EURO Journal on Decision Processes

González P.J.

El Hierro Island, the youngest of the Canary Islands, and its latest eruption in 2011–2012 have been a playground of fruitful decade-long studies. In this book, we summarize and provide future venues of action to solve outstanding questions. The topics cover geological studies of Holocene volcanism so it informs past, present and future activity. Its active magmatic system from a petrological and geophysical lens. How monitoring of volcanic activity can be optimized and how to read the data streams in a meaningful way. The marine environmental effects of a submarine eruption are covered in detail, as well as how the society could be properly engage to reduce the risks associated to it, and appreciate and benefit from it. So, in each chapter the reader should find inspiration and future challenges waiting to be solved. Remaining puzzles pieces about how volcanism works and how it affects its environment. An effort to provide food for thoughts of future Canary Islands volcanological research, and in particular El Hierro.

Álvarez-Valero A.M., Sánchez-Guillamón O., Navarro I., Albert H., Sánchez A.P., Rodríguez J.A., Geyer A., Martí J., Ban M., Gómez-Ballesteros M., Catalán M., García N., Fraile-Nuez E., Casillas R., Martín-Luis M.C., et. al.

Active volcanoes are key laboratories to carry out detailed research -and monitoring- about the history of magmas before, during and after eruptions. Tagoro, the submarine active volcano at El Hierro Island (Canary archipelago), is a highly favorable case to assess and monitor its daily ongoing behaviour, as well as to study the links between the processes of magma genesis occurring at depth and their derived eruptive events at the surface. In this interdisciplinary research we combine new results of classical petrology (petrography, geochemistry, and thermodynamics) on the volcanic products expelled by Tagoro during the 2011–2012 eruption, with a high-resolution (5 m grid) bathymetry model carried out during 2017, and recent data from magnetometry, to refine the current knowledge of this eruption. Our results mainly reveal (i) slight magma differentiation and mixing processes at c. 12 km depth during a continuous eruptive pulse; (ii) a similar magmatic evolution and residence times at depth between previous and 2011–2012 eruptions on the island; (iii) an insignificant interaction of external fluids with the magma at depth or within the ascent conduit; (iv) a present-day magnetometric anomaly under the Tagoro’s area; (v) a minimum volume estimate for the magma withdrawn from the plumbing system at depth.

Fraile-Nuez E., Santana-Casiano J.M., González-Dávila M., González-Vega A., Vázquez J.T., Sotomayor-García A., Ferrera I., Santana-González C., Eugenio F., Marcello J., Hernández-León S., Bakalis E., Rueda J.L., Gómez-Ballesteros M., Álvarez-Valero A.M., et. al.

The shallow Tagoro submarine volcano monitoring represents a unique opportunity not only for improving our sparse understanding of submarine volcanic processes in specific scientific fields as physical and chemical oceanography or marine geology but also its interactions over the marine biology in one of the richest marine ecosystems in Europe. This chapter aims to summarize the most relevant physical–chemical, geological and biological changes that occurred in the marine ecosystem of El Hierro island, at the Marine Reserve Punta de La Restinga—El Mar de Las Calmas, due to the genesis of the new underwater volcano Tagoro (27º37′07″N–017º59′28″W) in October 2011. During the first six months of the eruption, extreme physical–chemical perturbations caused by this event, comprising thermal increase from up to + 18.8 °C, water acidification with a pH decrease of 2.9 units, deoxygenation to anoxic levels and extremely high metal enrichment among others, resulted in significant and dramatic alterations of the marine ecosystem. After March 2012, once the eruptive phase was finished, the new submarine volcano entered an active hydrothermal phase involving the release of heat with smaller but still significant and important thermal anomalies of up to + 2.55 °C around the craters, density decrease of − 1.43 kg m−3, pH decrease of − 1.25 units, and high concentrations of metals and inorganic nutrients similar to upwelling zones. These enrichments are still active up to date, producing clear signs of marine recovery not only in the benthonic strata but also in the whole water column compared with pre-eruptive data. Since its eruption ten years ago, an unprecedented monitoring effort has turned into the longest and most complete multidisciplinary time-series for the study of a shallow submarine volcano, with the realization of 31 oceanographic multidisciplinary expeditions that systematically measure more than 40 different physical–chemical and biological variables. All this information and the results obtained during the evolution of the process could serve as a baseline for better understanding future or similar submarine eruptions worldwide.

