Assessing the potential for future explosive activity from ......Teide–Pico Viejo products have overspilled the easter Cañadas caldera rim at El Portillo, partially inﬁlling the

Journal of Volcanology and Geothermal Research 178 (2008) 529–542

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Journal of Volcanology and Geothermal Research

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Assessing the potential for future explosive activity from Teide–Pico Viejostratovolcanoes (Tenerife, Canary Islands)

J. Martí ⁎, A. Geyer 1, J. Andujar, F. Teixidó, F. CostaInstitute of Earth Sciences “Jaume Almera”, CSIC, Lluis Solés Sabarís s/n, 08028 Barcelona, Spain

⁎ Corresponding author.E-mail address: [email protected] (J. Martí).

1 Now at the Department of Earth Sciences, Universit

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.07.011

a b s t r a c t
a r t i c l e i n f o
Article history:
Since the onset of their eru Received 20 June 2007Accepted 21 July 2008Available online 27 July 2008
Keywords:Teide–Pico Viejo stratovolcanoesexplosive volcanismTenerifehazard assessment

ptive activity within the Cañadas caldera, about 180 ka ago, Teide–Pico Viejostratovolcanoes have mainly produced lava flow eruptions of basaltic to phonoltic magmas. The productsfrom 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 toassess the possible evolution Teide–Pico Viejo stratovolcanoes and their potential for future explosiveactivity, we have analysed their recent volcanic history, assuming that similar episodes have the highestprobability of occurrence in the near future. Explosive activity during the last 35000 years has beenassociated with the eruption of both, mafic (basalts, tephro–phonolites) and felsic (phono–tephrites andphonolites) magmas and has included strombolian, violent strombolian and sub-plinian magmaticeruptions, as well as phreatomagmatic eruptions of mafic magmas. Explosive eruptions have occurredboth from central and flank vents, ranging in size from 0.001 to 0.1 km3 for the mafic eruptions and from0.01 to b1 km3 for the phonolitic ones. Comparison of the Teide–Pico Viejo stratovolcanoes with theprevious cycles of activity from the central complex reveals that all them follow a similar pattern in thepetrological evolution but that there is a significant difference in the eruptive behaviour of these differentperiods of central volcanism on Tenerife. Pre-Teide central activity is mostly characterised by large-volume(1–N20 km3, DRE) eruptions of phonolitic magmas while Teide–Pico Viejo is dominated by effusiveeruptions. These differences can be explained in terms of the different degree of evolution of Teide–PicoViejo compared to the preceding cycles and, consequently, in the different pre-eruptive conditions of thecorresponding phonolitic magmas. A clear interaction between the basaltic and phonolitic systems isobserved from the products of phonolitic eruptions, indicating that basaltic magmatism is the driving forceof the phonolitic eruptive activity. The magmatic evolution of Teide–Pico Viejo stratovolcanoes willcontinue in the future with a probably tendency to produce a major volume of phonolitic magmas, with anincreasing explosive potential. Therefore, the explosive potential of Teide–Pico Viejo cannot be neglectedand should be considered in hazard assessment on Tenerife.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Spatial and temporal distribution of recent volcanism on Tenerifedemonstrates that the island is a highly active volcanic zone andthat future eruptionsmay occur frommany different vent sites on theisland. Despite the high risk that even small volume eruptionsmight represent today for such a highly populated and touristic area(Blong, 1984; Tilling and Lipmann, 1993; Simkin et al., 2001), thequantification of eruption risk remains a challenging task yet to beaccomplished.

Our present understanding of the impact of volcanic eruptions onpopulated areas comes from the study of damages caused by recent

y of Bristol, UK.

l rights reserved.

and historical eruptions. Unfortunately, the last eruption on Tenerifeoccurred nearly 100 years ago, a period of time excessively long for thehumanmemory. This is particularly relevant in places such us Tenerifewhere the demographic expansion and the territorial occupationwithnew settlements and infrastructures, has experienced a vertiginousincrease due to the economic progress derived frommassive arrival oftourism during the last decades. The fact that volcanic eruptions arenot as frequent as in other similar areas, such as Hawaii, Azores, orReunion, does not help to perceive volcanism as a real threat. In fact,volcanic eruptions on Tenerife are separated by tens to hundreds ofyears, or even more than one thousand years in the case of the centralvolcanic complex (Carracedo et al., 2003, 2007).

Moreover, historical eruptions all them correspond to relativelysmall basaltic eruptions that have produced cinder cones, reducedlapilli and ash deposits and lava flows several kilometers in length,causing a relatively low damage. However, the same type of eruptionswould cause today a significantly higher impact, as the increase in

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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: maficand 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. Coordinatesrefer to 20 km squares of the Spanish national grid (UTM projection).

530 J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) 529–542

population and infrastructure has made Tenerife much more vulner-able. This is particularly dramatic if we consider a potential eruptionfrom the Teide–Pico Viejo stratovolcanoes, which could be muchlarger than any of the historical eruptions and could also involverelatively violent explosive episodes, as it is revealed by its mostrecent past eruptive history (Ablay and Martí, 2000).

