Journal of Volcanology and Geothermal Research

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1 Journal of Volcanology and Geothermal Research 178 (2008) Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: Assessing the potential for future explosive activity from Teide Pico Viejo stratovolcanoes (Tenerife, Canary Islands) J. Martí, A.Geyer 1, J. Andujar, F. Teixidó, F. Costa Institute of Earth Sciences Jaume Almera, CSIC, Lluis Solés Sabarís s/n, Barcelona, Spain article info abstract Article history: Received 20 June 2007 Accepted 21 July 2008 Available online 27 July 2008 Keywords: Teide Pico Viejo stratovolcanoes explosive volcanism Tenerife hazard assessment Since the onset of their eruptive activity within the Cañadas caldera, about 180 ka ago, Teide Pico Viejo stratovolcanoes have mainly produced lava flow eruptions of basaltic to phonoltic magmas. The products from these eruptions partially fill the caldera, and the adjacent Icod and La Orotava valleys, to the north. Although less frequent, explosive eruptions have also occurred at these composite volcanoes. In order to assess the possible evolution Teide Pico Viejo stratovolcanoes and their potential for future explosive activity, we have analysed their recent volcanic history, assuming that similar episodes have the highest probability of occurrence in the near future. Explosive activity during the last years has been associated with the eruption of both, mafic (basalts, tephro phonolites) and felsic (phono tephrites and phonolites) magmas and has included strombolian, violent strombolian and sub-plinian magmatic eruptions,aswellasphreatomagmaticeruptionsofmafic magmas. Explosive eruptions have occurred both from central and flank vents, ranging in size from to 0.1 km 3 for the mafic eruptions and from 0.01 to b1 km 3 for the phonolitic ones. Comparison of the Teide Pico Viejo stratovolcanoes with the previous cycles of activity from the central complex reveals that all them follow a similar pattern in the petrological evolution but that there is a significant difference in the eruptive behaviour of these different periods of central volcanism on Tenerife. Pre-Teide central activity is mostly characterised by large-volume (1 N20 km 3, DRE) eruptions of phonolitic magmas while Teide Pico Viejo is dominated by effusive eruptions. These differences can be explained in terms of the different degree of evolution of Teide Pico Viejo compared to the preceding cycles and, consequently, in the different pre-eruptive conditions of the corresponding phonolitic magmas. A clear interaction between the basaltic and phonolitic systems is observed from the products of phonolitic eruptions, indicating that basaltic magmatism is the driving force of the phonolitic eruptive activity. The magmatic evolution of Teide Pico Viejo stratovolcanoes will continue in the future with a probably tendency to produce a major volume of phonolitic magmas, with an increasing explosive potential. Therefore, the explosive potential of Teide Pico Viejo cannot be neglected and should be considered in hazard assessment on Tenerife Elsevier B.V. All rights reserved. 1. Introduction Spatial and temporal distribution of recent volcanism on Tenerife demonstrates that the island is a highly active volcanic zone and that future eruptions may occur from many different vent sites on the island. Despite the high risk that even small volume eruptions might represent today for such a highly populated and touristic area (Blong, 1984; Tilling and Lipmann, 1993; Simkin et al., 2001), the quantification of eruption risk remains a challenging task yet to be accomplished. Our present understanding of the impact of volcanic eruptions on populated areas comes from the study of damages caused by recent Corresponding author. address: joan.marti@ija.csic.es (J. Martí). 1 Now at the Department of Earth Sciences, University of Bristol, UK. and historical eruptions. Unfortunately, the last eruption on Tenerife occurred nearly 100 years ago, a period of time excessively long for the human memory. This is particularly relevant in places such us Tenerife where the demographic expansion and the territorial occupation with new settlements and infrastructures, has experienced a vertiginous increase due to the economic progress derived from massive arrival of tourism during the last decades. The fact that volcanic eruptions are not as frequent as in other similar areas, such as Hawaii, Azores, or Reunion, does not help to perceive volcanism as a real threat. In fact, volcanic eruptions on Tenerife are separated by tens to hundreds of years, or even more than one thousand years in the case of the central volcanic complex (Carracedo et al., 2003, 2007). Moreover, historical eruptions all them correspond to relatively small basaltic eruptions that have produced cinder cones, reduced lapilli and ash deposits and lava flows several kilometers in length, causing a relatively low damage. However, the same type of eruptions would cause today a significantly higher impact, as the increase in /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.jvolgeores

