affected by vaporization of ingested snow where they overran snowpack. Such pyroclastic currents can erode substrates formed of snow or ice through a combination of mechanical and thermal processes at the bed, thus enhancing the spread of these flows across snowpack and generating mixed avalanches and lahars. Grain-size characteristics of these initial pyroclastic currents and overburden pressures at their bases favored thermal scour of snow and coeval fluidization. These flows scoured substrate snow and generated secondary slurry flows, whereas subsequent flows did not. Some secondary flows were wetter and more laharic than others. Where secondary flows were quite watery, recognizable mixed-avalanche deposits were small or insignificant, and lahars were predominant. Where such flows contained substantial amounts of snow, mixed-avalanche deposits blanketed medial reaches of valleys and formed extensive marginal terraces and axial islands in distal reaches. Flows that contained significant amounts of snow formed cogenetic mixed avalanches that slid across surfaces protected by snowpack, whereas water-rich axial lahars scoured channels. Correlations of planimetric area (A) versus volume (V) for pyroclastic deposits with similar origins and characteristics exhibit linear trends, such that A=cV2/3, where c is a constant for similar groups of flows. This relationship was tested and calibrated for dome-collapse, column-collapse, and surgelike flows using area-volume data from this study and examples from Montserrat, Merapi, and Mount St. Helens. The ratio A/V2/3=c gives a dimensionless measure of mobility calibrated for each of these three types of flow. Surgelike flows are highly mobile, with c≈520; column-collapse flows have c≈150; and dome-collapse flows have c≈35, about that of simple rock avalanches. Such calibrated mobility factors have a potential use in volcano-hazard assessments.
Lava dome
Stratovolcano
Peléan eruption
Lahar
Effusive eruption
Pyroclastic fall
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Phreatomagmatic eruption
Scoria
Dome (geology)
Strombolian eruption
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Lava dome
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Dense-rock equivalent
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Explosive volcanic eruption is one of the most hazardous natural phenomena. During explosive eruptions, a mixture of volcanic ash and gases is ejected from a volcanic vent into the atmosphere. For hazard risk assessment, it is important to comprehensively explain various observed data during eruptions and to understand the dynamics of explosive eruptions and the mechanism of volcanic ash dispersal. We have developed a pseudo-gas model of eruption cloud dynamics and ash dispersal. Our model has successfully reproduced the heights of eruption cloud and the distribution of fall deposits during large eruptions such as the Pinatubo 1991 eruption and those during small eruptions such as the Shinmoe-dake 2011 eruption. For more accurate estimates of volcanic hazard risks, two-way coupled models of multiphase flow are required.
Phreatic eruption
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Vulcanian eruption
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Volcanology
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Dacite
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The tiny Caribbean island of Montserrat has been in a state of crisis since the Soufriere Hills Volcano (SHV) began its current eruption in July 1995. With its main town, Plymouth, destroyed by pyroclastic flows in 1997, the islanders who have remained have had to rebuild their society on the northern half of the island under varying degrees of threat from the volcano to the south. During this time, the Montserrat government continues to receive advice on the volcanic hazards from the Montserrat Volcano Observatory (MVO). The continuing eruption has provided a wealth of research opportunities for many international groups who sought to study the growth and repeated partial destruction of a Peleean andesitic lava dome. There have been three episodes of dome growth: November 1995 through March 1998, November 1999 through July 2003, and August 2005 to present. Pyroclastic flows and explosions have been the main source of hazard.The pyroclastic flows have been generated mainly by gravitational collapse of the lava dome, but also by collapse of explosive ash columns. The dome collapses tend to occur from the area of the dome where new lava is being added. Similarly collapses are more likely when the rate of lava extrusion varies. Also, the propensity of explosive evacuation of the magma in the conduit is partly controlled by the magma supply rate, with high rates favoring explosions.
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Lava dome forming volcanic eruptions are common throughout the world. They can be dangerous; nearly all dome-forming eruptions have been associated with explosive activity (Newhall and Melson, 1983). Most explosions are vulcanian with eruption plumes reaching less than 15km, and with a Volcanic Explosivity Index (VEI) <3 (for a definition of VEI see Newhall and Self 1982). Large Plinian explosions with a VEI ≥ 4 do sometimes occur in association with dome-forming eruptions. Many of the most significant volcanic events of recent history are in this category. The 1902-1905 eruption of Mt. Pelee, Martinique; the 1980-1986 eruption of Mount St. Helens, USA; and the 1991 eruption of Mt. Pinatubo, Philippines all demonstrate the destructive power of VEI ≥ 4 dome-forming eruptions. Hazards related to dome-forming eruptions are numerous and range from dome-collapse and column-collapse pyroclastic flows and surges to tephra fall to directed blasts, lahars, and landslides.
Global historical analysis is a powerful tool for decision-making as well as for scientific discovery. In the absence of monitoring data or a knowledge of a volcano’s eruptive history, global analysis can provide a method of understanding what might be expected based on similar eruptions. Important scientific information has been gleaned from disparate collections of dome-forming eruption hazard information, and modeling of volcanic phenomena often requires extensive data for development and calibration.
This study investigates the relationship between large explosive eruptions (VEI ≥ 4) and lava dome-growth from 1000 BCE to present and develops a world-wide database of all relevant information, including dome growth duration, pauses between episodes of dome growth, and extrusion rates. Data sources include the database of volcanic activity maintained by the Smithsonian Institute (Global Volcanism Program) and all relevant published review papers, research papers and reports. Hazards related to dome-forming eruptions, including pyroclastic falls, rockfalls, tephra fall, lahars, and debris avalanches have also been catalogued for Soufriere Hills Volcano, Montserrat. Analysis of the databases has provided useful information regarding the relationship between extrusion rates and large explosions, the identification of patterns in eruptive frequency between different volcanoes, and the timing of large explosions in relation to dome growth. Relational databases will be compiled to allow users to query the database, and additional dome-forming eruption hazard data is requested from any interested parties.
Volcanic hazards
Lava dome
Peléan eruption
Dome (geology)
Martinique
Dense-rock equivalent
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Effusive eruption
Volcanology
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Merapi is Indonesia's most dangerous volcano with a history of deadly eruptions. Over the past two centuries, the volcanic activity has been dominated by prolonged periods of lava dome growth and intermittent gravitational or explosive dome failures to produce pyroclastic flows every few years. Explosive eruptions, such as in 2010, have occurred occasionally during this period, but were more common in pre‐historical time, during which a collapse of the western sector of the volcano occurred at least once. Variations in magma supply from depth, magma ascent rates and the degassing behaviour during ascent are thought to be important factors that control whether Merapi erupts effusively or explosively. A combination of sub‐surface processes operating at relatively shallow depth inside the volcano, including complex conduit processes and the release of carbon dioxide into the magmatic system through assimilation of carbonate crustal rocks, may result in unpredictable explosive behaviour during periods of dome growth. Pyroclastic flows generated by gravitational or explosive lava dome collapses and subsequent lahars remain the most likely immediate hazards near the volcano, although the possibility of more violent eruptions that affect areas farther away from the volcano cannot be fully discounted. In order to improve hazard assessment during future volcanic crises at Merapi, we consider it crucial to improve our understanding of the processes operating in the volcano's plumbing system and their surface manifestations, to generate accurate hazard zonation maps that make use of numerical mass flow models on a realistic digital terrain model, and to utilize probabilistic information on eruption recurrence and inundation areas.
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Phreatic eruption
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