The quantification of explosive eruption risk scenarios in denselypopulated regions is a necessary task that should be undertaken in allactive volcanic regions but, unfortunately, we are still far from satis-factorily achieving it inmost cases. In some cases, hazard assessment isdifficult to be carried out simply because the lack of knowledge of thepast volcanic history. In other cases, however, volcanism is notperceived as a potential problem, being only regarded as an attractionfor tourism or a source of economic benefit, thus hiding the needto conduct hazard assessment. Tenerife is not an exception to thisgeneral rule, although during the last years significant efforts havebeen made to improve our understanding of Tenerife volcanism (seee.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 thecharacterisation of the Teide–Pico Viejo explosive volcanism. Weanalyse the recent volcanic activity of these twin stratovolcanoes,assuming that similar episodes have the highest probability ofoccurrence in the near future. We describe the main characteristicsof the Teide–Pico Viejomagmatic system and discuss its control on theeruptive behaviour of the volcanoes. The different explosive eruptiontypes occurred on Tenerife in the recent past are identified and theirpotential for occurring in the future is discussed on the basis of thepossible evolution of Teide–Pico Viejo.

2. Geological background

The geological evolution of Tenerife involves the construction oftwo 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 formsabout the 90% of the volume of the island, continuing at present itssubaerial construction through two rift zones (Santiago Rift Zone andDorsal Rift Zone). The central complex (Fig. 3) comprises the Cañadasedifice (b4 Ma–0.18 Ma), a composite volcano characterised byabundant explosive eruptions of highly evolved phonolitic magmas,and the active Teide–Pico Viejo twin stratovolcanoes (0.18 Ma topresent). These last have evolved from basaltic to phonolitic and havemostly undergone effusive activity. The Cañadas caldera, in which theTeide–Pico Viejo stratovolcanoes stand (Fig. 3), truncated the Cañadasedifice and was transformed by several vertical collapses, which wereoccasionally associated with lateral collapses of the volcano flanks(Martí et al., 1994, 1997; Martí and Gudmundsson, 2000).

3. Stratigraphy and volcanic evolution of Teide–PicoViejo stratovolcanoes

The structure and volcanic stratigraphy of the Teide–Pico Viejostratovolcanoes were characterised by Ablay and Martí (2000), basedon a detailed field and petrological study. We address the reader tothat work for a more complete description of the stratigraphicevolution of Teide–Pico Viejo. More recently, Carracedo et al. (2003,

Fig. 2. Simplified volcano–stratigraphy of Tenerife. Ages from Ancochea et al. (1990) and Martí et al. (1994).

531J. Martí et al. / Journal of Volcanology and Geothermal Research 178 (2008) 529–542

2007) have provided the first group of isotopic ages from Teide–PicoViejo products, and we will use them in the following sections.

Ablay and Martí (2000) interpreted the internal structure of theTeide–Pico Viejo stratovolcanoes using borehole and geophysical data.They proposed that in the central and eastern parts of the Cañadascaldera the Teide–Pico Viejo sequence is more than 500 m thick whilein the western sector of Cañadas caldera it is less than 300 m. Thisgeometry has recently been confirmed by magnetotelluric studies(Pous et al., 2002; Coppo et al., 2008). At the north-eastern end recentTeide–Pico Viejo products have overspilled the easter Cañadas calderarim at El Portillo, partially infilling the La Orotava valley (Fig. 3). Ablayand Martí (2000) defined the stratigraphy of the northern sector ofTeide–Pico Viejo using surface geological relationships, geomorpho-logical observations, and data reported fromwater galleries in the Icod

Fig. 3. Shaded relief of the Tener

valley (Coello, 1973; Coello and Bravo, 1989). In the NW the Teide–PicoViejo 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 inwater galleries (Coello and Bravo, 1989). The location and geometry ofthe Icod valley headwall and the northern Cañadas caldera margin arenot constrained, however, by these data, and still represent amatter ofconsiderable 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 maficto intermediate products with volumetrically subordinated felsicproducts (Ridley, 1970, 1971; Ablay, 1997; Ablay and Martí, 2000).

ife central volcanic complex.

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 theTeide–Pico Viejo stratovolcanoes (Fig. 4). The thick sequence of maficto intermediate products that forms the lower to middle part of theTeide–Pico Viejo stratovolcanoes in the central part of the Cañadascaldera, is inferred to correspond with the products of Teide volcano(Ablay and Martí, 2000). In the eastern Cañadas caldera, mafic scoriasand lavas intercalatedwith the products of Teide are attributed to localmonogenetic centres. The identification of Teide lava units in bore-holes drilled at the central and eastern sectors of the caldera (Ablayand Martí, 2000), at the same altitude of ~1700 m, suggests that thecentral/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 areinferred to derive mainly from Pico Viejo volcano (Ablay and Martí,2000). Pico Viejo is interpreted to post-date the main part of thegrowth of Teide mafic stage (Fig. 4).

An interesting aspect of the evolution of Teide and Pico Viejostratovolcanoes is the configuration of their summit craters. In bothcases, there is clear evidence of the existence of several summitcollapses. Collapses affecting Teide volcano are interpreted to havetruncated the summit and formed the paired scarps that areobservable on its southern slope, while those affecting Pico Viejoformed andmodified the summit caldera (Ablay andMartí, 2000). Theouter pair of scarps on the Teide summit (Fig. 5) is interpreted as agraben-like subsidence of the summit on steeply inward-dippingfaults corresponding to the exposed scarps. Following Ridley (1971)and Ablay and Martí (2000), two vertical collapse episodes areinferred to have affected the Pico Viejo summit. Material making upthe 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 southernblock and its unconformable relationship with previous depositssuggest that it is material that originally filled completely an earliersummit caldera. The older and younger summit calderas areinterpreted as funnel shaped calderas, attributed to vertical collapsealong steeply inward-dipping fractures.