2 530 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 1. Simplified geological and topographic map of Tenerife illustrating the general distribution of visible vents. DH: Diego Hernández; DRZ: Dorsal rift zone; G: Guajara; MB: Montaña Blanca; MG: Montaña Guaza; RDG: Roques de García; T: Teide volcano; PV: Pico Viejo volcano; SRZ: Santiago rift zone; SVZ: Southern volcanic zone. black symbols: mafic and intermediate vents; white symbols: felsic vents; stars: historic and sub-historic vents; circles: other vents. Thick black lines show the lines of sections given in Fig. 4. Coordinates refer to 20 km squares of the Spanish national grid (UTM projection). population and infrastructure has made Tenerife much more vulnerable. This is particularly dramatic if we consider a potential eruption from the Teide Pico Viejo stratovolcanoes, which could be much larger than any of the historical eruptions and could also involve relatively violent explosive episodes, as it is revealed by its most recent past eruptive history (Ablay and Martí, 2000). The quantification of explosive eruption risk scenarios in densely populated regions is a necessary task that should be undertaken in all active volcanic regions but, unfortunately, we are still far from satisfactorily achieving it in most cases. In some cases, hazard assessment is difficult to be carried out simply because the lack of knowledge of the past volcanic history. In other cases, however, volcanism is not perceived as a potential problem, being only regarded as an attraction for tourism or a source of economic benefit, thus hiding the need to conduct hazard assessment. Tenerife is not an exception to this general rule, although during the last years significant efforts have been made to improve our understanding of Tenerife volcanism (see e.g. Martí and Wolff, 2000 and references herein; Carracedo et al., 2003, 2007). The aim of this paper is to provide a first approach to the characterisation of the Teide Pico Viejo explosive volcanism. We analyse the recent volcanic activity of these twin stratovolcanoes, assuming that similar episodes have the highest probability of occurrence in the near future. We describe the main characteristics of the Teide Pico Viejo magmatic system and discuss its control on the eruptive behaviour of the volcanoes. The different explosive eruption types occurred on Tenerife in the recent past are identified and their potential for occurring in the future is discussed on the basis of the possible evolution of Teide Pico Viejo. 2. Geological background The geological evolution of Tenerife involves the construction of two main volcanic complexes (Figs. 1 and 2): a basaltic shield complex (N12 Ma to present, Abdel-Monem et al., 1972; Ancochea et al., 1990; Thirlwall et al., 2000); and, a central complex (b4 Ma to present, Fuster et al., 1968; Araña, 1971; Ancochea et al., 1990; Martí et al., 1994). The basaltic shield complex is mostly submerged and forms about the 90% of the volume of the island, continuing at present its subaerial construction through two rift zones (Santiago Rift Zone and Dorsal Rift Zone). The central complex (Fig. 3) comprises the Cañadas edifice (b4 Ma 0.18 Ma), a composite volcano characterised by abundant explosive eruptions of highly evolved phonolitic magmas, and the active Teide Pico Viejo twin stratovolcanoes (0.18 Ma to present). These last have evolved from basaltic to phonolitic and have mostly undergone effusive activity. The Cañadas caldera, in which the Teide Pico Viejo stratovolcanoes stand (Fig. 3), truncated the Cañadas edifice and was transformed by several vertical collapses, which were occasionally associated with lateral collapses of the volcano flanks (Martí et al., 1994, 1997; Martí and Gudmundsson, 2000). 3. Stratigraphy and volcanic evolution of Teide Pico Viejo stratovolcanoes The structure and volcanic stratigraphy of the Teide Pico Viejo stratovolcanoes were characterised by Ablay and Martí (2000), based on a detailed field and petrological study. We address the reader to that work for a more complete description of the stratigraphic evolution of Teide Pico Viejo. More recently, Carracedo et al. (2003,

3 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 2. Simplified volcano stratigraphy of Tenerife. Ages from Ancochea et al. (1990) and Martí et al. (1994). 2007) have provided the first group of isotopic ages from Teide Pico Viejo products, and we will use them in the following sections. Ablay and Martí (2000) interpreted the internal structure of the Teide Pico Viejo stratovolcanoes using borehole and geophysical data. They proposed that in the central and eastern parts of the Cañadas caldera the Teide Pico Viejo sequence is more than 500 m thick while in the western sector of Cañadas caldera it is less than 300 m. This geometry has recently been confirmed by magnetotelluric studies (Pous et al., 2002; Coppo et al., 2008). At the north-eastern end recent Teide Pico Viejo products have overspilled the easter Cañadas caldera rim at El Portillo, partially infilling the La Orotava valley (Fig. 3). Ablay and Martí (2000) defined the stratigraphy of the northern sector of Teide Pico Viejo using surface geological relationships, geomorphological observations, and data reported from water galleries in the Icod valley (Coello, 1973; Coello and Bravo, 1989). In the NW the Teide Pico Viejo products spill out of the Cañadas caldera into the Icod valley (Fig. 3). A maximum thickness of 680 m has been attributed to the Teide Pico Viejo products in the Icod valley, based on observations made in water galleries (Coello and Bravo, 1989). The location and geometry of the Icod valley headwall and the northern Cañadas caldera margin are not constrained, however, by these data, and still represent a matter of considerable debate (Bravo, 1962; Fuster et al., 1968; Araña, 1971; Coello, 1973; Martí et al., 1994; Carracedo, 1994; Watts and Masson, 1995; Martí et al., 1997; Ancochea et al., 1999; Cantagrel et al., 1999; Martí and Gudmundsson, 2000; Watts and Masson, 2001). The Teide Pico Viejo stratovolcanoes consist dominantly of mafic to intermediate products with volumetrically subordinated felsic products (Ridley, 1970, 1971; Ablay, 1997; Ablay and Martí, 2000). Fig. 3. Shaded relief of the Tenerife central volcanic complex.

