Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

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Journal of Volcanology and Geothermal Research

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Geological and geomechanical models of the pre-landslide volcanic edifice of Güímar and mega-landslides ()

J. Seisdedos a,d, M. Ferrer b, L.I. González de Vallejo a,c,d,⁎ a Prospección y Geotecnia S.L., Madrid, b Área de Investigación en Peligrosidad y Riesgos Geológicos, Instituto Geológico y Minero de España (IGME), Madrid, Spain c Departamento de Geodinámica, Universidad Complutense de Madrid (UCM), Madrid, Spain d Instituto Volcanológico de Canarias (INVOLCAN), , Tenerife, , Spain article info abstract

Article history: The geological and geomechanical characterizations of the volcanic rock mass succession affected by the Received 1 August 2011 Güímar and La Orotava mega-landslides (Tenerife, Canary Island) are presented here for the first time. The Accepted 8 June 2012 estimated subaerial volume of rocks mobilized by each of these landslides is in the range of 30–50 km3, Available online 17 June 2012 one of the largest known in the world. Field data, gallery surveys and borehole drilling have allowed the main types of materials and their structural arrangement to be identified. Based on these information a geo- Keywords: logical model of the pre-landslide volcanic edifice is proposed. A palaeo-morphological reconstruction has Volcanic landslides fi Geomechanical models been produced and possible slope angles and heights of the pre-landslide volcanic edi ce are presented. Güímar and La Orotava mega-landslides Five main lithological units have been identified in the emerged part of the edifice, three of them forming Tenerife the flanks and two forming the structural axis of the island. On the flanks, lava flows predominate with dif- Canary Islands ferent degrees of alteration and proportion of dikes increasing near the structural axis. The predominant ma- terials on the structural axis are pyroclastic deposits, lava flows and dikes. In the submarine edifice four main lithological units have been distinguished, formed by hyaloclastites, pillow-lavas, dikes and gravitational de- posits (slope and basin facies). The geomechanical characterization of these materials has been obtained from field data, boreholes, laboratory tests and literature review. The geological and geomechanical models obtained provide the fundamental basis for the explanation of the instability processes that generated the Güímar and La Orotava mega-landslides. © 2012 Elsevier B.V. All rights reserved.

1. Introduction et al., 2002; Acosta et al., 2003), Stromboli (Pasquaré et al., 1993; Kokelaar and Romagnoli, 1995; Tibaldi, 1996, 2001; Apuani et al., Many of the world's volcanoes have suffered large gravitational 2005a, 2005b), French Polynesia (Wolfe et al., 1994; Clément et al., landslides on their flanks during their geological history. These insta- 2002; Clouard and Bonneville, 2004), Cape Verde (Elsworth and bility processes are related to the rapid growth of the volcanic edifices Day, 1999; Masson et al., 2008), Ischia (Chiocci and Alteriis, 2006) and are part of their natural evolution. The occurrence of these pro- or Vulcano island (Tommasi et al., 2007). The flanks of volcanic edi- cesses on volcanic island flanks has been investigated in the Hawaiian fices may fail and slide in the last phases of their growth when they Ridge (Moore, 1964; Duffield et al., 1982; Lipman et al., 1988; Moore reach critical dimensions or when specific triggering factors occur, et al., 1989, 1994; Morgan et al., 2003), La Réunion (Chevallier and e.g. large explosive eruptions with associated seismicity (Voight and Bachelery, 1981; Duffield et al., 1982; Lénat et al. 1989; Labazuy, Elsworth, 1997; Tibaldi, 2001). When the materials from the mega- 1996; Ollier et al., 1998; Bret et al., 2003; Merle and Lénat, 2003; landslides fall into the sea, extensive debris avalanches are deposited Oehler et al., 2004, 2005), Lesser Antilles (Roobol et al., 1983; on the seabed. Boudon et al., 1984; Semet and Boudon, 1994; Mattioli et al., 1995; In the Canaries at least ten large palaeo-landslides have occurred Deplus et al., 2001), Ritter Island and Bismarck Ridge (Johnson, over the last 1.5 million years (Krastel et al., 2001; Masson et al., 1987), Tristan da Cunha (Holcomb and Searle, 1991), the Canary 2002; Acosta et al., 2003; Fig. 1). The landslides which caused the Islands (Holcomb and Searle, 1991; Watts and Masson, 1995; Güímar and La Orotava valleys, on the NE side of the island of Tenerife, Group, 1997; Urgeles et al., 1997, 1998; Krastel et al., 2001; Masson are two exceptional cases. The valleys are both “U”-shaped depressions, 10 km wide, bounded by impressive vertical scarps (500–600 m high) (Fig. 2). The landslides that generated these valleys, during the Pleisto- ⁎ Corresponding author at: Universidad Complutense de Madrid (UCM), Antonio – 3 Novais 2, 28040 Madrid, Spain. Tel.: +34 913502007; fax: +34 913503876. cene, mobilized a subaerial volume of rocks in the range of 30 50 km . E-mail address: [email protected] (L.I. González de Vallejo). However, although these landslides are among the largest in the world

0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.06.013 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 93