Eruptions at Teide and Pico Viejo stratovolcanoes have occurredfrom their central vents but also from a multitude of vents distributedradially around their flanks in three preferential directions: NE, NW,and S. Mafic and phonolitic magmas have been erupted indistinctivelyfrom these vents. In these directions the position and relative age offlank vents define several radial eruptive fissures on the slopes of thetwo volcanoes.

The Santiago del Teide and Dorsal rift zones (Fig. 1), which are wellexpressed outside the Cañadas caldera, are inferred to be linked beneaththe Cañadas caldera and Teide–Pico Viejo stratovolcanoes (Carracedo,1994; Ablay and Martí, 2000). Some flank vents at the western side ofPico Viejo are located on eruption fissures that are sub-parallel tofissures further down the Santiago rift, and define themain rift axis. Onthe eastern side of Teide some flank vents define eruption fissuresorientated parallel to the upper Dorsal Ridge (Ablay and Martí, 2000).

The location of the Teide and Pico Viejo stratovolcanoes is notinterpreted to result only from the intersection of the two rift systems.On the contrary, it is believed that the location of the Icod valleyheadwall, assumed to lie on the north side of the Cañadas caldera, andthe existence of intersecting, steeply-dipping ring fracture systemslimiting the Cañadas caldera depressions, have played a major role indefining the position of the two stratocones (Martí et al., 1994; Martí

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 areindicated by arrows (crater 1 and crater 2). The present crater (crater 3) is also indicated.


and Gudmundsson, 2000; Ablay and Martí, 2000). This idea issupported by gravimetric andmagnetotelluric 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 issummarised in Fig. 7. The mafic to intermediate lava sequence in thecentral Cañadas caldera is interpreted to comprise the first products ofTeide, erupted shortly after the formation of the last part of the

Fig. 6. Picture of the Pico Viejo crater. The rest of lava plateau that can be observed at the soutthe formation of the present caldera. Ablay and Martí (2000) attribute this collapse to the dBlancos eruption from the northern flank of the volcano (see Fig. 3).

Cañadas caldera. The products of these initial eruptions are inter-calated with mafic scoria and lavas of monogenetic cones formed atthe eastern side of the caldera. Mafic volcanism is inferred to havebeen continuous in the eastern Cañadas caldera throughout thegrowth of Teide volcano (Ablay andMartí, 2000). Pico Viejo is inferredto have developed as a satellite vent of Teide volcano during eruptionsof intermediate and phonolitic products.

hern margin (left) of the crater is dawn-faulted 150 m indicating vertical collapse duringecompression of the Pico Viejo magma chamber by lateral drainage during the Roques

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).



4. Teide–Pico Viejo magmatic system

Fromwhat has been shown in the preceding section and consideringthe 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 andDanobeitia,1998;Dañobeitia andCanales, 2000) data,weproposeanoverallmodel of thebehaviourof themagmatic system(s)driving volcanism onTenerife (Fig. 8). This figure constitutes the base tounderstand the potential eruption scenarios that may be defined forTeide–Pico Viejo stratovolcanoes.

The Tenerife central complex (i.e. Canadas edifice and the Teide–PicoViejo stratovolcanoes) is characterised by products whose compositionsrange from basanites to phonolites., in contrast with volcanism in thebasaltic shield that is mostly mafic. Products from subaerial basalticvolcanism show a wide range of compositions that suggest differentsource regions for the basaltic magmas and processes of crystalfractionation, assimilation and mixing, occurring at different sites inthe interior of Tenerife and its underlying lithosphere (Neumann et al.,1999; Thirlwall et al., 2000). Some basaltic magmas have reached thesurface nearly directly from their source region, without showingevidence of large-scale differentiation, while others clearly do. More-over, the products of this magmatic differentiation and the differenttypes of crustal and mantle xenoliths that they contain, suggest theexistence of different levels at depth where some primary basalticmagmas temporarily arrest and evolve. Available geophysical data agreewith the existence of significant mechanical discontinuities belowTenerife, mainly those at the mantle–crust boundary and at the base ofthe volcanic pile (Watts et al., 1997; Dañobeitia and Canales, 2000;Canales et al., 2000), which may account for the storage anddifferentiation sites of basaltic magmas.

Eruption of basaltic magmas on Tenerife has been mainlycontrolled by the Santiago del Teide and the Dorsal rift systems

Fig. 8. Schematic representation of the Tenerife ma

(Figs. 1 and 2), which have been active at least since the earliestsubaerial volcanic episodes dated at N12 Ma (Abdel-Monem et al.,1972, Ancochea et al., 1990). In addition, basaltic volcanism is alsoresponsible for the formation of several fields of monogenetic cones inareas far from the rift zones and at the interior of the Cañadas calderasuch as the Southern volcanic zone (Fig. 1).