4 532 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 4. Geological cross sections of the Teide Pico Viejo stratovolcanoes (simplified from Ablay and Martí, 2000). Felsic products, however, predominate in the recent output of the Teide Pico Viejo stratovolcanoes (Fig. 4). The thick sequence of mafic to intermediate products that forms the lower to middle part of the Teide Pico Viejo stratovolcanoes in the central part of the Cañadas caldera, is inferred to correspond with the products of Teide volcano (Ablay and Martí, 2000). In the eastern Cañadas caldera, mafic scorias and lavas intercalated with the products of Teide are attributed to local monogenetic centres. The identification of Teide lava units in boreholes drilled at the central and eastern sectors of the caldera (Ablay and Martí, 2000), at the same altitude of ~1700 m, suggests that the central/eastern caldera floor was flat along the length of the section (Fig. 4). The flat top to the present Cañadas caldera floor (Fig. 4) supports this. Products occupying the western Cañadas caldera are inferred to derive mainly from Pico Viejo volcano (Ablay and Martí, 2000). Pico Viejo is interpreted to post-date the main part of the growth of Teide mafic stage (Fig. 4). An interesting aspect of the evolution of Teide and Pico Viejo stratovolcanoes is the configuration of their summit craters. In both cases, there is clear evidence of the existence of several summit collapses. Collapses affecting Teide volcano are interpreted to have truncated the summit and formed the paired scarps that are observable on its southern slope, while those affecting Pico Viejo formed and modified the summit caldera (Ablay and Martí, 2000). The outer pair of scarps on the Teide summit (Fig. 5) is interpreted as a graben-like subsidence of the summit on steeply inward-dipping faults corresponding to the exposed scarps. Following Ridley (1971) and Ablay and Martí (2000), two vertical collapse episodes are inferred to have affected the Pico Viejo summit. Material making up the southern block (Fig. 6) also underlies the summit caldera floor, suggesting that it has been down-faulted to form the present caldera. The sub-horizontal attitude of the pile of lavas forming the southern block and its unconformable relationship with previous deposits suggest that it is material that originally filled completely an earlier summit caldera. The older and younger summit calderas are interpreted as funnel shaped calderas, attributed to vertical collapse along steeply inward-dipping fractures. Eruptions at Teide and Pico Viejo stratovolcanoes have occurred from their central vents but also from a multitude of vents distributed radially around their flanks in three preferential directions: NE, NW, and S. Mafic and phonolitic magmas have been erupted indistinctively from these vents. In these directions the position and relative age of flank vents define several radial eruptive fissures on the slopes of the two volcanoes. The Santiago del Teide and Dorsal rift zones (Fig. 1), which are well expressed outside the Cañadas caldera, are inferred to be linked beneath the Cañadas caldera and Teide Pico Viejo stratovolcanoes (Carracedo, 1994; Ablay and Martí, 2000). Some flank vents at the western side of Pico Viejo are located on eruption fissures that are sub-parallel to fissures further down the Santiago rift, and define the main rift axis. On the eastern side of Teide some flank vents define eruption fissures orientated parallel to the upper Dorsal Ridge (Ablay and Martí, 2000). The location of the Teide and Pico Viejo stratovolcanoes is not interpreted to result only from the intersection of the two rift systems. On the contrary, it is believed that the location of the Icod valley headwall, assumed to lie on the north side of the Cañadas caldera, and the existence of intersecting, steeply-dipping ring fracture systems limiting the Cañadas caldera depressions, have played a major role in defining the position of the two stratocones (Martí et al., 1994; Martí

5 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 5. Picture of Teide taken from the east showing a south north profiles of the volcano. The two outer scarps that correspond to the southern borders of two former craters are indicated by arrows (crater 1 and crater 2). The present crater (crater 3) is also indicated. and Gudmundsson, 2000; Ablay and Martí, 2000). This idea is supported by gravimetric and magnetotelluric data (Ablay and Kearey, 2000; Araña et al., 2000; Pous et al., 2002; Coppo et al., 2008). The eruptive history of the Teide Pico Viejo stratovolcanoes is summarised in Fig. 7. The mafic to intermediate lava sequence in the central Cañadas caldera is interpreted to comprise the first products of Teide, erupted shortly after the formation of the last part of the Cañadas caldera. The products of these initial eruptions are intercalated with mafic scoria and lavas of monogenetic cones formed at the eastern side of the caldera. Mafic volcanism is inferred to have been continuous in the eastern Cañadas caldera throughout the growth of Teide volcano (Ablay and Martí, 2000). Pico Viejo is inferred to have developed as a satellite vent of Teide volcano during eruptions of intermediate and phonolitic products. Fig. 6. Picture of the Pico Viejo crater. The rest of lava plateau that can be observed at the southern margin (left) of the crater is dawn-faulted 150 m indicating vertical collapse during the formation of the present caldera. Ablay and Martí (2000) attribute this collapse to the decompression of the Pico Viejo magma chamber by lateral drainage during the Roques Blancos eruption from the northern flank of the volcano (see Fig. 3).

6 534 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 7. Summary of the relative stratigraphy of the Teide Pico Viejo stratovolcanoes, updated from Ablay and Martí (2000). Geochronological data from Carracedo et al (2003, 2007).