Galindo I., Montoya-Montes I., García López-Davalillo J.C., Sarro R., Llorente M., Sánchez N., Santamarta J.C., Cruz-Pérez N., Ortega A., Mateos R.M.

Secondary volcanic hazards (SVH) are not usually considered in volcanic hazard analysis, nor are they specifically included in volcanic risk management plans. However, SVH may cause more damage than primary volcanic hazards (PVH). The magmatic unrest on El Hierro Island in 2011–2012 is a perfect example of how SVH can be one of the leading causes of damage during magmatic unrest. Rockfalls are common on the island of El Hierro, mainly controlled by the heterogeneous lithology and the steep topography. Heavy rainfall and strong wind are usually the main triggering factors. However, during the 2011–2012 El Hierro Island magmatic unrest most of them were triggered due to earthquakes. Rock falls caused roadblocks and damage to road infrastructure. Two reports analysing rockfall hazards and associated risks during the emergency were based on expert knowledge and highlighted the need for, a comprehensive inventory of rockfalls, their processes characterisation, recurring event timing estimations and an analysis of their triggering factors. The need for better rockfall understanding on El Hierro island and the extensive experience of the local Civil Protection agents that were already working with citizens during the volcanic emergency, led to the development of a Rockfall Citizen’s Observatory. The observatory aims to engage citizens in the study of this geological hazard, hence providing a substantial increase in the amount of high quality data for rockfall risk analysis.

Sotomayor-García A., Rueda J.L., Sánchez-Guillamón O., Urra J., Martín-Arjona A., González-Porto M., Vazquez J.T., Palomino D., López-González N., Fernández-Salas L.M., Santana-Casiano J.M., González-Dávila M., Fraile-Nuez E.

Since Tagoro volcano erupted in 2011, several impacts have been associated to the volcano formation process, some of which are still present to date. This chapter is a review of the marine environmental perturbations caused by Tagoro volcano as a new geological structure, but thoroughly onto the partly annihilated benthic and demersal pre-existing biota, and the colonizing dynamics during the recovery process. Shallow recent volcanic activity in the NE Atlantic is uncommon, thus, Tagoro provides a unique opportunity to study, from the very beginning, the evolution of the unusual shallow hydrothermal systems, and the establishment of new marine habitats and associated biota. Distinct habitat types, with different associated deposit products (volcaniclastic aprons, lava balloons, lava ponds, etc.), have been described, and a description of the colonizing biota has been made from the available published works as well as from underwater imagery and benthic dredge samples taken during several field expeditions (Vulcano0313, 1013, 0314 and Vulcana0417). Those habitats included hard (rocky) and mixed (loose) substrate habitats, but also extreme habitats with hydrothermal vents and bacterial mats, accompanied by significant physical and chemical anomalies. Habitat preference by the observed taxa among the volcanic edifice has been explored through nMDS analyses, and a comparative analysis with published data of the typical fauna of the region (La Restinga, Mar de Las Calmas, Marine Reserve, El Hierro Island), permitted to foresee the very first steps and direction of the recovery of the benthic and demersal communities. The impact caused by Tagoro onto the nearest littoral benthic communities, the ichthyofauna and the local fisheries have been described as well. Finally, some recommendations and further steps are given in order to adequately monitor the successional trend and environmental status of the benthic and demersal communities.

Vázquez J., Sánchez Guillamón O., Palomino D., Fernández Salas L.M., Bárcenas P., Gómez-Ballesteros M., Tello M.O., López-González N., Presas-Navarro C., Fraile-Nuez E.

