Keywords

1 Underwater Eruption of El Hierro

The submarine eruption of El Hierro at the end of 2011 is the first of the twenty-first century in the Canary Islands and is the only one in historical period on the island. The eruption of the Tagoro volcano was an eruptive event very well documented by multiple agencies (e.g. Instituto Volcanológico de Canarias (INVOLCAN), Instituto Geográfico Nacional (IGN), Instituto Geológico y Minero de España (IGME), Instituto Español de Oceanografía (IEO), Consejo Superior de Investigaciones Científicas (CSIC), etc.) and managed by Civil Protection with the aim of reducing damage to the population. The eruption originated on the seabed, at a depth of 400 m, in the vicinity of the fishing village of La Restinga (Fig. 1). The economic activity of this area is based on the primary and tertiary sectors, with fishing and diving tourism being the most developed activities.n. However, this volcanic eruption in the vicinity of this area so closely linked to the sea caused direct and indirect consequences on the economy of the residents of La Restinga. The economy of the residents of La Restinga. Even so, thanks to technological and scientific advances, excellent measurements have been obtained before and during the underwater eruption. The locations of the earthquakes, the deformation of the island, the values of diffuse outgassing of volcanic gases, thermal images and bathymetric representations, allowed us to estimate with great precision the volume of the emitted material. In this sense, although the volcanic hazards linked to the eruption (seismicity, terrain deformation and gas emission) generated risks that affected to a greater or lesser extent the Herreña population, there were no human casualties.

Fig. 1
A map depicts the location of the Tagoro Volcano to the south of La Restinga on El Hierro island.

Location of the Tagoro Volcano south of La Restinga in El Hierro

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The aim of this chapter is to analyze how the eruption developed from the pre-eruptive to the post-eruptive period and the importance of this volcanic event for the development of the Herreño geopark.

1.1 Pre-Eruptive Period

The pre-eruptive period of the submarine eruption of the island of El Hierro lasted 83 days, starting on 17 July 2011 after the onset of the seismic shock recorded by the geophysical network of the National Geographic Institute (IGN) (Padilla et al. 2013; Pérez-Torrado et al. 2012a, b; Domínguez Cerdeña et al. 2018; Melián et al. 2014; López et al. 2012; Hernández et al. 2013; Pérez et al. 2012, 2014, 2015; Padrón et al. 2013; Ibáñez et al. 2012; Carracedo et al. 2012; Rivera et al. 2013, Sandoval-Velasquez et al. 2021; Rodríguez-Losada et al. 2015; García-Yeguas et al. 2014; Blanco et al. 2015). The seismicity produced had a migratory character (Fig. 2). Throughout the 3-month pre-eruptive period, 12,000 seismic events occurred, migrating in a pattern from the north (El Golfo) to the south (El Julan and the Mar de las Calmas) of the island indicating the movement of magma at depth and the search for the weakest pathways in the crust to emerge (Ibáñez et al. 2012).

Fig. 2
A map depicts the locations of San Simon well, La Restinga village, submarine eruption, and 3 seismic phases on the island of El Hierro.

Source Melián et al. (2014)

Location of seismic events recorded by IGN (www.ign.es) up to 5 March 2012 on the island of El Hierro. The colors show different seismic phases described by Ibáñez et al. (2012); the star in red shows the location of the submarine eruption.

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Together with seismicity, the IGN recorded ground deformation and an increase in endogenous gas emissions (Pérez et al. 2014; Melián et al. 2014). However, seismicity starred the pre-eruptive period, with earthquakes reaching magnitudes of up to 4.4, creating uncertainty about the place of origin of the new volcano due to its constant spatial migration. On October 8, the largest seismic event in the pre-eruptive stage originated at a depth of 15 km and about 3–4 km from La Restinga. This earthquake was caused by the opening of a hydraulic fracture when magma was injected into the cortical levels (Pérez-Torrado et al. 2012a, b). Some authors claim that the beginning of the eruption was on 10 October, when the harmonic tremor started (Martí et al. 2013). However, the first visible evidence of the eruption appeared on October 12 (Pérez et al. 2014). They are the change in color of the seawater, which went from light green to dark brown in the vicinity of La Restinga, as a result of the chemical interaction of seawater with the discharge of hydrothermal fluids at high temperature and magmatic gases. This volcanic manifestation was called the mancha (Pérez et al. 2014). The new volcano (Fig. 1) under construction started to be installed on the southern slope of the submarine base of El Hierro.