7 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Teide Pico Viejo magmatic system From what has been shown in the preceding section and considering the available petrological (Wolff, 1985, 1987; Wolff and Toney, 1993; Ablay, 1997; Ablay et al., 1998; Neumann et al., 1999; Simonsen et al., 2000; Thirlwall et al., 2000; Wolff et al., 2000; Zafrilla, 2001; Bryan et al., 2002; Triebold et al., 2006; Andujar, 2007), and geophysical (Watts et al., 1997; Canales and Danobeitia,1998; Dañobeitia and Canales, 2000) data, we propose an overall model of the behaviour of the magmatic system(s) driving volcanism on Tenerife (Fig. 8). This figure constitutes the base to understand the potential eruption scenarios that may be defined for Teide Pico Viejo stratovolcanoes. The Tenerife central complex (i.e. Canadas edifice and the Teide Pico Viejo stratovolcanoes) is characterised by products whose compositions range from basanites to phonolites., in contrast with volcanism in the basaltic shield that is mostly mafic. Products from subaerial basaltic volcanism show a wide range of compositions that suggest different source regions for the basaltic magmas and processes of crystal fractionation, assimilation and mixing, occurring at different sites in the interior of Tenerife and its underlying lithosphere (Neumann et al., 1999; Thirlwall et al., 2000). Some basaltic magmas have reached the surface nearly directly from their source region, without showing evidence of large-scale differentiation, while others clearly do. Moreover, the products of this magmatic differentiation and the different types of crustal and mantle xenoliths that they contain, suggest the existence of different levels at depth where some primary basaltic magmas temporarily arrest and evolve. Available geophysical data agree with the existence of significant mechanical discontinuities below Tenerife, mainly those at the mantle crust boundary and at the base of the volcanic pile (Watts et al., 1997; Dañobeitia and Canales, 2000; Canales et al., 2000), which may account for the storage and differentiation sites of basaltic magmas. Eruption of basaltic magmas on Tenerife has been mainly controlled by the Santiago del Teide and the Dorsal rift systems (Figs. 1 and 2), which have been active at least since the earliest subaerial volcanic episodes dated at N12 Ma (Abdel-Monem et al., 1972, Ancochea et al., 1990). In addition, basaltic volcanism is also responsible for the formation of several fields of monogenetic cones in areas far from the rift zones and at the interior of the Cañadas caldera such as the Southern volcanic zone (Fig. 1). In contrast, phonolitic activity is mostly restricted to the central complex, which started to grow about b4 Ma ago. Development of phonolitic magmatism is associated with the formation of shallow magma chambers at the interior of Tenerife, probably when the basaltic shield structure reached a height sufficient to stop the ascent of basaltic magmas through the centre of the island (Martí and Gudmundsson, 2000). The position of the phonolitic shallow magma chambers has changed significantly during the entire evolution of the central complex, as suggested by experimental petrology data (Andujar, 2007). Variations in the location of the magma chambers were probably induced by changes in the local stress field after each major caldera-forming episode (Martí and Gudmundsson, 2000). The sub-radial pattern of eruptive fissures associated with Teide and Pico Viejo felsic flank vents suggests the influence of the shallow, tumescent magma chambers beneath these volcanoes (Ablay and Martí, 2000). Petrological results support this, indicating that Teide Pico Viejo phonolites were stored, prior to eruption, at a shallow depth of ~1 2 km below sea-level (Ablay et al., 1998; Andujar, 2007). Petrologically most of the phonolitic eruptions from Teide Pico Viejo and Cañadas show signs of magma mixing, suggesting that eruptions were triggered by intrusion of deep basaltic magmas into shallow phonolitic reservoirs (Wolff, 1985; Araña et al., 1994; Triebold et al., 2006). On a few instances, basaltic eruptions shortly preceded (days to months??) phonolitic eruptions that show a genetic link and evidence for magma mixing (Araña et al., 1994). In these cases, basaltic activity may have thus been precursory to phonolitic activity. Geochronological data show that the time for renewal of phonolitic volcanism after each caldera collapse during the construction of the Fig. 8. Schematic representation of the Tenerife magmatic system (see text for more explanation).

8 536 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 9. Eruptions types occurred on Tenerife during the last 250 ka. Cañadas edifice is of about 200 ka (Martí et al., 1994; Martí and Gudmundsson, 2000). This result coincides with the time for phonolite melt generation and differentiation inferred from U-series isotope variations in the Teide Pico Viejo stratovolcanoes (Hawkesworth et al., 2000, 2001, 2004). Martí and Gudmundsson (2000) suggested that 200 ka is the time required to develop a new shallow magma chamber able to generate repeated eruptions of phonolitic magmas, after destruction of a previous chamber by caldera collapse. This interpretation is also supported by the occurrence of exclusively mafic volcanism in the central sector of Tenerife after each caldera collapse. In fact, when phonolitic volcanism is reinstalled again in the centre of the island, basaltic volcanism remains restricted to the periphery of the Cañadas caldera, thus suggesting the development of a 'shadow zone' beneath the caldera, caused by the phonolitic magma chambers. In summary, volcanism on Tenerife is fed by both the deep basaltic and the shallow phonolitic magmatic systems (Fig. 8). The basaltic system drives primitive and modified mafic magmas to the surface through the rift zones, but also through other secondary pathways to the south and north of the island. It also feeds the central zone of Tenerife where mafic magmas may assimilate and mix with magma stored at shallow depth and trigger phonolitic eruptions at the central complex. Occasionally, basaltic magmas can pass through the central system and reach the surface as flank eruptions or even as central eruptions, as can be observed from the old crater of Teide and the Pico Viejo caldera. 5. Teide Pico Viejo stratovolcanoes versus the Cañadas edifice In order to understand the recent eruptive history of Teide Pico Viejo stratovolcanoes and to define their possible future evolution, we have first investigated the explosive activity that occurred during the last 250 ka (Fig. 9). This is the minimum period that covers all the eruption types that can be distinguished in the geological record of the Tenerife central complex. During this period, explosive activity has been mostly associated with the eruption of phonolitic magmas, but it is also represented by strombolian and violent strombolian episodes during rift and flank basaltic eruptions and by a few phreatomagmatic mafic explosions from the summit crater of the twin stratovolcanoes. Phonolitic volcanism has been restricted to the central complex. Initially concerning only the Cañadas edifice phonolitic volcanism has appeared in more recent times also at Teide Pico Viejo stratovolcanos. A notable contrast exists in eruptive style between Teide Pico Viejo, which is mostly effusive and the Cañadas edifice, which is mostly explosive, although the composition of phonolitic magmas involved in both volcanic complexes is very similar (Andujar, 2007). Phonolitic pre-teide Pico Viejo volcanism, represented by the last episodes of the Diego Hernandez Formation (Edgar et al., 2007) from the Cañadas edifice, corresponds to explosive eruptions ranging in volume from 1 to N20 km 3 (Fig. 10). They have generated highly complex successions of plinian fall, surge and flow deposits and several of the eruptions produced widespread and internally complex ignimbrite sheets that were associated occasionally with caldera collapse (Martí et al., 1994; Bryan et al., 1998; Brown et al., 2003; Edgar et al., 2007). Phreatomagmatism occurred most frequently in the opening phase of the eruptions but also recurred repeatedly throughout many of the sequences. A periodicity of b5 to 30 ka characterises Diego Hernandez Formation phonolitic eruptions. Most of them were triggered by injection of mafic magma into existing phonolitic magma bodies. Age and volume estimates for the last pre-caldera phonolitic sequence