Fig. 1. Large landslide deposits on the submarine flanks of the Canary Islands. Debris avalanches in (Hi): El Golfo (1), El Julán (2), Las Playas I, II (3, 4); (LP): Santa Cruz (5), Playa de la Veta complex (6), Cumbre Nueva (7); Tenerife (TF): Icod (8), Roques de García (9), La Orotava (10), Güímar (11), Bandas del Sur (12), Anaga (13), Teno (14); Gran Canaria (GC): Roque Nublo (15), Horgazares (16), North west coast (17), North coast (18); Fuerteventura (Fu): East Canary Ridge (19), Jandia (20). Data from Krastel et al. (2001), Masson et al. (2002) and Acosta et al. (2003). The rectangle indicates the study area where the Güímar and La Orotava valleys are. (UTM 28N/WGS84/Meters). by volume, their causes and failure mechanisms are still not fully from generic theoretical data. Later, attention was focused on the understood. analysis of the condition of the materials forming the flanks (Reid et In the last years, engineering geological and rock mechanical al., 2001) and on the study of the physical and mechanical properties methods have been applied to the study of instability processes on of the volcanic materials using rock mechanic methods (Watters et the flanks of volcanic edifices with considerable success (Paul et al., al., 2000; Concha-Dimas, 2004; Zimbelman et al., 2004). More recent 1987; Elsworth and Voight, 1995; Iverson, 1995; Hürlimann, 1999; investigations have pointed out the importance of differentiating and Reid et al., 2001; Concha-Dimas, 2004; Zimbelman et al., 2004; characterizing specific lithological units, and establishing the struc- Apuani et al., 2005a, 2005b; Moon et al., 2005; Schiffman et al., tural distribution of the geological model (Okubo, 2004; Apuani 2006; Van Berlo, 2006, 2007; Thompson et al., 2008). The methods et al., 2005a, 2005b; Moon et al., 2005; Schiffman et al., 2006; Van and software for stability analysis (limit equilibrium or stress–strain Berlo, 2006; Del Potro and Hürlimann, 2008; Thompson et al., methods), traditionally used in slope stability analysis, provide rele- 2008). Table 1 shows a summary of the different geological models vant information on the influence of the factors involved in the insta- proposed by different authors and their most relevant characteristics. bility processes (conditioning and triggering factors) and the This paper presents the results of the geological and geo- characteristics of the failure mechanisms. For this type of analysis, mechanical investigations carried out on the volcanic rock mass suc- geological and geomechanical models representative for the pre- cession involved in the paleo-landslides of Güímar and La Orotava landslide volcanic edifices are needed. These models have to consider and the representative models obtained. These models are funda- the distribution, characteristics and geomechanical properties of the mental for the stability analysis of the flanks of the volcanic edifices lithological units of the volcano affected by the landslide, as well as and to determine the causes and failure mechanisms of these mega- its original geometry and hydrogeological conditions. Therefore the landslides. results of the subsequent stability analysis depend largely on the va- lidity and reliability of these models. 2. Geological setting The first models for the analysis of instability processes of volcano flanks were very simple (Paul et al., 1987; Elsworth and Voight, 1995; Tenerife is the largest island of the Canaries (2034 km2) and also Iverson, 1995; Hürlimann, 1999). They only distinguished a single where the maximum height is reached (3817 m, Teide volcano). Its lithological unit and their geomechanical properties were taken volcanic activity began in the late Miocene, around 12 Ma ago. The

Fig. 2. View from the NE of the La in Tenerife. This “U”-shaped valley, 10 km wide, that was generated by a large paleo-landslide occurred in the Pleistocene. The dashed line indicates the boundary of the valley marked by impressive scarps (500–600 m high). In the background is the Teide volcano (3718 m high). 94

Table 1 239 Research Geothermal and Volcanology of Journal / al. et Seisdedos J. Geological and geomechanical models of volcanic edifices in relation to the stability of their flanks including Tenerife geological model proposed in this paper.

Location Geological model Geomechanical properties Referencesb

Volcano Volcanic Continental Lithology Structure Hydrogeology Bibliographical Laboratory In Empirical Others or island island volcano references tests situ rock (e.g. geophysical Asingle Several lithological units Sub-aerial Submarine structural units namea tests mechanics investigations) lithological structural Sub-aerial Sub-marine criteria unit units

Differentiated Differentiated Differentiated Differentiated Structural Flanks Hyaloclastites Pillow Plutonic Water by the degree by the by the degree by the axis and lavas complex table of alteration composition of alteration composition gravitational deposits

SH X X X1 HX X XX 2 H,R (IV) X X XX 3 IV X X X XX 4 TX X XX 5 MR, MH X X X X XXX 6 MR X X X X X 7 SH, B,O X X X8 CX XX XX XXX 9 C X XX XX XXXX 10 HX X X XX XX X XXX 11 SX X X X XX X X XX 12 –