The main consequences for the population were due to the multiple earthquakes and their high magnitudes. In this sense, on September 23rd the yellow warning level was established in the traffic light which was composed of three colours: red, yellow and green. The most significant measures were the closure of the Los Roquillos tunnel (between the municipalities of La Frontera and El Valverde) and the evacuation of La Restinga. However, in early October 2011 as seismic activity experienced a decrease in frequency and magnitude, the residents of La Restinga returned to their homes. On October 8, 2011 a volcano-tectonic event of higher magnitude (4.4 Ml) occurred and two days later the volcanic tremor started (Ibáñez et al. 2012). The major seismic events caused landslides, rock falls and fear among the population, however, they did not cause major damage among the island's residents. However, the day before the eruption, the residents of La Restinga were evacuated to Valverde as a precaution and to improve the management of the volcanic crisis. volcanic crisis.

1.2 Course—Characteristics of the Volcanic Event

During the eruptive stage from October 12, 2011 to March 5, 2012 (206 days) a total of 2500 earthquakes and harmonic tremor (www.ign.es) were recorded. After the start of the eruption, the number of earthquakes was decreasing as the energy accumulated by the magmatic intrusion at depth was released through the eruption. However, on October 20, in the area of El Golfo, the largest number of events and the highest magnitudes were recorded, accompanied by a greater release of pyroclastic material from the eruptive source. These events were interpreted at first as a possible opening of a new eruptive mouth in the north of the island, but no other eruption occurred in the vicinity of it (Perez-Torrado et al. 2012a, b).

The ground deformation experienced as a result of the eruption was of the order of 40 mm in the vertical component and 50 mm in the horizontal component (www.ign.es), with the largest deformation recorded at La Frontera (Fig. 3), with intrusion volumes for the year 2011 estimated at 2.1 × 107 m3 (Pérez et al. 2014).

Fig. 3
3 graphs depict the horizontal and vertical displacements in E W, N S, and U D components for the FRON station from 2011 to 2013.

Source Pérez et al. (2014)

Time series of GPS coordinates for the FRON station from 2011 to the end of 2013, whose reference station is located at GMAS (Maspalomas). Solid red circles represent the elevation determined by the IGN (www.ign.es); first row- horizontal displacement in East–West component; second row-horizontal displacement in North–South component; third row- vertical displacement.

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The geochemical data also indicated an increase in the diffuse emission of CO-type volcanic gases2 (Fig. 4), H2S, and a significant change in 3He/He4 ratio and 222Rn activity values (Melián et al. 2014).

Fig. 4
6 maps of El Hierro island depict the emission of C O 2 gases from 22 July 2011 to 25 March 2012. The highest emission is on 24 November 2011.

Figure Melián et al. (2014)

CO flux maps 2 of El Hierro Island constructed from an average of 100 samples using the sGs method.

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In the early stages of the eruption, floating pyroclasts appeared on the sea surface. These volcanic bombs and slags, which were seen for the first time on 15 October 2011, were named Restingolites (Perez-Torrado et al. 2012a, b). It is the first time that pyroclast with white, siliceous cores and black basanitic crust has been studied and documented (Perez-Torrado et al. 2012a, b). Its peculiarity is due to the enormous chemical contrast represented by the product itself. This material emerged from the volcano to the water surface in the first eruptive phases and, contrary to materials with basaltic components, floated in the sea. After the first week of the eruption, the Restingolites stopped appearing on the water surface and on the nearby beaches, and another volcanic product, the “lava baloons” or hollow volcanic bombs, were found inside them (Perez-Torrado et al. 2012a, b). Their collection was complicated because the seawater invaded the interior of the pyroclasts and they lost their buoyancy. This type of volcanic product has been previously documented during submarine eruptions (Clague et al. 2000; Gaspar et al. 2003). In addition to the volcanic materials emitted, discoloration of the water due to gases from the volcanic activity itself was evident, discoloring to different shades of brown, red and green (Fig. 5).

Fig. 5
A photograph of the submarine volcano depicts the discoloration of the water to different shades.

Photograph taken at the source of the submarine volcano by INVOLCAN scientific staff. INVOLCAN SCIENTIFIC STAFF

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On November 8, 2011 the largest earthquake associated with the eruption of La Restinga was recorded; 2 km from the north coast of the island, with a depth of 21 km and magnitude 4.6Ml (Ibáñez et al. 2012). Since November 24, the volcanic activity had a decrease, noting a drop in seismicity. The newly formed volcano was emitting pillow lavas at depth along with pyroclasts. From 13 February 2012 the tremor was decreasing in amplitude and volcanic activity was markedly reduced, indicating that the end of the eruption was approaching (Ibáñez et al. 2012). After several days without apparent volcanic activity, on March 5, 2012 the IGN reported the end of the eruption.