9 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 10. Minimum volume estimates of the phonolitic eruptions occurred during the last phonolitic pre-caldera cycle (data from Table 2, Edgar et al., 2007). Note that each largevolume eruption (Fasnia and Abrigo) is preceded by a number of small ones, being the second large eruption more voluminous than the first one. Black squares, eruptions with known (isotopic) age. White circles, eruptions with unknown age, placed in the diagram according to their relative stratigraphic position. (Edgar et al., 2007) indicate that a number of relatively smallervolume eruptions preceded each large one (Fig. 10). Phonolitic activity in the Teide Pico Viejo stratovolcanoes, which started to be built within the Cañadas caldera at about 180 ka ago, only began about b35 ka ago (Ablay, 1997; Ablay and Martí, 2000). It developed from the central vents and from vents on the flanks of the two stratovolcanoes, with a periodicity of around years, according to the isotopic ages published by Carracedo et al. (2003, 2007). Phonolitic eruptions from Teide and Pico Viejo range in size from 0.01 to 1 km 3 and have mostly generated thick lava flows and domes, some of them associated with minor explosive phases developing block and ash deposits and clastogenic lavas, and some subplinian eruptions, such as the Montaña Blanca (MB) at the eastern flank of Teide, 2000 years ago (Ablay et al., 1995). Recent basaltic eruptions have principally occurred along the NE SW and NW SE rift zones with a periodicity of years during the last millennium (Romero, 1989; Carracedo et al., 2003, 2007). They are relatively scarce at the interior of the caldera due to a shadow effect for mafic magma ascent imposed by the presence of shallow phonolitic reservoirs. However, some significant basaltic eruptions have occurred within the caldera, along the caldera floor, on the flanks or from the central vents of the Teide Pico Viejo stratovolcanoes. All basaltic eruptions have developed explosive strombolian to violent strombolian phases leading to the construction of cinder and scoria cones and occasionally producing intense fire fountaining and violent explosions with the formation of ash-rich eruption columns. Violent basaltic phreatomagmatic eruptions have also occurred from the central craters of the Teide Pico Viejo stratovolcanoes, generating high-energy pyroclastic surges, which are now visible on the northern flank of Teide volcano and around and inside the caldera crater of Pico Viejo. As it is indicated by the stratigraphy, petrology and structural constraints (Martí et al., 1994; Martí and Gudmundsson, 2000; Ablay and Martí, 2000)), the evolution of the Teide Pico Viejo stratovolcanoes cannot be separated from the rest of the Cañadas edifice, as opposed to the former idea that both volcanic complexes probably behaved independently (Fuster et al., 1968; Araña, 1971). In fact, the comparison between the upper part of the Cañadas edifice and the Teide Pico Viejo stratovolcanoes reveals important features that allow us to understand the most recent evolution of the central volcanic complex in Tenerife (Martí et al., 1994, 1997; Martí and Gudmundsson, 2000; Ablay and Martí, 2000). Towards the end of the main episode in the construction of the basaltic shield volcano (12 to b4 Ma) volcanic activity concentrated in the central part of Tenerife. It involved the formation of shallow magma chambers and the construction of a central volcanic complex through a series of cycles (Fig. 11) following a similar sequence of events. This includes: 1) the continuous ascent of mantle-derived basaltic magmas; 2) the formation of a discrete shallow phonolitic magma chamber that will favor a predominance of phonolitic eruptions and the existence of a shadow zone for basaltic eruptions in the central part of the island; 3) the caldera-forming event, that leads to the destruction of the volcanic edifice and the partial or total destruction (or cooling) of the associated shallow magma chamber; 4) the eruption of basaltic magmas in the central part of the island; 5) the formation of a new shallow magma chamber in a different location, which leads to, migration of the locus of phonolitic volcanic activity to other sectors of the central part of Tenerife (Martí and Gudmundsson, 2000). These results allow us to interpret the evolution of the post-caldera Teide Pico Viejo stratovolcanoes from a new perspective, and more specifically in terms of cyclicity and potential for future eruptions. The Teide Pico Viejo stratovolcanoes must be considered as representing of the last constructive episode in the sequence of events described above. This implies that the central volcanic complex has to be considered still active and probably approaching to a more active state, with an increase in the production of phonolitic magma. In fact, magma eruption rate for the period corresponding to the construction of Teide Pico Viejo stratovolcanoes, compared to the total for the subaerial evolution of Tenerife is higher (Ancochea et al., 1990). This does not support the idea that Teide Pico Viejo stratovolcanoes should not be regarded as active volcanoes, as has recently been suggested by Carracedo et al (2003, 2007). On the contrary, it indicates that depending on the deep magma supply and the structural stability of the volcanic edifices, Teide Pico Viejo stratovolcanoes will continue their eruptive activity. The fact that the volume of Teide Pico Viejo phonolites is considerably smaller than the corresponding to the previous phonolitic cycles only means that Teide Pico Viejo have just Fig. 11. Temporal and spatial evolution of the phonolitic eruptive cycles from the Tenerife central volcanic complex (from Martí and Gudmundsson, 2000). We include Teide Pico Viejo stratovolcanoes and the three preceding cycles from the Cañadas edifice, which were included by Martí et al. (1994) in the Upper Group and identified as three separated formations, Ucanca, Guajara and Diego Hernández.