IV X X X X X X X X X 13 92 (2012) 240 PX X X X XX X X XX 14 HX XXXXXXXX X 15 HX XXX XXX XXXXX 16 TX XXXXXXXXXXXXXX 17 –

a Name of the volcano or island: SH) Mount St. Helens, H) Hawaii, R) Reunion, IV) Idealized volcano, T) Tenerife, MR) Mount Rainer, MH) Mount Hood, B) Bandai, O) Ontake, C) Citlaltepetl, S) Stromboli, P) La Palma. 110 b References: 1). Paul et al., 1987,2)Iverson, 1995,3)Elsworth and Voight, 1995,4)Voight and Elsworth, 1997,5)Hürlimann, 1999,6)Watters et al., 2000,7)Reid et al., 2001,8)Reid, 2004,9)Concha-Dimas, 2004, 10) Zimbelman et al., 2004, 11) Okubo, 2004, 12) Apuani et al., 2005a, 2005b, 13) Oehler et al., 2005, 14) Van Berlo, 2006, 15) Schiffman et al., 2006, 16) Thompson et al., 2008, 17) present research. J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 95 stratigraphy and chronological relations of the island have been ex- contrast to the mainly acid type volcanism of the Cañadas edifice tensively investigated by Fúster et al. (1968), Coello (1973), Abdel- (Ancochea et al., 1990, 1999). Monem et al. (1972), Ancochea et al. (1990, 1995, 1999) and Martí According to Cantagrel et al. (1999), the estimated age of the et al. (1994). The oldest emerged materials appear in the edifices of Güímar landslide is less than 0.84 Ma, and 0.54–0.69 Ma for La Roque del Conde (11.6–6.4 Ma), Anaga (8.0–3.2 Ma) and Teno (7.4– Orotava landslide. These ages were established using K/Ar techniques 4.5 Ma). They are formed mainly by basalts with some trachytes (Ibarrola et al., 1993; Ancochea et al., 1995, 1999). Alternatively, new and phonolites. The subsequent volcanic activity was focused mainly 40Ar/39Ar dating results for lava flows lying over the Güímar landslide on two large edifices, Cañadas and Dorsal edifices. The large Cañadas deposits, give an age of 1–1.1 Ma (Ferrer et al., 2008; Seisdedos, edifice, made up of trachybasalts, phonolites, trachytes and basalts, 2008). So that the Güímar landslide occurred more than 1–1.1 Ma was built between 3.5 Ma and 0.15 Ma. The Cañadas caldera had sev- ago, it is earlier than what was stated in previous studies. eral collapse phases, associated with large ignimbrite emissions. This Associated to the Güímar and La Orotava landslides there are the edifice was also affected by large landslides (Tigaiga>2.3 Ma, Roques debris avalanche deposits found in the interior of the galleries exca- de Garcia 0.6–0.7Ma and Icod>0.15 Ma; Cantagrel et al., 1999). The vated in the valleys (first described by Bravo, 1962). The origin of volcanic activity in the Dorsal edifice, situated in the eastern part of these breccias, made up of a sandy-clay matrix which includes rock this central caldera, began more than 1 Ma. The Güímar and La blocks of very different composition, size and shape, has been attrib- Orotava landslides mainly affected this edifice originating two large uted to the Güímar and La Orotava landslides (Navarro and Coello, depressions (valleys). The Cañadas caldera depression was later filled 1989). Fig. 4 shows a geological cross section along the Güímar valley. by the Teide volcano, which is still active. At the same time a series of The location of the landslide deposits that separate pre-landslide and small volcanoes erupted at scattered locations throughout the island post-landslide materials can be seen. (Fig. 3). A possible sketch of the geological evolution of the area where the Güímar and La Orotava valleys were originated is shown in Fig. 5.In 3. Güímar and La Orotava landslides the first stage (1) the Dorsal edifice is formed. This early stage of the Dorsal edifice is called “ edifice” (Navarro, 2005). In the sec- The Güímar and La Orotava valleys are both trough-shaped with a ond stage (2) the landslide that originated the Güímar valley took flat floor, around 10–11 km wide and long, open seawards and with place, affecting the “Arafo edifice”. (3) A subsequent phase of intense their bisector trending ESE and NNW respectively (Fig. 3). The volcanism of the Dorsal edifice filled the valley depression. The so- headwalls of these two valleys coincide in correspondence of the NE called “Cho-Marcial edifice” is formed in this stage (Navarro, 2005). Dorsal, the main volcanic rift zone of the island, also called structural (4) A new instability process took place later generating La Orotava axis. In this area the maximum heights of the valleys are reached, in- valley with the landslide that affected the Dorsal and the Cañadas ed- creasing to the SW between 1700 m and 2400 m. The impressive lat- ifices. Finally the depression of La Orotava was filled by materials eral scarps of both valleys, with a mean height of 500 m, reaching from new eruptions. 600 m at some points, are composed of pre-landslide rocks. They The presence of large masses of landslide (debris avalanche) de- are noticeably symmetrical with steep gradients (higher than 30°), posits lying on the sea bed surrounding the island of Tenerife has which contrast significantly with the gentle slopes of the valley floors, been considered as an unquestionable evidence of these large land- formed by the post-landslide lava flows which filled the depressions. slides (Holcomb and Searle, 1991; Watts and Masson, 1995; Teide The Güímar and La Orotava valleys affected the Dorsal edifice and, to Group, 1997; Krastel et al., 2001; Masson et al., 2002; Acosta et al., a lesser extent, the Cañadas edifice (just to the west of La Orotava 2003). According to Acosta et al. (2003) La Orotava submarine debris valley, Fig. 3). The Dorsal edifice presents a basaltic composition, in avalanche deposits cover an area of approximately 2200 km2,reaching

Fig. 3. Schematic geological map of Tenerife (modified from Ancochea et al., 2004). The rectangle indicates the study area. PS: Pleistocene, PO: Pliocene, MI: Miocene. P: Profile referred in Fig. 4. D: Outcrop of submarine materials, near the locality of Igueste de San Andrés, where three boreholes were carried out. (UTM 28N/WGS84/Meters). 96 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