The Spanish Institute of Oceanography has realized nineteen oceanographic cruises in order to monitor the geomorphological changes during the submarine eruption of the Tagoro volcano and later evolution. The major geomorphological features were achieved fundamentally by the use of Multibeam EM710 echosounder data. Eruption was characterized by two main phases, the first one alternate stages of vertical growth and denudation by development of basal and southern flank collapses of the main edifice took place; the second phase was characterized by a fissure growth with a NNW-SSE trend. The eruption produced a main volcanic edifice rising from 400 to 88 m water depth. The edifice consists of four attached cones extended and at least fifteen emission vents. This edifice has a quasi-circular base and its final morphology was modulated by the activity of emission vents during the second phase which produced a NNW-SSE elongated summit line. Both vertical growth and instability phases were conditioned by preexistent southwestwards gradient of the seafloor slope and its initial location into a gully on the southern submarine island flank. In the proximal area, morphology is also characterized in by four ridges that correspond to semi-buried residual scars of different collapse phases. On the SW flank an apron of mixed lavas, pyroclastic and debris flows were deposited along more than 5 km length. These deposits were channeled throughout the previous gully and three parts are differentiated: proximal apron from the cone to an intermediate ravine located at 2.5 km away from the base of the main edifice where its maximum thickness occur in an accumulation front, the intermediated ravine and the distal apron fan deposits from the mouth of the ravine to 1800 m depth. The Tagoro volcano was built during a monogenetic eruption dominated by pyroclastic and lava balloon emissions, with lava emissions in the deepest vents. Its evolution alternating constructive and destructive stages and its morphology being similar to that of long-lived volcanoes located on a steep seafloor and dominated by pyroclastic emissions.

Vegas J., Galindo I., Vázquez J., León R., Sánchez N., Martín-González E., Romero C.

The submarine eruption of La Restinga, now known under the name of Tagoro volcano, began in early October 2011 offshore the southern coast of El Hierro island and ended in early March 2012. This eruption produced a volcanic cone, hornitos, a thick pyroclastic apron that prograded towards the base of the volcanic edifice and volcanic products that emerged from the sea floor over sea level, such as lava balloons and low-density vesicular pyroclasts (xeno-pumices/restingolitas) that popped up in the sea surface. All these submarine volcanic elements and processes, which have been monitored and studied by direct and indirect scientific techniques, have made it possible to gain scientific knowledge of the whole eruption, taking into consideration that no other submarine eruption had been studied before in Spain. Tagoro is the youngest Spanish submarine volcano included in the national geoheritage inventory (Spanish Inventory of Geological Sites of Interest), being representative and the best example of an underwater eruption, as well as one that has been studied and monitored by various research teams since the beginning of the magmatic unrest. It is essential that geoheritage inventories include submarine geosites as an essential part of geological knowledge of the planet. For this reason, a new methodology has been designed for assessment because most of the criteria used in terrestrial geosites are not applicable. This submarine volcano, despite its inaccessibility, is a geotouristic resource for the El Hierro UNESCO Global Geopark (UGGp), which uses new technologies for its interpretation, through virtual reality presentation so that the visitors can access it at the Interpretation Centre. It is necessary to promote educational and tourist use by means of new technologies such as augmented reality, an e-library and a specialized section on its website that can be easily accessed with QR codes located in La Restinga port and in the UGGp network of hotels and rural houses to connect with the audience. Its protection is guaranteed as it lies within the ‘Punta de La Restinga-Mar de Las Calmas’ Marine Reserve, but the geological values of Tagoro should be specifically included in the regulations as one of the natural resources that contribute to this protected marine area.

Guillén C., Romero M.C., Galindo I.

Hydrovolcanism, resulting from the interaction of magma with water, is a frequent volcanic process found in many geotectonic settings and fundamental in the growth of oceanic islands. Examples of shallow to intermediate-depth hydrovolcanic activity are frequent in the Canary Islands and characterise volcanism from the seamount stage and emergent island stage to historical volcanism. We present a review of the existing information on subaqueous shallow and intermediate-depth hydrovolcanic eruptions on El Hierro prior to the 2011–2012 Tagoro hydrovolcanic eruption. New information is provided based on the study of subaerial and intermediate-depth volcanic deposits, as well as on the analysis of possible historical submarine eruptions. Our findings show that there is evidence of several shallow and intermediate-depth submarine eruptions around El Hierro Island, some having occurred in historical times. Knowledge of these eruptions is essential not only to better understand the evolution of the island, to establish future coastal and submarine eruptive scenarios, but also to improve volcanic risk analysis. This work highlights the fact that, when dealing with volcanic islands, it is essential to include not only the coastline as a boundary, but also to consider the subaerial and submarine part of the islands in a continuous fashion, always taking into account sea level changes in the past as well as in the future.

Domínguez Cerdeña I., Charco M., González-Alonso E., del Fresno C., Benito-Saz M.A., García-Cañada L.