The IEO made a bathymetric sweep in which the geomorphology of the slope before and after the volcano emerged can be appreciated. The IEO vessel Ramón Margalef, in the last bathymetric campaign, established that the volcanic cone was 88 m below sea level and the volume of the emitted material was 145 × 106 m3 (Perez-Torrado et al. 2012a, b).

The peculiarities of the volcanic event of El Hierro are marked mainly by the volcanic products originated in it as well as by the migratory activity of the seismicity.

During this stage, the main concerns of the authorities were the explosiveness of the eruption as the volcanic cone grew and approached the water surface. The eruption may have been explosive (surtseian activity) and considered hydromagmatic due to the contact of magma with water. In the case that the eruption had emerged, it could have violently released gases and could have been explosive, so estimating the height of the new volcano with bathymetric studies was of vital importance.

1.3 Post Eruption

The eruption of the Tagoro volcano did not produce any fatalities despite the fact that it was located 1 km off the coast of the village of La Restinga. Seismicity lasted until the following years with high activity and deformation continued until 2013. A decrease in the amount of endogenous gases emitted into the atmosphere was observed. However, this fissural eruption could have had a greater impact on the population and their property if it had had an explosive activity.

2 The Geopark of El Hierro

2.1 Tourism in the Context of the Submarine Eruption

The submarine eruption of the Tagoro volcano had, as has been pointed out, important consequences on the Herreña economy and especially on tourism (Arístegui 2015). During the underwater eruption the main source of the island's economy suffered a severe blow. In December 2011, figures showed that 70% of businesses were closed due to the crisis (https://elpais.com/sociedad/2012/01/19/actualidad/1327003592_267319.html). The figures provided by the Canary Islands Institute of Statistics (ISTAC) prove this. Taking as a reference the year before the eruption, the occupancy rate data for this municipality in October 2010 was 14.5%, in the same month but a year later, when the eruption occurred, the occupancy rate fell to 0.7%. This led to the declaration of a state of social emergency in the three Herreño municipalities and even caused emigration to other islands. Although there were public voices from the locals saying that the volcano was not dangerous, fear and uncertainty set in and caused the unprecedented collapse of this economic activity on the island (https://elpais.com/sociedad/2012/01/19/actualidad/1327003592_267319.html).

This hard time was fostered, in part, by the media's lack of knowledge of this type of event. Parallel to the eruption, there was intense media pressure that caused a false level of alarm to be perceived. In addition, social networks were the focus of hoaxes for visitors, as falsely catastrophic situations were described. These events caused serious damage to the image of El Hierro and caused the tourists present to leave the island (López Moreno 2013). So much so, that the occupancy rate for the year 2011 suffered a slight decline of 1%, compared to the previous year. However, the crisis was even greater the following year, as this percentage fell from 18% in 2011 to 13% in 2012 (ISTAC). If we consider the activity rate for the municipality of El Pinar, to which La Restinga belongs, during the third quarter of 2011 it can be seen that it suffered a decrease of 3.78% compared to the previous quarter of the same year. This is the second largest decline in the activity rate only surpassed by the world crisis COVID-19 (ISTAC).

The underwater eruption caused diving and fishing activities to come to a standstill. The eruption caused sharp temperature gradients, acidification of the water, sulphur and iron concentration (Arístegui 2015). In this context, the Cabildo of El Hierro wanted to reactivate the tourism sector and when the risk traffic light went down to yellow, it wanted to install three viewpoints of geological interest for the observation of the eruption. This was intended to counteract the massive cancellations of bookings in the last months of 2011 (https://www.hosteltur.com/151880_hierro-promociona-fenomeno-volcanico-comom-nuevo-atractivo-turistico-html) through geotourism.

2.2 The Birth of the Geopark of El Hierro

When the eruption ended on March 5, 2012, a new era for tourism on the island began at the same time. The actions that were carried out had as a maxim to take advantage of this fact to relaunch the visit of people. It was then that the idea of turning the island into a geopark was born. This management figure is promoted by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and focuses on a sustained local development that is allowed to manage both cultural and natural heritage, with special emphasis on the volcanic geodiversity of the island (www.elhierrogeoparque.es).