In contrast, phonolitic activity is mostly restricted to the centralcomplex, which started to grow about b4 Ma ago. Development ofphonolitic magmatism is associated with the formation of shallowmagma chambers at the interior of Tenerife, probably when thebasaltic shield structure reached a height sufficient to stop the ascentof basaltic magmas through the centre of the island (Martí andGudmundsson, 2000). The position of the phonolitic shallow magmachambers has changed significantly during the entire evolution of thecentral complex, as suggested by experimental petrology data(Andujar, 2007). Variations in the location of the magma chamberswere probably induced by changes in the local stress field after eachmajor caldera-forming episode (Martí and Gudmundsson, 2000). Thesub-radial pattern of eruptive fissures associated with Teide and PicoViejo felsic flank vents suggests the influence of the shallow,tumescent magma chambers beneath these volcanoes (Ablay andMartí, 2000). Petrological results support this, indicating that Teide–Pico Viejo phonolites were stored, prior to eruption, at a shallow depthof ~1–2 km below sea-level (Ablay et al., 1998; Andujar, 2007).

Petrologically most of the phonolitic eruptions from Teide–PicoViejo and Cañadas show signs of magma mixing, suggesting thateruptions were triggered by intrusion of deep basaltic magmas intoshallow phonolitic reservoirs (Wolff, 1985; Araña et al., 1994; Trieboldet al., 2006). On a few instances, basaltic eruptions shortly preceded(days to months ??) phonolitic eruptions that show a genetic link andevidence for magmamixing (Araña et al., 1994). In these cases, basalticactivity may have thus been precursory to phonolitic activity.

Geochronological data show that the time for renewal of phonoliticvolcanism after each caldera collapse during the construction of the

gmatic system (see text for more explanation).

Fig. 9. Eruptions types occurred on Tenerife during the last 250 ka.


Cañadas edifice is of about 200 ka (Martí et al., 1994; Martí andGudmundsson, 2000). This result coincides with the time for phonolitemelt generation and differentiation inferred from U-series isotopevariations in the Teide–Pico Viejo stratovolcanoes (Hawkesworth et al.,2000, 2001, 2004). Martí and Gudmundsson (2000) suggested that200 ka is the time required to develop a new shallow magma chamberable to generate repeated eruptions of phonolitic magmas, afterdestruction of a previous chamber by caldera collapse. This interpreta-tion is also supported by the occurrence of exclusively mafic volcanismin the central sector of Tenerife after each caldera collapse. In fact, whenphonolitic volcanism is reinstalled again in the centre of the island,basaltic volcanism remains restricted to the periphery of the Cañadascaldera, thus suggesting the development of a 'shadow zone' beneaththe caldera, caused by the phonolitic magma chambers.

In summary, volcanism on Tenerife is fed by both the deep basalticand the shallow phonolitic magmatic systems (Fig. 8). The basalticsystem drives primitive and modified mafic magmas to the surfacethrough the rift zones, but also through other secondary pathways to thesouth and north of the island. It also feeds the central zone of Tenerifewhere mafic magmas may assimilate and mix with magma stored atshallow depth and trigger phonolitic eruptions at the central complex.Occasionally, basaltic magmas can pass through the central system andreach the surface as flank eruptions or even as central eruptions, as canbe 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–PicoViejo stratovolcanoes and to define their possible future evolution, we

have first investigated the explosive activity that occurred during thelast 250 ka (Fig. 9). This is the minimum period that covers all theeruption types that can be distinguished in the geological record of theTenerife central complex. During this period, explosive activity hasbeen mostly associated with the eruption of phonolitic magmas, but itis also represented by strombolian and violent strombolian episodesduring rift and flank basaltic eruptions and by a few phreatomagmaticmafic 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 hasappeared 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 mostlyexplosive, although the composition of phonolitic magmas involved inboth volcanic complexes is very similar (Andujar, 2007). Phonoliticpre-Teide–Pico Viejo volcanism, represented by the last episodes ofthe Diego Hernandez Formation (Edgar et al., 2007) from the Cañadasedifice, corresponds to explosive eruptions ranging in volume from 1to N20 km3 (Fig. 10). They have generated highly complex successionsof plinian fall, surge and flow deposits and several of the eruptionsproduced widespread and internally complex ignimbrite sheets thatwere 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 phaseof the eruptions but also recurred repeatedly throughout many of thesequences. A periodicity of b5 to 30 ka characterises Diego HernandezFormation phonolitic eruptions. Most of them were triggered byinjection of mafic magma into existing phonolitic magma bodies. Ageand volume estimates for the last pre-caldera phonolitic sequence

Fig. 10.Minimum volume estimates of the phonolitic eruptions occurred during the lastphonolitic pre-caldera cycle (data from Table 2, Edgar et al., 2007). Note that each large-volume eruption (Fasnia and Abrigo) is preceded by a number of small ones, being thesecond large eruption more voluminous than the first one. Black squares, eruptionswith known (isotopic) age. White circles, eruptions with unknown age, placed in thediagram according to their relative stratigraphic position.

Fig. 11. Temporal and spatial evolution of the phonolitic eruptive cycles from theTenerife central volcanic complex (from Martí and Gudmundsson, 2000). We includeTeide–Pico Viejo stratovolcanoes and the three preceding cycles from the Cañadasedifice, which were included by Martí et al. (1994) in the Upper Group and identified asthree separated formations, Ucanca, Guajara and Diego Hernández.


(Edgar et al., 2007) indicate that a number of relatively smaller-volume eruptions preceded each large one (Fig. 10).

