Final Report Summary - DFAM (Decompression and fragmentation of andesitic magmas during explosive events at open-vent volcanoes)
DFAM
FP7-PEOPLE-2012-IEF
Scientist in charge: Prof. K. Cashman, School of Earth Science, University of Bristol
Fellow: Dr. J. Eychenne, School of Earth Science, University of Bristol
Introduction and objectives
This Marie Curie Fellowship has focused on deciphering the primary and secondary mechanisms that generate ash during explosive volcanic eruptions, with a specific emphasis on the processes controlling the formation of plumes associated with the propagation of pyroclastic density currents (PDCs). Transport and deposition of volcanic ash represent the most widespread hazard related to explosive eruptions, with societal impacts both on the ground close to and downwind from source (destruction of crops, livestock and buildings, health issues for humans), and in the atmosphere (airspace disruption). Volcanic ash deposits are also used by volcanologists to interpret the mechanisms leading to, and controlling, explosive eruptions, in order to understand and be able to predict the behaviour of a volcano. A critical requirement in assessing volcanic ash hazard and quantifying related risk, as well as in being able to interpret eruptive dynamics from volcanic ash, is the accurate identification and characterization of the sources of ash injected in the atmosphere and transported downwind.
Ash injection and dispersion in the atmosphere during explosive eruptions is primarily associated with the formation of buoyant convective columns above vents. In this context, a mixture of particles and gas is ejected from the conduit in a high velocity jet following fragmentation of magma ascending from depth. Development of convective plumes from propagating PDCs is another important mechanism of volcanic ash intrusion into the atmosphere. PDCs are dry mixtures of gas, particles and entrained air that travel as fast moving density currents away from the eruptive vent. While spreading, PDCs can generate secondary plumes by buoyant rise of a mixture of hot gas and fine-grained material. Such plumes are entitled here ‘co-PDC plumes’.
Efforts to understand the generation and transport of Co-PDC plumes are important for several reasons: 1) Co-PDC plumes represent a serious hazard close to source as proved where small PDCs are very frequent (e.g. Tungurahua 2006, Sinabung, 2014), 2) distinguishing between vent-derived and Co-PDC ash in fallout deposits is often difficult but critical for the interpreting the dynamics of explosive eruptions, and 3) the contribution of Co-PDC ash is not accounted for in ash dispersion models used operationally during eruptive crises to decide on potential airspace closure.
The objectives of this project were:
Objective 1: Characterise physically co-PDC ash.
Objective 2: Assess to what extent Co-PDC ash contribute to fallout deposits (i.e. generated by deposition of volcanic particles transported in the atmosphere).
Objective 3: Assess what are the mechanisms controlling the sedimentation of Co-PDC ash from volcanic plumes, and more generally what controls the sedimentation of fine ash.
Objective 4: Determine the physical processes governing the initiation and development of Co-PDC plumes.
Work performed
The work was organised in four different tasks: (1) characterise the mechanisms of ash injection, dispersion and sedimentation during a well-documented large magnitude explosive eruption which had an important co-PDC contribution (the May 18, 1980 Mount St. Helens eruption was chosen; Objs. 1, 2 and 3); (2) review and compare the characteristics of previously studied co-PDC deposits, and their importance in their respective eruptive sequence (Objs. 1, 2 and 3); (3) design analogue scaled experiments to study the initiation of co-PDC plumes (Obj. 4); (4) develop tools to simulate numerically the sedimentation of fine ash from volcanic plumes (Obj. 3).
Ash samples from the May 18, 1980 Mount St. Helens eruption were supplied by a collaborator from the University of Oslo (Dr. A. Durant). The review and comparison of co-PDC deposits was performed in collaboration with Dr. S. Engwell (INGV, Pisa, now BGS, Edinburgh). The apparatus for the analogue experiments was built in the workshop of the School of Earth Science of the University of Bristol and set up in the Geological Fluid Dynamic laboratory of the same school. Interest in numerical modelling was brought through new collaborations with the UK Met Office, INGV Pisa, and the Cascades Observatory of the USGS.
