Keywords

1 Formation and Large Units of the Relief Island

The Canary Islands originated more than 70 My ago from intraplate volcanism in the African Plate (Anguita and Hernán 2000). For decades their formation has caused numerous controversies on which the scientific community has not fully agreed. In general terms, they can be divided into two types of ideas: thermal and tectonic. The thermal ones are related to a mantle plume, also called “hot spot” (Anguita and Hernán 2000). The second idea states that tectonics plays a major role in the origin of the islands. These theories are the propagating fracture and uplifted block theories.

The hot spot or mantle plume theory extrapolates the model of creation of Hawaiian volcanism to Canary volcanism, that is, a magmatic flow that is injected from the mantle to the crust, forming the islands in the vertical of this focus (Sandoval-Velasquez et al. 2021). Among the thermal theories is the theory of the blob or “bubble” model, which is based on a series of magma droplets or bubbles that underlie the archipelago, which are injected into the lithosphere to produce volcanic events. This theory would explain the magmatic cycles that have occurred in the Canary Islands, as well as the geochemical diversity, which would have a simple explanation as a consequence of the heterogeneity of these bubbles (Anguita and Hernán 2000).

On the other hand, the propagating fracture theory links the formation of the islands to the Moroccan Atlas Mountains through a shear fault. The uplifted block theory argues that compressional tectonics led to crustal thickening, causing the uplift of the blocks that formed the islands (Sandoval-Velasquez et al. 2021). Finally, the unified model tried to integrate tectonic and thermal theories to explain the complexity of the formation of the Canary Islands (Sandoval-Velasquez et al. 2021).

El Hierro is the youngest island of the whole Archipelago, with an age of about 1.56 My, which corresponds to submarine construction, although the oldest subaerial rocks have been dated at 1.12 My (Gee et al. 2001). The formation of the island is considered to have occurred rapidly due to the intense processes that have taken place (Gómez Sáinz de Aja et al. 2010). One of the singularities of El Hierro is the clarity of massive lateral landslides and rift volcanism. These facts have resulted in a star-shaped island with structural axes every 120º. The first volcanic landmark was the Tiñor volcano, which was active between 1.2 and 0.88 My ago (Guillou et al. 1996; Barrera Morate and García Moral 2011). The rapidity of its formation produced instabilities in the platform that led to gravity sliding (Gómez Sáinz de Aja et al. 2010).

This was followed by a significant period of inactivity and then, between 0.54 and 0.17 My, a second stage of formation began in the Tiñor slide basin, corresponding to the eruptions of the El Golfo volcanic edifice. The materials ejected by this volcano completely buried the Tiñor landslide scar and a large part of the preceding volcanic edifice. At the end of this new stage, there were two volcanoes separated by an almost vertical landslide scarp (Carracedo et al. 2001).

Subsequently, rift volcanism began, a stage characterized by several events. The emission centers are grouped in the main structural axes of the island, which are more concentrated in the center and south, and are more dispersed on the east and west flanks. Between 0.5 and 0.3 My was the El Julan gravity slip (Carracedo et al. 2001), of which there is no evidence on the surface, but there is evidence on the ocean floor. Between 0.54 and 0.17 My ago, a new lateral collapse took place to the northeast, at San Andrés (Day et al. 1997), which was not completed and ended up generating a system of step faults. Next, between 0.17 and 0.14 My was the Las Playas gravity slide (Gee et al. 2001), which has the smallest volume of all. The largest landslide occurred at El Golfo, which is also the best example of a large-scale collapse (Gee et al. 2001). However, authors do not agree on the age, although some studies suggest that it could be between 0.013 and 0.017 My (Carracedo et al. 2001). After this mega-sliding, and also within the rift volcanism, there followed a stage of island formation characterized by recent volcanism that took place at various points in the El Golfo valley and filled in the escarpments of this valley. These episodes shaped the geography of the island as it is known today.

The last stage is limited to historical volcanism, represented by the eruption of the Tagoro volcano between 2011 and 2012. This is a submarine eruption that occurred SW of La Restinga and remained at just 88 m (Pérez-Torrado et al. 2012) from sea level. Therefore, volcanism on the island is still active. Both the ages of the different episodes of its formation, as well as the last volcanic landmark, suggest that the activity will continue in the future (Fig. 1).