Phonolitic activity in the Teide–Pico Viejo stratovolcanoes, whichstarted to be built within the Cañadas caldera at about 180 ka ago, onlybegan about b35 ka ago (Ablay, 1997; Ablay and Martí, 2000). Itdeveloped from the central vents and from vents on the flanks of thetwo stratovolcanoes, with a periodicity of around 250–1000 years,according to the isotopic ages published by Carracedo et al. (2003,2007). Phonolitic eruptions from Teide and Pico Viejo range in sizefrom 0.01 to 1 km3 and have mostly generated thick lava flows anddomes, some of them associated with minor explosive phasesdeveloping block and ash deposits and clastogenic lavas, and somesubplinian eruptions, such as the Montaña Blanca (MB) at the easternflank 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 100–200 years duringthe last millennium (Romero,1989; Carracedo et al., 2003, 2007). Theyare relatively scarce at the interior of the caldera due to a shadoweffect for mafic magma ascent imposed by the presence of shallowphonolitic reservoirs. However, some significant basaltic eruptionshave occurred within the caldera, along the caldera floor, on the flanksor from the central vents of the Teide–Pico Viejo stratovolcanoes. Allbasaltic eruptions have developed explosive strombolian to violentstrombolian phases leading to the construction of cinder and scoriacones and occasionally producing intense fire fountaining and violentexplosions with the formation of ash-rich eruption columns. Violentbasaltic phreatomagmatic eruptions have also occurred from thecentral craters of the Teide–Pico Viejo stratovolcanoes, generatinghigh-energy pyroclastic surges, which are now visible on the northernflank of Teide volcano and around and inside the caldera crater of PicoViejo.

As it is indicated by the stratigraphy, petrology and structuralconstraints (Martí et al., 1994; Martí and Gudmundsson, 2000; Ablayand Martí, 2000)), the evolution of the Teide–Pico Viejo stratovolca-noes cannot be separated from the rest of the Cañadas edifice, asopposed to the former idea that both volcanic complexes probablybehaved independently (Fuster et al., 1968; Araña, 1971). In fact, thecomparison between the upper part of the Cañadas edifice and theTeide–Pico Viejo stratovolcanoes reveals important features that allowus to understand the most recent evolution of the central volcaniccomplex 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) volcanicactivity concentrated in the central part of Tenerife. It involved theformation of shallow magma chambers and the construction of acentral volcanic complex through a series of cycles (Fig. 11) following asimilar sequence of events. This includes: 1) the continuous ascent ofmantle-derived basaltic magmas; 2) the formation of a discreteshallow phonolitic magma chamber that will favor a predominance ofphonolitic eruptions and the existence of a shadow zone for basalticeruptions in the central part of the island; 3) the caldera-formingevent, that leads to the destruction of the volcanic edifice and thepartial or total destruction (or cooling) of the associated shallowmagma chamber; 4) the eruption of basaltic magmas in the centralpart of the island; 5) the formation of a new shallow magma chamberin a different location, which leads to, migration of the locus ofphonolitic volcanic activity to other sectors of the central part ofTenerife (Martí and Gudmundsson, 2000).

These results allow us to interpret the evolution of the post-calderaTeide–Pico Viejo stratovolcanoes from a new perspective, and morespecifically in terms of cyclicity and potential for future eruptions. TheTeide–Pico Viejo stratovolcanoes must be considered as representingof the last constructive episode in the sequence of events describedabove. This implies that the central volcanic complex has to beconsidered still active and probably approaching to a more activestate, with an increase in the production of phonolitic magma. In fact,magma eruption rate for the period corresponding to the constructionof Teide–Pico Viejo stratovolcanoes, compared to the total for thesubaerial evolution of Tenerife is higher (Ancochea et al., 1990). Thisdoes not support the idea that Teide–Pico Viejo stratovolcanoesshould not be regarded as active volcanoes, as has recently beensuggested by Carracedo et al (2003, 2007). On the contrary, it indicatesthat depending on the deep magma supply and the structural stabilityof the volcanic edifices, Teide–Pico Viejo stratovolcanoes will continuetheir eruptive activity. The fact that the volume of Teide–Pico Viejophonolites is considerably smaller than the corresponding to theprevious phonolitic cycles only means that Teide–Pico Viejo have just


started to produce evolved magmas and that they will becomepredominant in the future.

6. The recent volcanic history (last 35 ka) of the Teide–Pico Viejostratovolcanoes and its explosive activity

If we concentrate now on the study of the volcanic activityoccurred on Tenerife during the last 35 ka, we will observe that thenumber of eruptions occurred during this period is rather important(Fig. 7 and Table 1). However, the total volume of magma erupted inthis period (1.5–3 km3) (Figs. 12 and 13) is small compared to the totalvolume of the central complex. Phonolitic magmas represent a 83% ofthat volume whereas the remainder consists of magmas of maficcompositions. Phonolitic eruptions have been less frequent but muchmore voluminous than mafic eruptions and their eruption rate isprogressively increasing in the period considered (Fig. 13). Allphonolitic activity has been concentrated at the central complex,from the central vents and flanks of the Teide–Pico Viejo stratovolca-noes (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 sameshallow magmatic system, which includes the establishment ofseveral magma chambers and apophasis at depths of about 5–6 kmbelow Teide–Pico Viejo. The fact that the eruptions may occurcentrally or from the flanks of the two stratovolcanoes depends onthe position of the corresponding magma pockets and the particularstress distribution around them (Martí et al., 2006; Andujar, 2007),but does not indicate the existence of different shallow systems thatcould suggest independent eruptive behaviours. Basaltic eruptionsduring this period have also occurred from the Teide–Pico Viejo

