Extracting total grain size distribution data for ash fall deposits from densely populated volcanoes (Chaparraristique volcano, El Salvador)

Lead Research Organisation: Durham University
Department Name: Earth Sciences


As recent eruptions in Iceland have shown, volcanic ash clouds present immediate hazards to people, infrastructure and aviation. How volcanic clouds are dispersed through the atmosphere is dependent on both the characteristics of the eruption (e.g., volume and rate of eruption, grain size, height of eruption column) and atmospheric conditions (e.g., wind speed and direction). Volcanic Ash Advisory Centres (VAAC) use computer models to forecast ash clouds dispersal during an eruption, but in order for these models to be accurate, they require high quality datasets gained from real-time observations (satellite, airborne and ground-based) and detailed datasets from previous similar eruptions (e.g. grainsize distribution). Such datasets are scarce and are a significant source of uncertainty in model outputs. There are only a handful of high quality datasets available globally and these are skewed towards large eruptions and silicic compositions. Smaller volume eruptions, which occur on a higher frequency and are still of considerable impact on a regional and sub-regional scale, are poorly represented in these datasets. This is due to the short-lived nature of the eruptions, which reduces opportunities for observation, and the thinner deposits are more quickly eroded. Opportunities to gather quantitative, field-derived data from a recent eruption is extremely valuable to science.
On the 29th December 2013, Chaparrastique volcano (aka Volcán San Miguel) in El Salvador, erupted and produced a ~10 km-high plume that drifted across the country and deposited ash > 100 km away. It is one of El Salvador's most active volcanoes and historical records indicate that this was its most powerful eruption for 300 years. Five thousand people were evacuated within a 3 km radius of the volcano. The volcano lies 12 km from El Salvador's third largest city, San Miguel. Despite the many historical eruptions, and the high risk posed, few quantitative data exist for the volcano and little is known about its geologic history. Ash layers from historic eruptions are poorly preserved due to the tropical climate.
This research will obtain a unique quantitative dataset on the deposits from the 2013 Chaparraristique eruption, to better understand the causes of the eruption, find out why it was more powerful than its past eruptions, and fully characterise the deposits before they are removed by rainfall. Some of the data will be used to create probabilistic hazard maps for the volcano. These will help the local authorities plan for similar eruptions from the volcano in the future under a range of wind conditions. Usefully, the eruption occurred during the dry season and the ash deposits remain reasonably well preserved on the ground. There is an urgent need collect data prior to the wet season at the end of April that will strip away the ash and increase the risk of lahars (rain-triggered ash slurries).
The research objectives are: (1) obtain a comprehensive ash fall deposit dataset; (2) construct thickness and grainsize maps for the ash fall deposit; (3) calculate important eruption parameters (total grain size distribution, constrain dispersion, eruption volume, column height and mass discharge rates); (4) calibrate these with visual and remote observations of 29th December eruption plume; (5) carry out textural, optical, morphological and geochemical characterisations of the ash to investigate eruption processes. Some of these data will be used to model the ash dispersion and develop probabilistic hazard assessments for Chaparrastique using freely available analytical Volcanic Ash Transport and Dispersal Models (VATDM). We will disseminate these results to interested authorities in El Salvador, and critical data such as grainsize distribution and refractive index will be distributed to the Washington VAAC and globally to ensure maximum impact. The results will help agencies worldwide in understanding the hazards posed by similar eruptions.

Planned Impact

The key stakeholder beneficiaries of this research will be:
(1) El Salvador government (Ministry of the Environment and Natural Resources);
(2) El Salvador population;
(3) Civil protection authorities;
(4) Washington Volcanic Ash Advisory Centre (VAAC);
(5) UK government scientists (NERC BGS);
(6) Wider scientific community (observatory scientists, meteorologists, and social scientists interested in hazard mitigation, i.e., scientists who have roles to play informing authorities on volcanic hazard).

The research will be of primary use to the Salvadoran scientists at the government Ministry of the Environment and Natural Resources (MARN) and the civil protection in San Salvador, who are charged with planning for and managing volcanic crises in the country. El Salvador is home to a large number of active volcanoes, many of which are capable of erupting in a similar manner to the 2013 Chaparrastique eruption. Recent investment in geophysical and atmospheric monitoring in the country means that they are well prepared to pick up signs of unrest at volcanoes, monitor the height and dispersal of ash clouds and communicate with Washington VAAC. But, critically for hazard assessments, there are scant resources to conduct fieldwork on volcanoes or on volcanic ash deposits. We will use this research to transfer knowledge to MARN scientists to equip them with the skills to be able to respond to volcanic eruptions in this manner, engage effectively with Washington VAAC and collect data that can be used to build probabilistic hazard models. These stakeholders will gain through having a more complete understanding of the 2013 eruption and through the co-produced improvements to scenario planning and risk mitigation that will arise out of this research. The El Salvador government will have a substantial dataset that they can use for probabilistic ash fall hazard mapping of Chaparrastique and other volcanoes in El Salvador (Volcan San Salvador, Volcan San Vicente) which pose a similar threat to neighbouring communities. This will help the government better prepare the populations for future eruptions and develop improved civil protection contingency plans.

NERC BGS are well disposed to place the results of this research into the hands of organisations that acutely need these sorts of datasets and case studies to help countries and communities prepare for and mitigate against for the impacts of volcanic eruptions. Data and results will also be shared with international projects, such as the Global Volcano Model (GVM, http://www.globalvolcanomodel.org/) that provides systematic evidence, data and analysis of volcanic hazards and risk, and the Volcanic Global Risk Identification and Analysis Project (VOGRIPA, http://www.bgs.ac.uk/vogripa/index.cfm) that provides systematic information on global volcanic activity, hazards and vulnerability that can be analysed to identify locations at high risk from volcanism and gaps in knowledge about hazards and risk. Project partner Sue Loughlin (Head of Volcanology, NERC BGS) sits on the management team of both GVM and VOGRIPA and is well placed to maximise the impact of the research in these areas. NERC BGS, under Loughlin, are also members of the Strengthening Resilience in Volcanic Areas project (STREVA, http://streva.ac.uk/): a multi-institutional, UEA-led initiative aimed at innovative interdisciplinary research to develop and apply a practical and adaptable volcanic risk assessment framework to reduce the negative consequences of volcanic activity on people and assets.


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Description We have undertaken detailed sampling of the eruption's deposits and are continuing the laboratory analysis. We have been able to describe the eruption in substantial detail using our analysis of the deposits together with eyewitness accounts, video footage and photographs. The eruption, although small, was complex and a number of significant events during the eruption have been recognised, described and interpreted. These include two initial large discrete explosions, the generation of a pyroclastic flow and the fallout of ash aggregates.

Petrological work has established that the eruption followed magma mixing between two compositionally distinct magmas.
Exploitation Route We expect our findings to be used by the Salvadoran government to improve hazard planning for volcanic eruptions in the country.
Sectors Aerospace, Defence and Marine,Education,Environment,Government, Democracy and Justice