The 2011–2014 volcanic episode of El Hierro has become an excellent laboratory for studying the magma plumbing system beneath the island, with tens of works in the literature focusing on the geophysical, geodetic and/or petrological perspective. This chapter mainly focuses on the results obtained through the analysis of the seismicity and ground deformation during the different periods of the activity, including the three months of pre-eruptive unrest, the 5-month-long submarine volcanic eruption, and the magmatic intrusions during the following 2 years after the end of the eruption. All these results, together with those from other petrological and gravimetric studies, have allowed us to obtain a joint description of magma transport and storage at El Hierro Island. Most interpretations point to the presence of a main deep magma storage system below the center of the island feeding both the eruption and the later sill-like magmatic intrusions. The stagnation of the post-eruptive magma accumulations could indicate the presence of stress barriers inhibiting the ascent of magma. This structure may have an important influence on the growth of the island, with similar a contribution from magma storage under the crust as from the eruption itself. Despite all the accumulated knowledge, there are still open questions to be addressed.

López C., Luengo-Oroz N., Felpeto A., Torres-González P.A., Meletlidis S., García-Cañada L., Sainz-Maza S., Del Fresno C., Benito-Saz M.A., Blanco M.J.

Observational data recorded during the reawakening of volcanic activity in El Hierro, which ended with 2011–2012 Tagoro eruption, allow us to study the precursory activity associated with this event. This volcanic activity occurred between 1971 Teneguía eruption and the 2021 eruption in Cumbre Vieja (both in La Palma island) and represents the first such event in the Canary Islands that has been fully recorded from beginning to end by a multi-parameter monitoring network. The lack of previous instrumental data makes these data unique and a case study for a broader understanding of monogenetic volcanism. In this chapter we review previous works to collect the precursors of the historical eruptions in the Canaries, discussing which were most relevant to forecast the evolution of the eruptive processes. We find solid signs of the undergoing magmatic process in El Hierro in the evolution of the seismicity, the surface deformations and gas emissions, the changes in media properties and the appearing of gravity anomalies. Results can contribute to a better interpretation of the observational data and the correct volcanic risk assessment of future events.

González-Vega A., Arrieta J.M., Santana-Casiano M., González-Dávila M., Santana-González C., Mercado J.M., Escánez-Pérez J., Presas-Navarro C., Fraile-Nuez E.

The shallow submarine volcano Tagoro releases high amounts of inorganic nutrients (N, P, Si, Fe) into the surrounding waters. These emissions have been intensely monitored during both the eruptive stage (October 2011–March 2012) and the post-eruptive hydrothermal stage (March 2012—ongoing). The obtained seven-years dataset comprises over 3300 water samples analysed for concentrations of silicate (Si(OH)4), phosphate (PO4), nitrate + nitrate(NO3− + NO2−), ammonium (NH4+), and iron (Fe(II)) in the area affected by the volcanic emissions (> 250 km2) as well as outside of this area for reference. This chapter provides an overview on the main results obtained from this comprehensive dataset, as well as contextualizing these results by comparison with other volcanic and hydrothermal nutrient sources in the world. The results show that the hydrothermal emissions from Tagoro volcano cause nutrient enrichments both in the vicinities of the vents (88–130 m depth) and the water column up to 50 m above the seabed. The emissions can occasionally also be injected into the mixed layer and might even reach the surface. The transport of these emissions presents a flux of comparable magnitude to other important nutrient fluxes in the region, such as those related to the NW-African coastal upwelling, when compared per unit of area. We highlight the importance of accounting for shallow hydrothermal and volcanic inputs worldwide as significant nutrient sources that can exert an important influence on the ecosystems of superficial nutrient-poor waters.

Meletlidis S., Becerril L., Felpeto A.

El Hierro, the youngest island of the Canary Islands, emerged from the ocean floor only 1.2 Ma ago and it is still in its youngest stage (shield volcanism). During its growth, the island passed from the submarine stage to the subaerial activity, constructing an island that had been modified by at least six large destructive flank collapses or giant landslides, a common feature in the development of the basaltic oceanic islands. Throughout the period of its evolution, it showed different eruptive styles, developing two main edifices. The last 160 kyrs the activity has been characterized by the rift volcanism, resulting in a three-arms rift construction. This chapter provides with a comprehensive picture of the evolution of the island and generate a baseline of knowledge that will encourage a further study on volcanic processes related to the hot spot volcanism. In this work, we describe, not only the geological evolution of the island, but also review the existed petrological, geochemical and geochronological data. Furthermore, we use this data to generate the probable future eruptive scenarios. We conclude with an updated and synthesized review of the recent 2011–2012 submarine eruption.