Table 1List of the eruptions occurred on Tenerife during the last 35,000 years, identifiedaccording to their age (from Carracedo et al., 2003, 2007) and/or total volume (DRE)(estimated from geological mapping)

Age (ka) Volume (DRE, km3)

Chinyero 0.041 0.006Chahorra 0.152 0.010Pico Viejo (phreatic) 0.152 ?Garachico 0.244 0.028Arafo 0.245 0.034Fasnia 0.245 0.003Siete Fuentes 0.245 0.0008Montaña Boca Cangrejo 0.350 ?Montaña Reventada 0.990 0.054Lavas Negras 1.240 0.425Roques Blancos 1.790 0.182Cieglo hoya de los ajos 1.850 0.030Los Hornitos 1.930 ?La Angostura 2.020 ?Montaña Blanca 2.020 0.038El Boquerón / La Angostura 2.420 0.077El Ciego volcano 2.600 ?Montaña de Chío 3.620 ?La Abejera (lower) 4.79 0.139La Abejera (upper) 5.17 ?Cuevas del Ratón 5.37 ?Montaña Liferfe 7.4 0.173Montaña Las Lajas 8 ?Montaña Negra–Los Tomillos 8.220 ?El Portillo upper vent 11 ?El Portillo lower vent 12 ?Montaña del Blanco 12.810 ?Southern early Pico Viejo Coladas 14.630 ?Montaña Majua 17 0.039Northern early Pico Viejo Coladas 17.570 ~0.130Pahoe-hoe Pico Viejo Coladas 26 ~0.160Northern Flank Pico Viejo basaltic eruptions 27.030 ?Old Teide Coladas (Orotava valley) 31 ~0.120Old Teide Coladas (San Marcos beach) 32 ~0.230Old Teide phonolites 32.360 ?

stratovolcanoes as well as away from them, mainly through the activerift zones.

Eruptive activity from Teide–Pico Viejo stratovolcanoes and the riftzones during the last 35 ka has produced a number of differentexplosive eruptions, including strombolian, violent strombolian andsub-plinian magmatic eruptions, as well as phreatomagmatic erup-tions of mafic magmas (Fig. 12).

Strombolian eruptions (VEI 1–2) associated with mafic magmasusually correspond to small volume events (0.001–0.01 km3 of DRE)that lead to the formation of cinder and scoria cones, several tens ofmeters high with associated tephra dispersal, frequently accompaniedby the emplacement of lava flows. Comparison with similar historicaleruptions that have occurred outside the central Teide–Pico Viejostratovolcanoes reveals that these explosive episodes may generateash columns several hundreds of meters high, which produce arelatively wide carpet of fine grained fallout material that is rapidlyeroded out bywind and rainfall (Romero,1989). Ballistic emplacementand accumulation of scoria and spatter from fire fountaining is alsoobserved in these eruptions. Similar scoria and spatter deposits arealso 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 theTeide–Pico Viejo stratovolcanoes during the period considered. Inmafic eruptions the volume of extruded magma may be one order ofmagnitude larger than in the strombolian episodes. Ballistic emplace-ment of big bombs (N1 m across) several hundreds of meters far fromthe vent is common in these eruptions. The construction of a relativelylarge (N0.01 km3) cinder and scoria cone characterise these eruptions,which may also include the emplacement of lava flows. The onlyhistorical eruption of these characteristics corresponds to theChahorra eruption in 1798, and witness descriptions report a columnof ash a few kilometers high during the first stages of the eruption(Romero,1989). Accumulation of most of the scoria and spatter aroundthe vents occurred through fire fountains that have been reported tohave reached a height of a few hundreds of meters. Some phonoliticeruptions from Teide, such as the Roques Blancos (Fig. 3), compriseinitial explosive episodes that may be also classified as violentstrombolian. Their main characteristics include the generation ofpumice cones with abundant pumice fragments up to 1 m in diameterand of a fallout deposit including lapilli to ash size fragments,extending for a few kilometers far from the vent and covering an areaof several km2.

At least two sub-plinian, eruptions of phonolitic composition (VEI3–4) have occurred from flank vents at Teide–Pico Viejo stratovolca-noes during recent times. The best known is the 2020 bp MontañaBlanca eruption that occurred at the eastern flank of Teide andproduced an eruption column of 10 km high from which a extendpumice fall deposit was emplaced towards the NE of the vent (Ablayet al., 1995; Folch and Felpeto, 2005). Similar deposits derived from adifferent eruption, from which the corresponding vent has not yetbeen identified, have been found at the north side of Teide. In bothcases, no pyroclastic flow or surge deposits have been observed in thestratigraphic record.

Three different phreatomagmatic (VEI 3) eruptions of maficmagmas have occurred at the Teide–Pico Viejo stratovolcanoes duringits more recent history. One of these eruptions occurred at the Pico–Viejo pre-caldera crater and generated a widely dispersed ballisticbreccia and a base surge deposit that covered most of its flanks. Theother two phreatomagmatic episodes occurred at the older Teidecraters and also generated base surges and ballistic breccias emplacedalong the flanks of the volcano, one of them corresponding to theprominent Las Calvas sector at the northern flank of Teide (Perez-Torrado et al., 2004). Finally, a phreatic episode occurred inside thePico Viejo caldera probably during the 1798 eruption (Ablay, 1997)generating a 150 m deep pit crater and a widespread ballistic brecciadeposit.