Fig. 4. Schematic geological cross-section along the Güímar valley corresponding to the P profile (see Fig. 3). G: Cueva de las Colmenas gallery (see Section 4.2). as far as 75 km away from the coast, and the submarine deposits from the separation between the slip-planes of the subaerial and subma- the Güímar landslide occupy an area of 2600 km2 reaching a distance rine avalanches. of 85 km from the coast. The volume of these debris avalanches on the ocean floor has been estimated to be around 120 km3 in the case of 4. Geological and geomechanical characterizations of the volcanic the Güímar landslide and less than 500 km3 in the La Orotava landslide rock succession (Krastel et al., 2001; Masson et al., 2002). In contrast, the volume of these landslides estimated from the valley sizes is much lower, in the The geological and geomechanical characterizations of the volca- range of 30–50 km3. Acosta et al. (2003) propose that the distinct nic rock succession involved in the Güímar and La Orotava landslides difference in volume between the inshore and the offshore data is have been carried out from field data obtained in outcrops and galler- areflection of the manner in which failures take place along the ies. These data have been compared and completed with a compre- flanks of volcanoes. They infer that failures in the Canary Islands hensive literature review (Seisdedos, 2008). took the form described by Jacobs (1995) for the Nuuanu Debris Av- alanche in Oahu, Hawaii. Here failure was in two synchronous or 4.1. Outcrop characterization near synchronous avalanches, one onshore and another offshore. Acosta et al. (2003) further propose that the location of the surface Rock masses forming the lateral and head scarps of the Güímar separating these two slip surfaces was controlled by the geology of and La Orotava landslides have been characterized in areas where the flank of the volcanoes. They infer that the transition from subaer- pre-landslide volcanic rocks crop out. Because of the difficulty to ac- ial lavas above to the submarine facies below, a transition marked by cess most of these reliefs, field work has been carried out in represen- volcaniclastic rocks and glassy lavas (DePaolo et al., 2001), defines tative accessible areas (Fig. 6). In situ geomechanical data collection

Fig. 5. Geological evolution approach of Güímar and La Orotava valleys (modified from Navarro, 2004). 1: Early stage of the Dorsal edifice (“Arafo edifice”). 2: Güímar landslide. 3: Güímar landslide infill (“Cho-Marcial” edifice). 4: La Orotava landslide. J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 97

Fig. 6. Geological map of the study area with geomechanical data collection and observation points. and observation points (marked with an asterisk) can be grouped as In summary, from the field observations the following consider- follows: ations can be established: the more recent or superficial materials for- ming the flanks are mainly massive lava flows and lava flows with 1 Southern scarp of Güímar (points 1 to 8 and 1*). scoria layers (Fig. 7A). The presence of pyroclastic deposits and 2 Northern scarp of Güímar (point 2*). dikes increases towards the inner parts of the flanks and in the struc- 3 Eastern scarp of La Orotava (points 9 and 17 to 21). tural axis of the island (Fig. 7B). In these areas lava flows have higher 4 Western scarp of La Orotava (points 22 to 26 and 3*). degree of alteration (Fig. 7C, D). 5 Head scarps of Güímar and La Orotava (points 10 to 16). In order to carry out the geomechanical characterization of the rock masses, the geomechanical RMR classification (Bieniawski, Table 2 summarizes the lithologies at each point. 1989) has been applied in a first approximation. This classification

Table 2 Lithologies present in the study outcrops and observation points (marked with an asterisk) (see Fig. 6).

Point Lava Lava flows with Pyroclastic Dikes Point Lava Lava flows with Pyroclastic Dikes Point Lava Lava flows with Piyoclastic Dikes flows scoria layers deposits flows scoria layers deposits flows scoria layers deposits

1X X X2* X 19 X 2 X X10 X X20X 3X11X21X 4X 12 X22 X 5 X X 13 X 23 X 6X X X14X 24 X 7 X X 15 X X X 25 X 8X16X26X 9X 17X 3* X 1* X 18 X

X Predominant lithologies. * Observation points. 98 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

Fig. 7. Representative views of the volcanic materials observed in some of the outcrops (Fig. 6). A: Lava flows with scoria layers in the exterior parts of the flanks (point 22). B: Pyroclastic deposit succession in the upper part of the structural axis (point 15). C: Highly altered and fractured lava flows in the structural axis (point 16). D: Altered and fractured lava flows with dikes intrusion in the inner part of the flank (point 5).

Table 3 Relative weights of observational and laboratory-determined parameters used to calculate RMR (Bieniawski, 1989).

1. Intact rock strength Point-load Index (MPa) >10 10–44–22–1 Uniaxial compressive >250 250–100 100–50 50–25 25–55–1 b1 strength, UCS (MPa) Rating 15 12 7 4 2 1 0 2. RQD (%) 100–90 90–75 75–50 50–25 b25 Rating 20 17 13 8 3 3. Average discontinuity spacing (m) >2 2–0.6 0.6–0.2 0.2–0.06 b0.06 Rating 20 15 10 8 5 4. Discontinuity condition 4a. Length (persistence) b1m 1–3m 3–10 m 10–20 m >20 m Rating 6 4 2 1 0 4b. Separation (aperture) None b0.1 mm 0.1–1.0 mm 1–5mm >5mm Rating 6 5 3 1 0 4c. Roughness Very rough Rough Slightly rough Smooth Slickensided Rating 6 5 3 1 0 4d. Infilling (gouge) None Hard filling Hard filling Soft filling Soft filling >5 mm b5mm >5 mm Rating 6 4 2 2 0 4e. Weathering Unweathered Slightly weathered Moder. Weathered Highly altered Decomposed Rating 6 5 3 1 0 5. Groundwater Inflow per 10 m tunnel None b10 10–25 25–125 >125 length (l/min)

Pore fluid pressure/σ1 0 b0.1 0.1–0.2 0.2–0.5 >0.5 General conditions Completely dry Damp Wet Dripping Flowing Rating 15 10 7 4 0

Rating values for each of the five parameters are summed to obtain cumulative RMR value. J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 99

Table 4 Rock Mass Rating (RMR) parameters measured in lava flows at the geomechanical data points (Fig. 6) and derived GSI.