10 538 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) started to produce evolved magmas and that they will become predominant in the future. 6. The recent volcanic history (last 35 ka) of the Teide Pico Viejo stratovolcanoes and its explosive activity If we concentrate now on the study of the volcanic activity occurred on Tenerife during the last 35 ka, we will observe that the number of eruptions occurred during this period is rather important (Fig. 7 and Table 1). However, the total volume of magma erupted in this period (1.5 3km 3 )(Figs. 12 and 13) is small compared to the total volume of the central complex. Phonolitic magmas represent a 83% of that volume whereas the remainder consists of magmas of mafic compositions. Phonolitic eruptions have been less frequent but much more voluminous than mafic eruptions and their eruption rate is progressively increasing in the period considered (Fig. 13). All phonolitic activity has been concentrated at the central complex, from the central vents and flanks of the Teide Pico Viejo stratovolcanoes (Fig. 7). Petrological investigation of these phonolitic products (Ablay, 1997; Ablay et al., 1998; Triebold et al., 2006; Andujar, 2007) demonstrate that all them belong to the development of the same shallow magmatic system, which includes the establishment of several magma chambers and apophasis at depths of about 5 6 km below Teide Pico Viejo. The fact that the eruptions may occur centrally or from the flanks of the two stratovolcanoes depends on the position of the corresponding magma pockets and the particular stress distribution around them (Martí et al., 2006; Andujar, 2007), but does not indicate the existence of different shallow systems that could suggest independent eruptive behaviours. Basaltic eruptions during this period have also occurred from the Teide Pico Viejo Table 1 List of the eruptions occurred on Tenerife during the last 35,000 years, identified according to their age (from Carracedo et al., 2003, 2007) and/or total volume (DRE) (estimated from geological mapping) Age (ka) Volume (DRE, km 3 ) Chinyero Chahorra Pico Viejo (phreatic) 0.152? Garachico Arafo Fasnia Siete Fuentes Montaña Boca Cangrejo 0.350? Montaña Reventada Lavas Negras Roques Blancos Cieglo hoya de los ajos Los Hornitos 1.930? La Angostura 2.020? Montaña Blanca El Boquerón / La Angostura El Ciego volcano 2.600? Montaña de Chío 3.620? La Abejera (lower) La Abejera (upper) 5.17? Cuevas del Ratón 5.37? Montaña Liferfe Montaña Las Lajas 8? Montaña Negra Los Tomillos 8.220? El Portillo upper vent 11? El Portillo lower vent 12? Montaña del Blanco ? Southern early Pico Viejo Coladas ? Montaña Majua Northern early Pico Viejo Coladas ~0.130 Pahoe-hoe Pico Viejo Coladas 26 ~0.160 Northern Flank Pico Viejo basaltic eruptions ? Old Teide Coladas (Orotava valley) 31 ~0.120 Old Teide Coladas (San Marcos beach) 32 ~0.230 Old Teide phonolites ? stratovolcanoes as well as away from them, mainly through the active rift zones. Eruptive activity from Teide Pico Viejo stratovolcanoes and the rift zones during the last 35 ka has produced a number of different explosive eruptions, including strombolian, violent strombolian and sub-plinian magmatic eruptions, as well as phreatomagmatic eruptions of mafic magmas (Fig. 12). Strombolian eruptions (VEI 1 2) associated with mafic magmas usually correspond to small volume events ( km 3 of DRE) that lead to the formation of cinder and scoria cones, several tens of meters high with associated tephra dispersal, frequently accompanied by the emplacement of lava flows. Comparison with similar historical eruptions that have occurred outside the central Teide Pico Viejo stratovolcanoes reveals that these explosive episodes may generate ash columns several hundreds of meters high, which produce a relatively wide carpet of fine grained fallout material that is rapidly eroded out by wind and rainfall (Romero, 1989). Ballistic emplacement and accumulation of scoria and spatter from fire fountaining is also observed in these eruptions. Similar scoria and spatter deposits are also observed in some phonolitic eruptions from Teide and Pico Viejo, mostly characterised by the emplacement of thick lava flows. Violent strombolian eruptions (VEI 2 3) have also occurred at the Teide Pico Viejo stratovolcanoes during the period considered. In mafic eruptions the volume of extruded magma may be one order of magnitude larger than in the strombolian episodes. Ballistic emplacement of big bombs (N1 m across) several hundreds of meters far from the vent is common in these eruptions. The construction of a relatively large (N0.01 km 3 ) cinder and scoria cone characterise these eruptions, which may also include the emplacement of lava flows. The only historical eruption of these characteristics corresponds to the Chahorra eruption in 1798, and witness descriptions report a column of ash a few kilometers high during the first stages of the eruption (Romero, 1989). Accumulation of most of the scoria and spatter around the vents occurred through fire fountains that have been reported to have reached a height of a few hundreds of meters. Some phonolitic eruptions from Teide, such as the Roques Blancos (Fig. 3), comprise initial explosive episodes that may be also classified as violent strombolian. Their main characteristics include the generation of pumice cones with abundant pumice fragments up to 1 m in diameter and of a fallout deposit including lapilli to ash size fragments, extending for a few kilometers far from the vent and covering an area of several km 2. At least two sub-plinian, eruptions of phonolitic composition (VEI 3 4) have occurred from flank vents at Teide Pico Viejo stratovolcanoes during recent times. The best known is the 2020 bp Montaña Blanca eruption that occurred at the eastern flank of Teide and produced an eruption column of 10 km high from which a extend pumice fall deposit was emplaced towards the NE of the vent (Ablay et al., 1995; Folch and Felpeto, 2005). Similar deposits derived from a different eruption, from which the corresponding vent has not yet been identified, have been found at the north side of Teide. In both cases, no pyroclastic flow or surge deposits have been observed in the stratigraphic record. Three different phreatomagmatic (VEI 3) eruptions of mafic magmas have occurred at the Teide Pico Viejo stratovolcanoes during its more recent history. One of these eruptions occurred at the Pico Viejo pre-caldera crater and generated a widely dispersed ballistic breccia and a base surge deposit that covered most of its flanks. The other two phreatomagmatic episodes occurred at the older Teide craters and also generated base surges and ballistic breccias emplaced along the flanks of the volcano, one of them corresponding to the prominent Las Calvas sector at the northern flank of Teide (Perez- Torrado et al., 2004). Finally, a phreatic episode occurred inside the Pico Viejo caldera probably during the 1798 eruption (Ablay, 1997) generating a 150 m deep pit crater and a widespread ballistic breccia deposit.