Fig. 1
A geological map depicts the areas of recent volcanism, post landslide, rifts volcanism, El Golfo edifice, and Tinor edifice of Hierro island.

Source Grafcan, self-elaboration

Geological mapa of El Hierro Island.

Full size image

1.1 Building Tiñor (1.2–0.88 My)

The El Tiñor volcanic edifice, formed during 0.3 My (Gómez Sáinz de Aja et al. 2010), is the first phase of subaerial formation on the island of El Hierro. Thanks to K–Ar and magnetostratigraphic dating methods, its formation is estimated in the Lower-Middle Pleistocene (Barrera Morate and García Moral 2011). The Tiñor volcanic complex developed very rapidly, with three phases of formation with three well-differentiated phases of formation (Barrera Morate and García Moral 2011). Finally, the large volumes of material emitted and its great height, caused a destabilization of the flank and, consequently, a large gravitational slide exposing the first phases of formation of the Tiñor.

The first stage took place 1.12 My ago with the shield construction of the Tiñor edifice, where volcanic material covered large extensions (Guillou et al. 1996; Carracedo et al. 2008). It was represented by basaltic lava flows of not very great power, with brecciated appearance and divergent dikes of plagioclase basalts and pyroclastic deposits along the eruptive fissures (Barrera Morate and García Moral 2011). The first stage of formation outcrops at the bottom of the Tiñor, Honduras, Balón and Playecillas ravines. The main outcrops are in the NE sector of the Tiñor and in the Las Playas escarpment (lower-middle part). Narrow subvertical dykes intercalate thin lava flows with pyroclasts and brecciated levels. Several buried cones and pyroclast levels can be seen intercalated with the lavas (Barrera Morate and García Moral 2011). They are mainly basaltic, basaltic and tephritic flows. The basalts are plagioclastic-olivine-pyroxenic (Gómez Sáinz de Aja et al. 2010). One of the most abundant geomorphological units, but because they are submerged and one of the least common, are the pillow lavas (pillow-lavas) that can be seen in Timijiraque Bay. Another unit present in the lower section of Tiñor is formed by tephra cones and some buried cones with hydromagmatic and strombolian phases (Barrera Morate and García Moral 2011).

The second stage of formation of the Tiñor edifice occupies larger volumes than the previous stage. It is the intermediate or tabular stage that formed the San Andrés plateau thanks to lavas of great power and with subhorizontal dip in the center of the same building in formation (Guillou et al. 1996; Carracedo et al. 2008). This plateau is dated to between approximately 1.04 and 1.7 My (Barrera Morate and García Moral 2011). The San Andrés plateau formation section outcrops both in the El Toril escarpment and on the Dar slopes, as well as in the Tiñor and Honduras ravines. The materials of the tabular section outcrop in Tamaduste and Puerto de La Estaca and are mainly basaltic, basanitic and tephritic lava flows (Gómez Sáinz de Aja et al. 2010). The compositional typology of the materials includes pyroxenic-olivine basalts, olivine-pyroxenic basalts, olivine basalts, trachybasalts, tephras and plagioclase basalts. The “aa” lavas from these eruptions create a stepped relief with slag intercalations. Within the plateau unit, two basic intrusive bodies are recognized (Barrera Morate and García Moral 2011).

After the end of the intermediate growth stage, between 1.04 and 0.88 My, there was an eruptive lull that culminated with the last stage of formation of the volcanic complex (Barrera Morate and García Moral 2011). After this eruptive pause, the formation of the Ventejís section took place, known as the late period, with a more explosive character. Wide craters were formed, pyroclasts were deposited interspersed with flows with pyroxene nodules, giving rise to the presence of the Ventejís stratovolcano with a predominant direction of growth towards NE. The formative stage is shown by the alignment of the Picos-Rivera-Moles buildings. The main material emitted was pyroclasts with smaller amounts of lava flows than previously, forming a stratovolcano-type unit (Barrera Morate and García Moral 2011). The Ventejís-Pico-Moles group of volcanoes is composed of basaltic and tephritic lava flows, as well as basaltic tephra edifices with some hydromagmatic intercalations. These materials outcrop in the western and southeastern part of the town of Valverde (Gómez Sáinz de Aja et al. 2010) or as infill of between Tiñor and Tamaduste. The pyroclasts represented by bombs, slags, ashes and lapilli are olivine and tephritic basalts. Among the products from the late stage of the Tiñor formation, well-preserved volcanic cones with evidence of hydromagmatic interactions can be distinguished (Barrera Morate and García Moral 2011). Finally, this stage ends with the Tiñor megathrust 0.88 My ago.