Fig.12. Cake diagrams showing the relative proportions of: a) number of eruptions of each type, b) total number ofmafic and felsic eruptions, and 3) total erupted volume ofmafic andfelsic magmas, during the last 35,000 years on Tenerife.


Although mainly-effusive phonolitic eruptions from Teide–Picobasically develop domes and/or lavas flows with a wide range of thick-nesses and extensions, in some cases these eruptions may generate an

Fig. 13. Minimum estimate of the cumulative volumes (DRE, km3) of felsic and mafic magmdiagram we have only considered those eruptions from which age and volume are available

explosive scenario that has to be also considered here. It refers to thecase in which the dome and/or the lava flow collapses gravitationallyforming pyroclastic density currents that generate block and ash

as erupted on Tenerife during the last 35,000 years. Data from Table 1. To construct the. This implies that the numbers obtained are absolute minimums.

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) andstretched 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 samecomposition.


deposits (Fig. 14). Some of these deposit can be recognised at thenorthern side of Teide–PicoViejo stratovolcanoes emplaced inside someof themain gullies. These deposits are always of relatively small volumecompared to the volume of some lavas, and can travel distances ofseveral kilometers. Thickness varies from less than1m to severalmetresdepending on the paleotography on which they are emplaced. In somecases they grade dawn slope into debris flow deposits.

In addition to the presence of block and ash deposits, it is worthmentioning that a large number of phonolitic lava flows from Teideand Pico Viejo really correspond to clastogenic lavas formed byagglutination and stretching of large juvenile fragments parallel to theflow direction. In some occasions these clastogenic lavas includeabundant non-welded pumices, suggesting that they derive fromexplosive episodes (e.g.: fire fountaning) rather than from purelyeffusive eruptions. Despite these clastogenic lavas have not yet beencharacterised in detail and would deserve a much more accuratestudy, we claim that phonolitic explosive activity from Teide–PicoViejo is muchmore common than it has traditionally been considered.

7. Discussion and conclusions

Apparently less frequent than effusive eruptions, explosive erup-tions involving mafic and phonolitic magmas have also occurredduring the most recent history of the Tenerife central complex. All theexplosive eruption types that we have been able to distinguish fromTeide and Pico Viejo during the last 35,000 years, would today cause aserious impact on the infrastructure and economy of the island.Indeed they would probably affect the air traffic, and some of themainenergy and water lifelines. Therefore, a hazard assessment pro-gramme should be conducted on Tenerife in order to quantify thefuture potential volcanic risk and to propose mitigation strategies.From the analysis of past activity, it can be seen that even the smallesteruptions that are likely to occur on Tenerife (e.g., Strombolianeruptions frommafic magmas) would generate enough ash to imposea serious threat to the civil aviation, a basic need for the survival of theisland. Eruptive activity from the central complex, a possibility thatshould not be ruled out at all, would however cause a more significantimpact.

The available data presented in the previous sections, suggest thatthe style and magma composition characterizing volcanic activity inTenerife and from Teide–Pico Viejo stratovolcanoes will continue toevolve in the future and that new eruptions should be expected.Moreover, the possibility for Teide–Pico Viejo to evolve into a morecomplex phonolitic system, with a higher explosive potential, shouldnot be discarded. This hypothesis is supported by the fact that theeruption rate during the last 200 ka on Tenerife has increased abovethe average for the whole subaerial period of activity of the island

(Ancochea et al., 1990). This does not necessary mean that theproduction rate of magma from the mantle has proportionallyincreased too, but confirms that it is not currently decreasing. Also,the similarity in the evolution patterns shown by the previous cyclesin the construction of the Tenerife central complex and that fromTeide–Pico Viejo suggests that themost recent cycle has just started togenerate phonolites and that the production of these magmas is likelyto increase in the future.

The style of mafic explosive eruptions that have occurred onTenerife during the last 35,000 years has been controlled by thecharacteristics of the erupting magmas. Mafic magmas show verylittle variation in their physical properties through time. Thus we canexpect similar eruptions in the future. These eruptionsmay occur fromthe same vent sites as in the past, most probably from the rift zonesand with a much lower probability within the caldera and on theflanks of Teide and Pico Viejo, or from central vents. Mafic eruptionsfrom the central vents have had a phreatomagmatic character as aresult of interaction of magma with a lake and/or a shallow aquiferformed either in the old craters from Teide or the Pico Viejo caldera.These conditions still prevail for Pico Viejo but have significantlychanged at Teide, where the lavas and scoria fallout from the lastphonolitic eruption have completely filled the old craters.

We have already mentioned the marked differences between theCañadas phonolitic eruptions and those from Teide–Pico Viejo. Thiscontrast in the eruptive behavior of phonolitic magmas must beconsidered in more detail if we want to assess the explosive potentialof future eruptions from Teide–Pico Viejo. New petrological data(Andujar, 2007) show that the Teide–Pico Viejo stratovolcanoes andthe Cañadas edifice are characterised by similar magmatic systemsthat include different magma chambers of different volumes, whichcoexist in time to feed minor and major eruptions. However, theCañadas edifice mostly generated explosive activity, whereas theTeide Pico Viejo stratovolcanoes have mainly developed effusiveactivity. These differences in the eruptive behavior may reflect thedifferent degree of petrological evolution of the two components ofthe central complex. (Ablay et al., 1998; Zafrilla, 2001; Andujar, 2007).The magmas erupted from Teide–Pico Viejo are moderately evolvedwhen compared with the highly evolved materials erupted from thepre-caldera Diego Hernandez formation (Ablay et al., 1998; Edgaret al., 2002, 2007). Thus, we suggest that the Teide–Pico Viejostratovolcanoes are currently in the initial phase of their magmaticevolution. Therefore, a longer time is required before the phonoliticmagmas achieve the same degree of evolution that is typical of thepre-caldera phonolites in order to develop highly explosive eruptions.