Achievements
Task (1) The fellow has unravelled the dynamics of the co-PDC plume formed at the start of the May 18, 1980 Mount St. Helens eruption by combining detailed grain size and componentry analyses with reanalysis data of the atmospheric wind field and satellite images of the spreading ash cloud. This led to a major publication in the Journal of Geophysical Research.
Task (2) The fellow and her collaborator S. Engwell compiled and reanalysed an exceptional amount of published data related to co-PDC ash. This is an unprecedented database, which has already produced a publication in an Elsevier book on volcanic ash intended for a community interested in the atmospheric hazard. It will also lead to two other publications in volcanological journals, which are in preparation. In addition, the fellow supervised a master student (R. Edwards) who as part of her research project, characterised co-PDC ash from different types of explosive events at Soufriere Hills, Montserrat. The data collected and presented in her thesis are in the process of being published.
Task (3) Another master research project supervised by the fellow was focused on running experiments in the newly built apparatus. Important observations and data were collected, which improved our understanding of the role of grainsize and temperature on formation of co-PDC plumes. The master student (S. Mitchell) produced a 1st marked thesis at the end of his project, which is in the process of being turned into a publication.
Task (4) The work performed in tasks (1) and (2) led to a new understanding of the sedimentation of fine ash from volcanic plume. In order to improve the current numerical models used by the volcanological community to simulate the dispersion and sedimentation of volcanic particles in the atmosphere, the fellow has initiated collaborations with numerical modellers to insert her findings in the models. This effort has not yet led to publishable outputs, but is pursued beyond the end of the fellowship, by both Dr. J. Eychenne, her collaborators, and the volcanology group of the University of Bristol through research projects led by Prof. K. Cashman. This represents a step finding for the scientific community working on ash dispersion, and has significantly broaden our research horizons.
Impact
The achieved research work has important impact for agencies modelling ash distribution and assessing and mitigating ash-related hazards. Publication of part of the achieved work in a book specifically intended for these communities insures efficient dissemination of these findings.
FP7-PEOPLE-2012-IEF
Scientist in charge: Prof. K. Cashman, School of Earth Science, University of Bristol
Fellow: Dr. J. Eychenne, School of Earth Science, University of Bristol
Introduction and objectives
This Marie Curie Fellowship has focused on deciphering the primary and secondary mechanisms that generate ash during explosive volcanic eruptions, with a specific emphasis on the processes controlling the formation of plumes associated with the propagation of pyroclastic density currents (PDCs). Transport and deposition of volcanic ash represent the most widespread hazard related to explosive eruptions, with societal impacts both on the ground close to and downwind from source (destruction of crops, livestock and buildings, health issues for humans), and in the atmosphere (airspace disruption). Volcanic ash deposits are also used by volcanologists to interpret the mechanisms leading to, and controlling, explosive eruptions, in order to understand and be able to predict the behaviour of a volcano. A critical requirement in assessing volcanic ash hazard and quantifying related risk, as well as in being able to interpret eruptive dynamics from volcanic ash, is the accurate identification and characterization of the sources of ash injected in the atmosphere and transported downwind.
Ash injection and dispersion in the atmosphere during explosive eruptions is primarily associated with the formation of buoyant convective columns above vents. In this context, a mixture of particles and gas is ejected from the conduit in a high velocity jet following fragmentation of magma ascending from depth. Development of convective plumes from propagating PDCs is another important mechanism of volcanic ash intrusion into the atmosphere. PDCs are dry mixtures of gas, particles and entrained air that travel as fast moving density currents away from the eruptive vent. While spreading, PDCs can generate secondary plumes by buoyant rise of a mixture of hot gas and fine-grained material. Such plumes are entitled here ‘co-PDC plumes’.