Site 1a 2b 3 4a 4b 4c 4d 4e 5 RMR GSIc

17 1710611637 5861 47 2020655654 7884 57 138 613637 5457 67 171043165106363 77 171043165106363 97 171063165106565 1326 1043563104949 1412201021365106969 157171061365157065 1643 8 61321103838 177201541365157671 1812201521165157772 1912171543166157974 2012201523366158277 2112201523366158277 227171043166156964 2312171543166157974 24122015431667 7477 25123 8 63166156055 2673 8 63365155651 F7–12 17–20 10–15 2–61–51–56 5–67–15 56–90 (68) 59–85 (63) A2–73–13 8–10613–52–61–34–15 30–66 (47) 36–61 (50)

Bold entries: Representative values for relatively fresh lava flows (F) and altered lava flows (A). a The intact rock strength has been determined by field indexes and from Schmidt hammer rebound. b RQD values obtained from correlations with Jv parameter (number of discontinuities per cubic meter) according to the expression RQD=115−3,3Jv. c GSI (Geological Strength Index) calculated from the RMR values by empirical correlation (Hoek and Brown, 1997): GSI=RMR−5 (considering a value of 15 to the water conditions).

takes the following geomechanical parameters into account to obtain Strength Index (GSI) has been also calculated from corresponding the RMR (Rock Mass Rating) quality index: uniaxial compressive chart assigned taking into account the structure and the discontinuity strength of intact rock, degree of fracturing, spacing and conditions surface conditions (Hoek et al., 1998)(Section 4.4) and from the RMR of discontinuities and groundwater conditions (Table 3). The results values by empirical correlation (Hoek and Brown, 1997). obtained by applying this classification to the lava flows of the geo- From the obtained data, representative RMR values for relatively mechanical data points are included in Table 4. This classification is fresh lava flows (F) and altered lava flows (A) have been established. not applicable to pyroclastic deposits and scoria layers. The Geological In the case of the fresh ones, it ranges from 56 to 90 (68 mean value)

Fig. 8. Galleries excavated in the Güímar and La Orotava valleys. The visited ones are (bold lines): (b) Bolaños, (pm) Pasada de Montelongo, (ps) Pino Soler, (c) Chimoche, (mb) Montaña Blanca, (a) El Aderno, (ba) Barranco de Amance, (d) El Drago, (cc) Cueva de las Colmenas and (sj) San José. 200 m contour intervals. Dashed line: water table corresponding to 1985 (previous to the water extractions), with 200 m contour intervals (bold contours every 1000 m) (Consejo Insular de Aguas de Tenerife). (UTM 28N/ WGS84/Meters). 100 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

inside the galleries were focused on the aspects of major interest: identification of predominant lithologies, degree of alteration and fracturing, number of dikes intruding the materials, presence of water and general geomechanical conditions. Fig. 9 shows the geolog- ical schemes of the volcanic rock successions crossed by these galler- ies. The increase of the degree of alteration and the intrusion of dykes with depth is shown in this figure. In summary, from the observations inside the galleries the follow- ing considerations can be advanced. Closer to the gallery pithead re- cent lava flows with scoria layers predominate with low degree of alteration and compaction. In the scoria layers large cavities are often developed (Fig. 10A). With depth compaction and alteration in- crease, and scoria layers are no longer identified. Pyroclastic deposits can be also observed (Fig. 10B). The debris avalanche deposits attrib- uted to the landslides are found in many galleries (Fig. 10C, D). These deposits separate post-landslide and pre-landslide materials. Rocks located deeper are intensely compacted, altered and fractured (Fig. 10E, F). Galleries had to be reinforced in the weaker sections, fre- quently associated with water flow (Fig. 10G). In these areas the pres- ence of dikes is very important (Fig. 10H). Depending on the number of dikes, three levels of intrusion intensity have been identified: low, with less than 4 dikes per 100 m of gallery length; intense, 4–10 dikes per 100 m; and very intense, with more than 10 dikes per 100 m.