11 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig.12. Cake diagrams showing the relative proportions of: a) number of eruptions of each type, b) total number of mafic and felsic eruptions, and 3) total erupted volume of mafic and felsic magmas, during the last 35,000 years on Tenerife. Although mainly-effusive phonolitic eruptions from Teide Pico basically develop domes and/or lavas flows with a wide range of thicknesses and extensions, in some cases these eruptions may generate an explosive scenario that has to be also considered here. It refers to the case in which the dome and/or the lava flow collapses gravitationally forming pyroclastic density currents that generate block and ash Fig. 13. Minimum estimate of the cumulative volumes (DRE, km 3 ) of felsic and mafic magmas erupted on Tenerife during the last 35,000 years. Data from Table 1. To construct the diagram we have only considered those eruptions from which age and volume are available. This implies that the numbers obtained are absolute minimums.

12 540 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) Fig. 14. Photographs of a clastogenic lava (left) and block and ash deposit (right) from Teide. Clastogenic lavas contain angular to rounded non-welded pumices (light color) and stretched welded pumice fragments (dark color). Blocks in the block and ash deposit correspond to phonolitic lavas and the matrix is a mixture of lapilli and ash of the same composition. deposits (Fig. 14). Some of these deposit can be recognised at the northern side of Teide Pico Viejo stratovolcanoes emplaced inside some of the main gullies. These deposits are always of relatively small volume compared to the volume of some lavas, and can travel distances of several kilometers. Thickness varies from less than 1 m to several metres depending on the paleotography on which they are emplaced. In some cases they grade dawn slope into debris flow deposits. In addition to the presence of block and ash deposits, it is worth mentioning that a large number of phonolitic lava flows from Teide and Pico Viejo really correspond to clastogenic lavas formed by agglutination and stretching of large juvenile fragments parallel to the flow direction. In some occasions these clastogenic lavas include abundant non-welded pumices, suggesting that they derive from explosive episodes (e.g.: fire fountaning) rather than from purely effusive eruptions. Despite these clastogenic lavas have not yet been characterised in detail and would deserve a much more accurate study, we claim that phonolitic explosive activity from Teide Pico Viejo is much more common than it has traditionally been considered. 7. Discussion and conclusions Apparently less frequent than effusive eruptions, explosive eruptions involving mafic and phonolitic magmas have also occurred during the most recent history of the Tenerife central complex. All the explosive eruption types that we have been able to distinguish from Teide and Pico Viejo during the last 35,000 years, would today cause a serious impact on the infrastructure and economy of the island. Indeed they would probably affect the air traffic, and some of the main energy and water lifelines. Therefore, a hazard assessment programme should be conducted on Tenerife in order to quantify the future potential volcanic risk and to propose mitigation strategies. From the analysis of past activity, it can be seen that even the smallest eruptions that are likely to occur on Tenerife (e.g., Strombolian eruptions from mafic magmas) would generate enough ash to impose a serious threat to the civil aviation, a basic need for the survival of the island. Eruptive activity from the central complex, a possibility that should not be ruled out at all, would however cause a more significant impact. The available data presented in the previous sections, suggest that the style and magma composition characterizing volcanic activity in Tenerife and from Teide Pico Viejo stratovolcanoes will continue to evolve in the future and that new eruptions should be expected. Moreover, the possibility for Teide Pico Viejo to evolve into a more complex phonolitic system, with a higher explosive potential, should not be discarded. This hypothesis is supported by the fact that the eruption rate during the last 200 ka on Tenerife has increased above the average for the whole subaerial period of activity of the island (Ancochea et al., 1990). This does not necessary mean that the production rate of magma from the mantle has proportionally increased too, but confirms that it is not currently decreasing. Also, the similarity in the evolution patterns shown by the previous cycles in the construction of the Tenerife central complex and that from Teide Pico Viejo suggests that the most recent cycle has just started to generate phonolites and that the production of these magmas is likely to increase in the future. The style of mafic explosive eruptions that have occurred on Tenerife during the last 35,000 years has been controlled by the characteristics of the erupting magmas. Mafic magmas show very little variation in their physical properties through time. Thus we can expect similar eruptions in the future. These eruptions may occur from the same vent sites as in the past, most probably from the rift zones and with a much lower probability within the caldera and on the flanks of Teide and Pico Viejo, or from central vents. Mafic eruptions from the central vents have had a phreatomagmatic character as a result of interaction of magma with a lake and/or a shallow aquifer formed either in the old craters from Teide or the Pico Viejo caldera. These conditions still prevail for Pico Viejo but have significantly changed at Teide, where the lavas and scoria fallout from the last phonolitic eruption have completely filled the old craters. We have already mentioned the marked differences between the Cañadas phonolitic eruptions and those from Teide Pico Viejo. This contrast in the eruptive behavior of phonolitic magmas must be considered in more detail if we want to assess the explosive potential of future eruptions from Teide Pico Viejo. New petrological data (Andujar, 2007) show that the Teide Pico Viejo stratovolcanoes and the Cañadas edifice are characterised by similar magmatic systems that include different magma chambers of different volumes, which coexist in time to feed minor and major eruptions. However, the Cañadas edifice mostly generated explosive activity, whereas the Teide Pico Viejo stratovolcanoes have mainly developed effusive activity. These differences in the eruptive behavior may reflect the different degree of petrological evolution of the two components of the central complex. (Ablay et al., 1998; Zafrilla, 2001; Andujar, 2007). The magmas erupted from Teide Pico Viejo are moderately evolved when compared with the highly evolved materials erupted from the pre-caldera Diego Hernandez formation (Ablay et al., 1998; Edgar et al., 2002, 2007). Thus, we suggest that the Teide Pico Viejo stratovolcanoes are currently in the initial phase of their magmatic evolution. Therefore, a longer time is required before the phonolitic magmas achieve the same degree of evolution that is typical of the pre-caldera phonolites in order to develop highly explosive eruptions. Experimental petrology data (Andujar, 2007) also demonstrate that the depths (minimum pressure) of the phonolitic magma chambers below Teide Pico Viejo and the Cañadas edifice are