Efforts to understand the generation and transport of Co-PDC plumes are important for several reasons: 1) Co-PDC plumes represent a serious hazard close to source as proved where small PDCs are very frequent (e.g. Tungurahua 2006, Sinabung, 2014), 2) distinguishing between vent-derived and Co-PDC ash in fallout deposits is often difficult but critical for the interpreting the dynamics of explosive eruptions, and 3) the contribution of Co-PDC ash is not accounted for in ash dispersion models used operationally during eruptive crises to decide on potential airspace closure.
The objectives of this project were:
Objective 1: Characterise physically co-PDC ash.
Objective 2: Assess to what extent Co-PDC ash contribute to fallout deposits (i.e. generated by deposition of volcanic particles transported in the atmosphere).
Objective 3: Assess what are the mechanisms controlling the sedimentation of Co-PDC ash from volcanic plumes, and more generally what controls the sedimentation of fine ash.
Objective 4: Determine the physical processes governing the initiation and development of Co-PDC plumes.
Work performed
The work was organised in four different tasks: (1) characterise the mechanisms of ash injection, dispersion and sedimentation during a well-documented large magnitude explosive eruption which had an important co-PDC contribution (the May 18, 1980 Mount St. Helens eruption was chosen; Objs. 1, 2 and 3); (2) review and compare the characteristics of previously studied co-PDC deposits, and their importance in their respective eruptive sequence (Objs. 1, 2 and 3); (3) design analogue scaled experiments to study the initiation of co-PDC plumes (Obj. 4); (4) develop tools to simulate numerically the sedimentation of fine ash from volcanic plumes (Obj. 3).
Ash samples from the May 18, 1980 Mount St. Helens eruption were supplied by a collaborator from the University of Oslo (Dr. A. Durant). The review and comparison of co-PDC deposits was performed in collaboration with Dr. S. Engwell (INGV, Pisa, now BGS, Edinburgh). The apparatus for the analogue experiments was built in the workshop of the School of Earth Science of the University of Bristol and set up in the Geological Fluid Dynamic laboratory of the same school. Interest in numerical modelling was brought through new collaborations with the UK Met Office, INGV Pisa, and the Cascades Observatory of the USGS.
Achievements
Task (1) The fellow has unravelled the dynamics of the co-PDC plume formed at the start of the May 18, 1980 Mount St. Helens eruption by combining detailed grain size and componentry analyses with reanalysis data of the atmospheric wind field and satellite images of the spreading ash cloud. This led to a major publication in the Journal of Geophysical Research.
Task (2) The fellow and her collaborator S. Engwell compiled and reanalysed an exceptional amount of published data related to co-PDC ash. This is an unprecedented database, which has already produced a publication in an Elsevier book on volcanic ash intended for a community interested in the atmospheric hazard. It will also lead to two other publications in volcanological journals, which are in preparation. In addition, the fellow supervised a master student (R. Edwards) who as part of her research project, characterised co-PDC ash from different types of explosive events at Soufriere Hills, Montserrat. The data collected and presented in her thesis are in the process of being published.
Task (3) Another master research project supervised by the fellow was focused on running experiments in the newly built apparatus. Important observations and data were collected, which improved our understanding of the role of grainsize and temperature on formation of co-PDC plumes. The master student (S. Mitchell) produced a 1st marked thesis at the end of his project, which is in the process of being turned into a publication.
Task (4) The work performed in tasks (1) and (2) led to a new understanding of the sedimentation of fine ash from volcanic plume. In order to improve the current numerical models used by the volcanological community to simulate the dispersion and sedimentation of volcanic particles in the atmosphere, the fellow has initiated collaborations with numerical modellers to insert her findings in the models. This effort has not yet led to publishable outputs, but is pursued beyond the end of the fellowship, by both Dr. J. Eychenne, her collaborators, and the volcanology group of the University of Bristol through research projects led by Prof. K. Cashman. This represents a step finding for the scientific community working on ash dispersion, and has significantly broaden our research horizons.
Impact
The achieved research work has important impact for agencies modelling ash distribution and assessing and mitigating ash-related hazards. Publication of part of the achieved work in a book specifically intended for these communities insures efficient dissemination of these findings.