4.3. Hydrogeological conditions

The groundwater galleries excavated in Tenerife provide relevant information about the hydrogeological characteristics of the edifices forming the island. The Water Council of Tenerife (Consejo Insular de Aguas de Tenerife, CIAT) is the responsible of the water planning and in charge of the hydrogeological data of the galleries of the island. This Council has also carried out relevant hydrogeological investigations to understand the hydrogeological regime of the island (Navarro and Farrujia, 1989). According to the observations carried out in this study inside the galleries and the data provided by the CIAT, the following considerations can be established. Lithological anisotropies characteris- tic of volcanic materials are directly reflected in the hydrogeological behavior and the groundwater regime. In Tenerife, these anisotropies are more evident at small scale. For example, along the same gallery, completely dry sections alternate with zones with an important water flow. These changes in the water regime are due to the different perme- Fig. 9. Geological schemes of the volcanic rocks successions crossed by the galleries in ability of the materials crossed in the galleries. In general, the more re- the La Orotava valley: (c) Chimoche, (b) Bolaños, (ps) Pino Soler, (pm) Pasada de Montelongo, (mb) Montaña Blanca; and the Güímar valley: (sj) San José, (ba) Barranco cent the rocks, the higher the permeability differences shown, because de Amance, (d) El Drago, (a) El Aderno, (cc) Cueva de las Colmenas. Dike intrusion rate the oldest rocks present a higher degree of compaction. At the scale of (number of dikes per 100 m of gallery length): low≤4 dikes; intense=4–10 dikes; the whole island, a simplified model can be established with layers very intense≥10 dikes. (See location map in Fig. 8). characterized by permeability decreasing with depth (Fig. 11A), taking into account the non homogeneity of these layers as mentioned above. and in the altered lavas from 30 to 66 (47 mean value). GSI values cal- This setting is not present in the structural axis or dorsal rift, character- culated from RMR for fresh lava flows range 59–85 (63 mean value) ized by the concentration of volcanic cones at the surface and a dense and for altered lava flows 36–61 (50 mean value). network of dikes associated to an intense fracturing. Both intrusion of dikes and fracturing change the original characteristics of the rocks 4.2. Galleries and their hydrogeological behavior. Permeability increases due to the presence of large open fractures and jointed dike networks which oper- The geological and geomechanical characterizations of the volcanic ate like effective vertical drainage ways (Figs. 10F, 12). These factors rocks succession have also been carried out inside several galleries ex- contribute to establish a vertical intercommunication in the core of cavated in both valleys. A total of 1051 galleries have been excavated the structural axis. However, more recent dikes, not affected by fractur- for over 170 years in search of groundwater on the island of Tenerife ing, behave as elements of low permeability, making difficult the trans- (Balcells, 2007). The galleries are tunnels with a small section versal groundwater flow. This creates an anisotropic medium in which (1.8 m2 — the oldest — and around 4 m2 the modern), with rising gra- longitudinal flow (direction of the dorsal rift) is enhanced rather than dients toward the interior of the volcanic edifice (between 5‰ and transverse flow. As a result the water table is raised and the thick- 7‰) and variable length (tens of meters to more than 6 km the lon- ness of the saturated zone increases (Fig. 11B). gest). Ten are the galleries visited and information of many others has been consulted. The visited ones are: Cueva de las Colmenas, El 4.4. Submarine materials Aderno, El Drago, Barranco de Amance and San José in the Güímar valley; Montaña Blanca, Pasada de Montelongo, Pino Soler, Bolaños The importance of the submarine materials in the instability processes and Chimoche in the La Orotava Valley (Fig. 8). The observations of volcanic island flanks was evidenced by DePaolo et al. (2001), J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 101

Fig. 10. Representative views of the volcanic materials observed inside the galleries (name of the gallery, distance to the pithead). A: Large cavity formed in a scoria layer (San José, 600 m). B: Alternation of pyroclastic deposits (Montaña Blanca, 3400 m). C and D: Landslide deposits (El Drago, 2565 m, and Pasada de Montelongo, 2700 m, respectively). E: Large open fracture with water flow (Barranco de Amance, 3125 m). F: Highly altered rocks (Cueva de las Colmenas, 2390 m). G: Reinforcement in jointed and weak sections associated with water flow. H: Overlapping dikes (El Drago, 2180 m). (See location map in Fig. 8). 102 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

Fig. 11. Schematic hydrogeological cross section of Tenerife (modified from Navarro and Farrujia, 1989). Dashed line: water table (WT), solid line: sea level (SL). A: Theo- retical hydrogeological model with lava layers associated to a decreasing permeability with depth. B: Layer model across a structural axis where the water table is raised be- cause of the dikes influence. In the structural axis core, large open fractures, jointed dikes and generalized fracturing contribute to increase the permeability and to estab- lish vertical communication.

Schiffman et al. (2006) and Thompson et al. (2008),inthelightof the Hawaiian Scientific Drilling Project's 3-km-deep drill core. Core sam- ples recovered in such drilling revealed that the upper 1 km of the sub- marine flank of Mauna Kea was composed mainly by hyaloclastites. Strength measurements on this rocks indicated that they were very Fig. 13. A: Outcrop of submarine materials located in Tenerife, near Igueste de San weak materials, with mean values for unconfined compressive strength Andrés (see Fig. 3). Pillow lavas and hyaloclastites can be distinguished. B: Uniaxial of 2.5 MPa, 4.6 MPa and 10.0 MPa. The strength of the hyaloclastites in- compression test on a hyaloclastites core sample. These rocks have low strength with creased with depth and grade of alteration (the alteration of minimum values of 5 MPa. C: Hyaloclastite core samples composed of clastic particles forming breccias. D: Slickenside surface in a hyaloclastite core sample. hyaloclastites appears to strengthened as opposed to weaken the flanks of the edifice). In 2006 an outcrop of submarine materials was identified in Tenerife, near the locality Igueste de San Andrés, in the Anaga edifice (Fig. 3), 4.5. Geomechanical properties where hyaloclastites with pillow lavas and basaltic dikes crop out (Fig. 13A). In the site three boreholes were drilled, one of them reaching Heterogeneity and anisotropy, characteristic of volcanic rocks, are 200 m depth. The main lithological units of this borehole are shown in reflected in the variability of their physical and mechanical proper- Fig. 14. Hyaloclastites are composed of clastic particles of irregular ties. The collection of geotechnical data in volcanic environments is shape and size ranging from 0.5 to 3 cm, forming green, gray or brown particularly difficult because of the geological complexity and the lo- colored breccias; many clasts show alteration halos (Fig. 13C). This ma- gistical difficulties that often limit the feasibility and reliability of terial is poorly consolidated and weakly cemented, especially in the sampling and carrying out in-situ surveys. For these reasons, in shallower parts. Voids and vacuoles are occasionally present with order to characterize the geomechanical properties of the rock sizes from 0.5 to 3 cm, and secondary minerals are observed inside masses and obtain representative ranges, the data collected from them. Fracture zones and slickenside surfaces have been identified field survey and in situ observation were complemented with infor- (Fig. 13D). mation from a comprehensive literature review and subsequent anal- Laboratory tests and in situ pressuremeter tests inside the boreholes ysis. The analysis is focused on the properties needed for the were carried out (e.g., Fig. 13B). The hyaloclastite rocks have low application of Hoek and Brown criterion (Hoek et al., 2002, and refer- strength and high deformability. Uniaxial strength results range from ences therein): unit weight (γ), uniaxial compressive strength (σci) 5 MPa to 60 MPa, with a mean value of 16 MPa, and pressuremeter and Geological Strength Index (GSI). (Hoek–Brown criterion is ap- moduli range from 5 MPa to 3200 MPa, with a mean representative plied in Section 5 in order to obtain the strength properties of the value of 560 MPa (Ferrer et al., 2011a, 2011b). rock masses.)