13 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) different. In the case of the Cañadas edifice large magma chambers (minimum volumes between km 3, Edgar et al., 2007) were located at shallow depths (3 4.5 km). However, in the case of Teide Pico Viejo their magma chambers are smaller (one to two orders of magnitude less) and located at a minimum depth of 6 km. However, for the Montaña Blanca eruption, the only currently well-documented explosive phonolitic eruption from Teide Pico Viejo stratovolcanoes, the magma reservoir was located at a depth of 4.8 km (Andujar, 2007). Therefore, we suggest that the explosivity of the Tenerife central complex is related to the degree of evolution of the phonolitic system. Larger and shallower magma chambers tend to facilitate the explosive disruption of phonolitic magma. The size of the phonolitic magma chambers increases with time as it is suggested by the data for to the last pre-caldera cycle (Edgar et al., 2007) (Fig. 11). This is likely to facilitate buoyancy of the phonolitic magma and its accumulation at shallower levels. In the case of the Teide Pico Viejo stratovolcanoes, the incipient state of their evolution has not yet permitted the development of large volumes of phonolitic magma, which has remained stored at deeper levels with a limited explosive potential. This is further evidenced by the present production (eruption rate) of phonolitic magmas from the Teide Pico Viejo stratovolcanoes, which is one to two orders of magnitude smaller than that of the last pre-caldera phonolitic cycle (Figs. 12 and 13). We suggest that either 1) the conditions to generate phonolites at present are inadequate, or 2) that the generation of phonolites has still not achieved optimal conditions. The increase in the total eruption rate for Teide Pico Viejo stratovolcanoes compared with that of the previous volcanic episodes of Tenerife (Ancochea et al., 1990), as well as the progressive increase of eruption rate of phonolites during the last years (Fig. 13), seems to support the second hypothesis. Furthermore, Tenerife phonolites mainly derive from crystal fractionation and/or recycling of differentiated rocks by mafic magmas (Ablay et al., 1998; Wolff et al., 2000; Zafrilla, 2001; Andujar, 2007). The incipiently explosive character of some of the last phonolitic episodes from Teide Pico Viejo, such as Roques Blancos or even the Lavas Negras, and the existence of recent subplinian eruptions would confirm this tendency towards an increase in the explosive potential of Teide Pico Viejo phonolites. Phonolitic eruptions from the Teide Pico Viejo stratovolcanoes may occur indistinctively from central vents or from the flanks. In both cases, eruptions show similar volumes and eruption characteristics, so that we consider that the vent position does not control the eruptive style, but that it plays an important role in the distribution of the eruption products. Volcanic hazard assessment on Tenerife, requires to consider the intimate relation that exists between mafic and phonolitic systems. Petrological data suggest that mafic magmatism drives phonolitic eruptions at the Teide Pico Viejo stratovolcanoes, as well as it has done so for the older Cañadas edifice. Each mafic episode (eruption) may cause the accumulation of a certain amount of deep magma at shallow depths. After several eruptions, the volume of accumulated mafic magma may re-energise residual phonolitic magma or even to assimilate older syenites and to trigger a new phonolitic eruption. Thus a period of several hundred to thousand years is necessary between each phonolitic eruptions in contrast to a much shorter interval between basaltic eruptions. In fact, if we assume that the rift zones intersect and communicate below the central complex (Carracedo, 1994; Ablay and Martí, 2000; Carracedo et al., 2003, 2007), it is not inconsistent to consider that part of the magma driving each mafic eruption through the rift zones or their periphery, may remain arrested below the residual phonolitic chamber(s) We suggest that as a result of this recurrent dyking process, thermal energy increases systematically to the point where phonolitic magma may reach eruptive conditions. When these conditions are definitively achieved, a new injection of mafic magma below the central zone may trigger a phonolitic eruption. Indeed, because Teide Pico Viejo phonolites are subsaturated (Andujar, 2007), they cannot trigger an eruption by themselves (i.e. as in a closed system), as supported by the invariable presence of products of magma mixing in many of the Teide Pico Viejo phonolites. We have shown that the magmatic evolution of the Teide Pico Viejo stratovolcanoes will continue in the future with a likely tendency to produce a major volume of phonolitic magma with an increased explosive potential. Thus in the near future eruptive activity is expected to show the same characteristics as that of the last 35 ka albeit with a tendency for an increased explosivity of phonolitic magmas.. Therefore the necessity to undertake with no delay a valid integrated volcanic hazard assessment can be achieved on the basis of the available data on past eruptions. Under these circumstances, we can conduct hazard assessment based on the information we can collect from the recent activity, as no substantial differences should be expected for the near future. However, although the level of expected volcanic hazards has not changed on Tenerife as a result of new scientific knowledge, the associated risks to the population and the infrastructures have increased exponentially in the last decades with no compensating mitigation strategies. Acknowledgment This research has been funded by the EC EXPLORIS (EVR1-CT ) and MEC TEGETEIDE (CGL E) projects. JM is grateful for the MEC grant PR References Abdel-Monem, A., Watkins, N.D., Gast, P.W., Potassium Argon ages, volcanic stratigraphy and geomagnetic polarity history of the Canary Islands: Tenerife, La Palma and Hierro. 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