The creation of the Geopark of El Hierro was included in the planning of medium and long term actions to boost the battered economy of El Hierro after the eruption of the submarine eruption. The actions were agreed in several areas such as the Council of Ministers of 28 October 2011, or orders PRE/293636/2011 and IET/460/2012 in which, among other issues, sectoral measures were established to support the promotion of tourism, industrial and business revitalization and the promotion of Information Technology and Communication in El Hierro. The aim of the official bodies was to achieve economic sustainability through the “El Hierro Geopark” brand with initiatives by both public and private business entities (Poch et al. 2015).

The Cabildo of El Hierro and the Ministry of Industry, Energy and Tourism agreed and developed a proposal for the candidacy of El Hierro to join the European Geoparks Network. This proposal was presented in Lesvos (Greece) from 3 to 14 September 2012 and organized by the University of the Aegean with the collaboration of the Global and European Geoparks Network (www.europapress.es/epsocial/responsables/noticia-herro-presienta-portugal-candidatura-entrar-red-europea-geoparques-20120919180113.html).

The process was long and the premises that had to be fulfilled were the following (https://proxecto.xeoparquecaboortegal.gal/es/que-es-que-es-un-geoparque/): That it was a geographically unified territory, with unique geological references and with a certain visual attraction and content for visitors. That there is an entity with management capacity in charge of carrying out an integral strategy related to the conservation of resources, research, education, tourism and economic and social development. That this strategy is in place even before the presentation of the candidacy, with actions to enhance the value of geology and measures to ensure maximum social participation, since geoparks should be built from the bottom up. That there are sufficient economic resources to support the action plan, which can then be carried out directly or with the collaboration of companies and organizations in the territory.

The administrations carried out different improvement works with the aim of responding to the premises to be fulfilled. Among others, the rehabilitation of the Cueva de la Pólvora, the recovery of the Bien de Interés Cultural Fuente de Isora, the opening of the volcanic tube of the Cueva de Guinea, the improvement of the visitor centre Árbol Garoé or the reopening of the Parque Cultural El Julan, with guided routes with ethnographic, historical and geological information. With all this, on 23 September 2014, more than two years after submitting its candidacy, El Hierro was declared a Geopark, thus becoming the first geopark in the Canary Islands. UNESCO took into account several aspects for its inclusion in the European and global network of geoparks, among which stand out that it was a Biosphere Reserve since 2000, that it has seven protected natural areas (Nature Reserves of Mencafete, Roques de Salmor, Tibataje, Protected Landscapes of Ventejís and Timijiraque, Rural Park of Frontera and Natural Monument of Las Playas) that represent 60% of the territory or the experience gained during the crisis of the submarine eruption (www.rtve.es/noticias/20140923/isla-hierro-declarada-geoparque-unesco/1016824.shtml).

2.3 A New Era for El Hierro

One of the main boosts for the economy of El Hierro after the submarine eruption was its declaration as a geopark and the promotion of volcanic geotourism on the island. The geopark area covers the entire emerged island and about 300 km2 of sea area around El Hierro. Thus, in addition to including the large landslides that are the protagonists in the definition of the geomorphology of El Hierro, the submarine eruption of the Tagoro volcano (Poch et al. 2015) is also included as one of the geozones of the geopark. All these initiatives and the passage of time have made diving regain strength through activities that have been developed for several decades, such as the Open Fotosub underwater photography contest. Likewise, from the public administrations, initiatives were carried out that also had to do in some way with the observation or study of underwater geological activity. For this reason, the Biosphere, Geology and Geopark Interpretation Centres were built (Poch et al. 2015).

3 Conclusion

The Tagoro eruption between 2011 and 2012 in El Hierro, being submarine in nature, did not cause major damage, although it could have been disastrous if it had originated on land. The only volcanic hazards that took place during the pre- and eruptive period were earthquakes of greater magnitude, however, they did not cause major damage to the population. The deformation of the terrain and the geochemical activity of the gases did not pose a problem for the citizens of the island. More scientific interest was aroused by the volcanic product never seen before-Restingolites, as well as the fact that it was the last volcanic eruption in the Canary Islands for 40 years and the first historical eruption in El Hierro.

On the other hand, the submarine eruption had a significant impact on the Herreña economy. The employment rate fell both for the year as a whole and during the month in which the eruption began. After the end of the eruption there was a turning point that changed the conception of the island and of tourism on El Hierro. In this context, the possibility of turning the island into a Geopark was raised. The attractiveness of its volcanic nature was an important factor, but the fact that the eruption took place below the surface was a major factor, as it added to the attraction it already had for diving activities prior to the eruption. The administrations at both regional and national level were coordinated so that the island ended up being part of the network of geoparks around the world, and this fact was ratified in 2014. In this way El Hierro became the first geopark in the Canary Islands, thus changing the image of the island forever.