Experimental petrology data (Andujar, 2007) also demonstratethat the depths (minimum pressure) of the phonolitic magmachambers below Teide–Pico Viejo and the Cañadas edifice are


different. In the case of the Cañadas edifice large magma chambers(minimum volumes between 10–20 km3, Edgar et al., 2007) werelocated at shallow depths (3–4.5 km). However, in the case of Teide–Pico Viejo their magma chambers are smaller (one to two orders ofmagnitude less) and located at a minimum depth of 6 km. However,for the Montaña Blanca eruption, the only currently well-documentedexplosive 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 centralcomplex is related to the degree of evolution of the phonolitic system.Larger and shallower magma chambers tend to facilitate the explosivedisruption of phonolitic magma. The size of the phonolitic magmachambers increases with time as it is suggested by the data for to thelast pre-caldera cycle (Edgar et al., 2007) (Fig. 11). This is likely tofacilitate buoyancy of the phonolitic magma and its accumulation atshallower levels. In the case of the Teide–Pico Viejo stratovolcanoes,the incipient state of their evolution has not yet permitted thedevelopment of large volumes of phonolitic magma, which hasremained 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 lastpre-caldera phonolitic cycle (Figs. 12 and 13). We suggest that either1) the conditions to generate phonolites at present are inadequate, or2) that the generation of phonolites has still not achieved optimalconditions. The increase in the total eruption rate for Teide–Pico Viejostratovolcanoes compared with that of the previous volcanic episodesof Tenerife (Ancochea et al., 1990), as well as the progressive increaseof eruption rate of phonolites during the last 35000 years (Fig. 13),seems to support the second hypothesis. Furthermore, Tenerifephonolites mainly derive from crystal fractionation and/or recyclingof differentiated rocks bymaficmagmas (Ablay et al., 1998;Wolff et al.,2000; Zafrilla, 2001; Andujar, 2007). The incipiently explosivecharacter of some of the last phonolitic episodes from Teide–PicoViejo, such as Roques Blancos or even the Lavas Negras, and theexistence of recent subplinian eruptions would confirm this tendencytowards an increase in the explosive potential of Teide–Pico Viejophonolites. Phonolitic eruptions from the Teide–Pico Viejo stratovol-canoesmay occur indistinctively from central vents or from the flanks.In both cases, eruptions show similar volumes and eruption charac-teristics, so that we consider that the vent position does not controlthe eruptive style, but that it plays an important role in the dis-tribution of the eruption products.

Volcanic hazard assessment on Tenerife, requires to consider theintimate relation that exists between mafic and phonolitic systems.Petrological data suggest that mafic magmatism drives phonoliticeruptions at the Teide–Pico Viejo stratovolcanoes, as well as it hasdone so for the older Cañadas edifice. Each mafic episode (eruption)may cause the accumulation of a certain amount of deep magma atshallow depths. After several eruptions, the volume of accumulatedmafic magma may re-energise residual phonolitic magma or even toassimilate older syenites and to trigger a new phonolitic eruption.Thus a period of several hundred to thousand years is necessarybetween each phonolitic eruptions in contrast to a much shorterinterval between basaltic eruptions. In fact, if we assume that the riftzones 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 drivingeach mafic eruption through the rift zones or their periphery, mayremain arrested below the residual phonolitic chamber(s) We suggestthat as a result of this recurrent dyking process, thermal energyincreases systematically to the point where phonolitic magma mayreach eruptive conditions. When these conditions are definitivelyachieved, a new injection of mafic magma below the central zone maytrigger a phonolitic eruption. Indeed, because Teide–Pico Viejophonolites are subsaturated (Andujar, 2007), they cannot trigger an

eruption by themselves (i.e. as in a closed system), as supported by theinvariable presence of products of magma mixing in many of theTeide–Pico Viejo phonolites.

We have shown that the magmatic evolution of the Teide–PicoViejo stratovolcanoes will continue in the future with a likelytendency to produce a major volume of phonolitic magma with anincreased explosive potential. Thus in the near future eruptive activityis expected to show the same characteristics as that of the last 35 kaalbeit with a tendency for an increased explosivity of phonoliticmagmas.. Therefore the necessity to undertake with no delay a validintegrated volcanic hazard assessment can be achieved on the basis ofthe available data on past eruptions. Under these circumstances, wecan conduct hazard assessment based on the information we cancollect from the recent activity, as no substantial differences should beexpected for the near future. However, although the level of expectedvolcanic hazards has not changed on Tenerife as a result of newscientific knowledge, the associated risks to the population and theinfrastructures have increased exponentially in the last decades withno compensating mitigation strategies.

Acknowledgment

This research has been funded by the EC EXPLORIS (EVR1-CT-2002-40026) and MEC TEGETEIDE (CGL2004-21643-E) projects. JM isgrateful for the MEC grant PR-2006-0499.

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