The rapid and voluminous growth of such a small area of the island caused the destabilization of the flank that originated the first gravity slide of the island in the NW of the newly formed edifice.

The later formative processes of the island have covered a large part of the Tiñor building although, thanks to the landslides, some of the materials of the three preceding stages have been left uncovered.

At present, the maximum height of this geological unit is 1137 m asl. There are materials from each of the formation sections: early stage/lower section, middle/tabular section and Ventejís section (Gómez Sáinz de Aja et al. 2010).

1.2 The Building El Golfo—Las Playas (0.54–0.18 My)

It is considered that at this time the volcanism associated with this building took place after the partial collapse of the Tiñor Building, creating paleo-reliefs. Volcanic activity was mainly focused on the escarpment of the El Golfo arch to the north of the island and the midlands of the Las Playas arch to the southeast.

The outcrops of the units of this volcanic edifice are scarce as they have been covered by volcanic material from later events. The first section outcrops in the northern part of the island, in the Hoya del Verodal area and at the base of the Las Puntas cliff. As for the second section, it can be seen on the slopes of both escarpments, in El Golfo and Las Playas, resting discordantly on the materials of the lower section. In this way, it can be seen that this volcanism took place mainly on landslide escarpments. There is no clear evidence about the central area of this volcanic edifice, although with the study of the phyllonian network, there is the hypothesis that this area is close to the Cruz de los Reyes (Barrera Morate and García Moral 2011).

The orogenesis of this large volcanic edifice occurred during 0.35 My (0.54–0.18 My), in the Middle Pleistocene (Carracedo et al. 2001). The activity starts in the geological setting of the Tiñor edifice and has a NE-SW progression. All authors agree on the evolution of this volcanic edifice, although the age dating varies according to the area studied. After the volcanic inactivity of the Tiñor edifice, there was a reactivation that emitted basaltic and trachybasaltic lava flows and tephra cones discordant with the previous units. These types of emissions and units created are considered as the units of the lower section of the El Golfo—Las Playas edifice. Subsequently, the units of the upper section such as olivine-pyroxenic basaltic flows, olivine basalts, trachybasalts, trachytic flows and mafic trachytes were deposited concordantly (Barrera Morate and Barrera Morate). mafic trachytes (Barrera Morate and García Moral 2011).

The lower section begins with the deposition of volcanic material on the slopes of the Tiñor landslide, which corresponds to the basaltic and hydromagmatic tephra cones. In addition, hydromagmatic volcanism occurred in some areas such as Sabinosa, leaving these buildings in discordance with the units above it.

The second section outcrops on the slopes of the escarpments of El Golfo and Las Playas. Both sections are dominated by basaltic, trachybasaltic and tephritic lava flows, the second section being differentiated by the presence of tephritic basanites and intercalations of basaltic, trachybasaltic, basanitic and tephritic tephra cones. In addition, in the upper part of the second section there are more acidic, alkaline and more evolved flows, such as trachytes and mafic trachytes (Carracedo et al. 2001).

The petrological composition is quite broad, although it depends on the study area and its emission centres. In general, there is an increase in alkalinity over time, with the last lava flows appearing to the SE of the Las Playas escarpment, dated at about 0.176 My (Barrera Morate and García Moral 2011). There is undoubtedly agreement between the different stages of formation of the island with some distinct substages depending on the author, although the petrological evolution varies a little more (Fuster et al. 1993; Carracedo et al. 1997; Guillou et al. 1996).

1.3 The Volcanism of the Rifts (0.15–0.012 My)

The volcanism of the rifts occurred 0.15 and 0.012 My ago along the three structural axes of the island (Fig. 2). It corresponds to the last stages of construction of El Hierro and affects much of its entire surface. The magma thrust broke the earth's crust creating a triple fracture (Carracedo et al. 2008). These fractures extend along the three main structural axes. These axes have a West-Northwest, North-Southwest and South-Southeast orientation, where they converge and are arranged in a series of eruptions that have shaped the Herreño territory up to the present day.

Fig. 2
A structural map depicts the hydromagmatic emission center, magmatic emission center, dykes, fault, landslide, and rifts of Hierro island.