Ferrera I., Arrieta J.M., Sebastián M., Fraile-Nuez E.

Underwater hydrothermal systems release nutrient-rich fluids that are favorable for microbial activity. This allows the growth of a plethora of microorganisms that can be found inhabiting hydrothermal plumes, metalliferous sediments, or forming dense microbial biofilms or mats on the surrounding of the vents. Most of the current knowledge on the microbiology associated with underwater volcanic activity comes from the study of deep-sea hydrothermal vents. However, much less is known about shallow underwater hydrothermal systems. The submarine volcanic eruption that took place near El Hierro (Canary Islands) in October 2011 and gave rise to the Tagoro volcano, located only 1.8 km from land and approximately 89 m below the sea surface, provided a rare occasion to study almost in real time the microbiology associated with an underwater eruptive process, from its birth to the current degassing state, allowing investigations of its effects on the surrounding pelagic and benthic communities. In this chapter, we summarize what is known 10 years after the formation of the Tagoro submarine volcano, and we discuss what questions should be pursued in order to foster our knowledge of the microbiology associated with this volcano and its surrounding waters.

Lavigne F., Mei E.T., Morin J., Humaida H., Moatty A., de Bélizal E., Hadmoko D.S., Grancher D., Picquout A.

Merapi is a two-sided, paradoxical volcano: on the one hand 1.8 million people live on its flanks. It is one of the most densely populated volcanoes on Earth, with population densities averaging 764 inhabitants per square kilometre within a 10 km radius from the summit. The main reasons for the high densities are land resources and associated livelihoods from agriculture, livestock, sand mining, and tourism. On the other hand, Merapi is also one of the world’s most active volcanoes. Dome-collapse pyroclastic density currents (PDCs) occur every few years (e.g. 1994, 2002, 2006), and more violent explosive episodes are generated with an average recurrence interval of several decades (e.g. 1872, 1930, 2010). Risk management at Merapi is based on volcanic hazard zonation (called KRB I, II, and III, from the less exposed to the most exposed), derived from its eruptive history. Since its first publication by the Volcanological Survey of Indonesia in 1978, the danger map has been updated twice, in 2002 and after the deadly eruption of Merapi in 2010. Most of the information is provided by scientists during the ‘raising awareness program’ phase and achieved in the framework of a Community-Based Disaster Risk Management (CBDRM), which empowers communities with self-developed ways of coping with crises due to natural hazards. In periods of emergency, the Center for Volcanology and Geological Hazard Mitigation provides four warning levels of volcanic activity. In 2010, Merapi produced its largest eruption since 1872, damaging around 12,000 buildings, claiming 367 lives, including 200 directly by PDCs, and triggering massive evacuations of up to 400,000 people, as counted in the evacuation camps.

Budi-Santoso A., Beauducel F., Nandaka I.G., Humaida H., Costa F., Widiwijayanti C., Iguchi M., Métaxian J., Rudianto I., Rozin M., Sulistiyani, Nurdin I., Kelfoun K., Byrdina S., Pinel V., et. al.

Merapi volcano has the most advanced and comprehensive monitoring system in Indonesia. Monitoring at the volcano started in 1920, when the Dutch East Indies government established the Vulkaanbewakingsdienst. Since then, monitoring, initially carried out visually, evolved rapidly using bespoke monitoring equipment and modern technology, and with intensive cooperation with scientists and institutions from abroad. At present, BPPTKG (Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi) in Yogyakarta has the mandate for hazard mitigation and the task of monitoring and providing early warnings to save local communities at risk from eruptions of Merapi. In carrying out its duties, BPPTKG manages a monitoring network consisting of various techniques, including seismic, deformation, gas and temperature monitoring, visual observations and attempts to improve monitoring techniques and methods, data handling and ways to provide early warning information. One of the fundamental recent changes in the Merapi monitoring system has been the transition in data processing from off-line-based to real-time, on-line-based techniques. Although the current Merapi monitoring system is presently at its most advanced, monitoring and anticipating changes in eruption styles, particularly those with weak precursors, such as phreatic eruptions, remain challenging.