Fig. 12. Detail of the geological and hydrogeological data obtained from Cueva de las Colmenas gallery. It can be observed that the water is especially associated with fractures and dikes (see location map in Fig. 8 and cross-section in Fig. 4). J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 103

Fig. 16. Uniaxial compressive strength of volcanic materials (logarithmic scale). 1: Massive lava flows; 2: Scoria layers; 3: Altered lava flows; 4: Pyroclastic deposits; 5: Dikes; 6: Hyaloclastites; 7: Pillow lavas. The fine lines and the gray points represent the extreme and the mean values of the reviewed literature (references in the text). The thick lines and the large black points represent the representative ranges and means selected in this study.

4.5.1. Unit weight Fig. 15 shows the characteristic unit weight values of the volcanic materials here considered. The data correspond to massive lava flows (Okubo, 2004; Apuani et al., 2005a, 2005b; González de Vallejo et al., 2006, 2008; Hernández et al., 2006; Van Berlo, 2006; Del Potro and Hürlimann, 2007, 2008), scoria layers (Apuani et al., 2005a, 2005b; Van Berlo, 2006; Del Potro and Hürlimann, 2007, 2008), altered lava flows (Reid et al., 2000, 2001; Zimbelman et al., 2003, 2004; Concha-Dimas, 2004; Van Berlo, 2006; Del Potro and Hürlimann, Fig. 14. Major lithologic units as described from the 200-m deep borehole. 2007, 2008), pyroclastic deposits (IGME, 1974; Peiró, 1997; Serrano Hyaloclastites are predominant with a global fraction of 70% versus massive basalts. and Olalla, 1998; Alvarado, 2003; Sigarán, 2003; Concha-Dimas, 2004; Apuani et al., 2005a, 2005b; González de Vallejo et al., 2006, 2008; Lomoschitz et al., 2006; Serrano et al., 2007), dikes (Moore, In the case of the unit weight and the uniaxial compressive 2001; Yokose and Lipman, 2004; Van Berlo, 2006), hyaloclastites strength a statistical analysis has been performed taking into account (Moore, 2001; Yokose and Lipman, 2004) and pillow lavas (Moore, published data. The results are represented in Figs. 15 and 16. The 2001). The most representative values selected have been obtained graphs include extreme and mean values of the examined literature considering the lower, upper and mean values. Representative unit (references bellow as further detailed), and the mean and most rep- weight ranges and means (in brackets) are: 19–27 (24) kN/m3 for resentative values selected in this study. In the case of the Geological massive lava flows, 13–16 (15) kN/m3 for scoria layers, 15–19 (17) Strength Index (GSI), field observations are considered the most rep- kN/m3 for altered lava flows, 10–14 (12) kN/m3 for pyroclastic resentative data (Fig. 17), but it has been also represented along with deposits, 27–30 (28) kN/m3 for dikes, 22–26 (24) kN/m3 for other bibliographic results for comparison (Fig. 18). hyaloclastites and 27–29 (28) kN/m3 for pillow lavas (in the case of dikes, hyaloclastites and pillow lavas, the unit weight is saturated).

4.5.2. Uniaxial compressive strength Fig. 16 shows the characteristic uniaxial compressive strength values of the volcanic materials. The data correspond to massive lava flows (Thomas et al., 2004; Apuani et al., 2005a, 2005b; González de Vallejo et al., 2006, 2008; Hernández et al., 2006; Van Berlo, 2006; Del Potro and Hürlimann, 2007, 2008), scoria layers (Apuani et al., 2005a, 2005b; Del Potro and Hürlimann, 2007, 2008; Rodríguez-Losada et al., 2007), altered lava flows (Zimbelman et al., 2003, 2004; Concha-Dimas, 2004; Del Potro and Hürlimann, 2007, 2008), pyroclastic deposits (IGME, 1974; Serrano and Olalla, 1998; González de Vallejo et al., 2006, 2008; Lomoschitz et al., 2006; Serrano et al., 2007), dikes (Van Berlo, 2006), hyaloclastites (Schiffman et al., 2006) and pillow lavas (Schiffman et al., 2006). The most representative values selected in this study, obtained by the means of the lower, upper and mean values of the literature Fig. 15. Unit weight of volcanic materials. 1: Massive lava flows; 2: Scoria layers; 3: Al- values, are (mean in brackets): 69–196 (99) MPa for massive lava tered lava flows; 4: Pyroclastic deposits; 5: Dikes; 6: Hyaloclastites; 7: Pillow lavas flows, 18–122 (33) MPa for scoria layers, 18–56 (34) MPa for altered (saturated unit weight in the case of 5, 6 and 7). The fine lines and the gray points rep- lava flows, 1–4 (2) MPa for pyroclastic deposits, 100–250 (175) MPa resent the extreme and the mean values from the reviewed literature (references in – – the text). The thick lines and the large black points represent the representative ranges for dikes, 1 20 (6) MPa for hyaloclastites and 95 270 (144) MPa for and means selected in this study. pillow lavas. 104 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