Source Grafcan. Self-elabortaion

Structuralmap of El Hierro.

Full size image

The activity of this last phase of formation began between 0.17 and 0.15 My ago. However, one of the most important processes related to this volcanism were the infill eruptions that occurred in the geographic setting of the El Golfo valley (Gómez Sáinz de Aja et al. 2010) and also in the aforementioned rift sectors, being a more moderate volcanism (Barrera Morate and García Moral 2011). The main characteristic of the volcanism in the interior of El Golfo is its location near the escarpment, as well as its duration, which is estimated to be no more than 10,000 years. The fact that there was little time for its formation is demonstrated by the fact that there are no important discordances or contrasts in its morphology (Barrera Morate and García Moral 2011). The eruptions that occurred during the Holocene of this phase led to the formation of lava deltas that gained ground to the sea (Barrera Morate and García Moral 2011) and caused the fossilization of many cliffs and the smoothing of the morphology of the coast.

In the rest of the Herreño territory where eruptions have occurred in this period, two stages can be distinguished: sub-recent and recent emissions (Barrera Morate and García Moral 2011). The former correspond to isolated eruptions in the structural axes of the three rifts. Some of the volcanoes belonging to this volcanism are the mountains Cueva del Guanche, Tomillar, Fara, Las Charquillas, Las Tabladas and Las Montañetas, among others (Barrera Morate and García Moral 2011). Recent emissions take place at the ends of the three rifts, in the western part (Punta del Verodal), in the northeast of the island (Hoya del Tamaduste) and in the south (La Restinga). All this activity meant, as mentioned above, an increase in the size of the island's perimeter due to the formation of the coastal platforms. The volcanoes that are part of these emissions are the Orchilla-Calderetón, Las Calcosas, Hoya del Verodal, Punta de la Dehesa, Chamuscada-Entremontañas, La Cancela and Aguajiro mountains, among many others (Barrera Morate and García Moral 2011).

During the historical volcanic activity there is only one eruption in 2011 and a supposed eruption in the year 1793, although not considered so far as a historical eruption (Romero Ruiz 1989). This volcanic eruption that may have taken place in the historical past, has been dated with the carbon 14 method, which establishes it in the year 1793—it is the so-called “Lomo Negro” eruption (Barrera Morate and García Moral 2011).

The only historical eruption on the island began on October 10, 2011 and lasted until March 2012. It was a submarine eruption that originated 400 m deep and 1.5 km SSW of the coast of La Restinga. The originated volcano was named “Tagoro”. Such volcanic eruption is the first of the twenty-first century in the Canary Islands, therefore, the study of the volcanic characteristics and its evolution are well documented (Barrera Morate and García Moral 2011; Pérez-Torrado et al. 2012; Domínguez Cerdeña et al. 2018; Melián et al. 2014; Pérez et al. 2012, 2014, 2015; Padrón et al. 2013; Carracedo et al. 2012; Rivera et al. 2013). Particularly interesting were the surface seawater discoloration phenomena and the presence of floating volcanic material. These are dark crusted pyroclasts, formed by basanites, of basaltic character and white cores, rich in silica and very porous (Rodríguez-Losada et al. 2014; Pérez-Torrado et al. 2012) of trachytic-riolitic character, finally named as “Restingolites”.

2 Conclusions

The island of El Hierro has gone through several formative stages over the last hundreds of thousands of years. These stages shaped the current relief, being interspersed between phases of formation and erosion to large dimensions. The first stage of the formation is known as the Tiñor volcanic edifice, composed of 3 formative sub-stages and a gravitational landslide that culminated its construction process. After a long period of inactivity, the island began a new stage of growth (El Golfo). The new volcanic materials were deposited in discordance with the previous materials on the escarpments of El Golfo and Las Playas.

Finally, the last stage of the island's formation encompasses rift volcanism. This is a process characterized by two types of events, on the one hand the volcanic activity itself produced in the three structural axes of the island, as well as in the valley of El Golfo, and on the other hand, three gravitational landslides (El Julan, Las Playas and El Golfo) that determined the current configuration of El Hierro.

As the volcanic activity in the Canary Islands is understood, being El Hierro one of the western islands, the youngest and adding the last eruption in La Restinga, it is confirmed that the process of growth of the island has not yet finished.