Fig. 17. Geological Strength Index (GSI) of the rock masses grouped into lithological units. Classification table from Hoek et al. (1998). 1: Massive lava flows; 2: Scoria layers; 3: Altered lava flows; 4: Pyroclastic deposits; 5: Dikes; 6: Hyaloclastites; 7: Pillow lavas.

4.5.3. Geological Strength Index (GSI) 2008), scoria layers (Serrano and Olalla, 1998; Apuani et al., 2005a, Fig. 17 shows the results of applying Geological Strength Index 2005b; Del Potro and Hürlimann, 2008), altered lava flows (Concha- classification (Hoek et al., 1998) to the volcanic materials involved Dimas, 2004; Del Potro and Hürlimann, 2007, 2008), pyroclastic de- in Güímar and La Orotava landslides observed in the outcrops and posits (Serrano and Olalla, 1998; Apuani et al., 2005a, 2005b), dikes galleries. The GSI values obtained from RMR values (Table 1) have (Van Berlo, 2006), hyaloclastites (Schiffman et al., 2006; Thompson been also considered. GSI for submarine materials has been obtained et al., 2008) and pillow lavas (Thompson et al., 2008). from the Igueste de San Andrés outcrop, in the case of hyalocastites; Table 5 summarized the most representative geomechanical prop- for the pillow lavas the GSI has been estimated and extrapolated erties obtained for the different volcanic rock types. from similar outcrops in La Palma island. Obtained GSI ranges and means are: 53–71 (63) for massive lava flows, 20–30 (25) for scoria 5. Geological model of the pre-landslide volcanic edifice layers, 40–55 (50) for altered lava flows, 15–25 (20) for pyroclastic deposits, 57–73 (65) for dikes, 17–32 (21) for hyaloclastites and A morphological reconstruction of the pre-landslide edifice was 50–70 (63) for pillow lavas. In Fig. 18 these results are compared to carried out from current geomorphological data from the valley those obtained by other authors for each material: massive lava slopes not affected by landslides and from the Dorsal edifice. An esti- flows (Concha-Dimas, 2004; Apuani et al., 2005a, 2005b; Van Berlo, mated height of 3500 m a.s.l. was considered, with slopes ranging 2006; Del Potro and Hürlimann, 2007, 2008; Thompson et al., from 12° to 20° in the emerged part of the edifice, while in the J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 105

Table 6 Rocks materials forming the lithological units in the emerged edifice of Tenerife (Fig. 19).

Lithological Massive Lava flows and Altered Pyroclastic Dikes units lava flows scoria layers lava flows deposits

1 60% 40% –– – 2 70% 30% –– – 3a 45% – 45% 10% – 3b –– 80% 10% 10% 4 30% 20% – 40% 10% 5 –– 40% 30% 30%

were assumed to be similar to those existing in the island before intensive exploitation of the aquifers (Fig. 8). According to the available data of the Insular Water Council of Tenerife, the depth of the water table has been Fig. 18. Geological Strength Index (GSI) of volcanic materials. 1: Massive lava flows; estimated at 600–700 m below the surface, except in areas nearest the fl 2: Scoria layers; 3: Altered lava ows; 4: Pyroclastic deposits; 5: Dikes; 6: Hyaloclastites; coast. In the central part of the edifice (structural axis) the water table is 7: Pillow lavas. The fine lines and the gray points represent the extreme and mean values of the reviewed literature (references in the text). The thick lines and the higher due to the presence of a large number of dikes (see Section 4.3). large black points represent the representative ranges and mean values selected in In the following sections we describe the characteristics of the this study from field observations and data (Fig. 17). geological model, including both the emerged and submarine volca- nic materials (Fig. 19).

Table 5 Geomechanical indexes and properties of volcanic rocks (means in brackets). 5.1. Emerged edifice volcanic materials

Material GSI σci (MPa) γdry γsat 3 3 (kN/m ) (kN/m ) The materials on the flanks of the emerged edifice are different from Lava flow massive layer 53–71 (63) 69–196 (99) 19–27 (24) 20–28 (25) those forming the structural axis. The flanks are formed by deposits out- Scoria layers 23–30 (25) 18–122 (33) 13–16 (15) 14–17 (16) poured from the eruptive centers of the structural axis, mainly lava Altered lava flows 40–55 (50) 18–56 (34) 15–19 (17) 16–20 (18) fl – – – – ows with scoria layers whose degree of alteration and compaction in- Pyroclastic deposits 15 25 (20) 1 4 (2) 10 14 (12) 12 16 (14) fl Dikes 57–73 (65) 100–250 (175) 25–30 (27) 27–30 (28) creases with depth. In the inner part of the anks, as approaching to the Hyaloclastite rocks 17–32 (21) 1–20 (6) 21–25 (23) 22–26 (24) structural axis, dike intrusion is more intense while it decreases with Pillow lavas 55–70 (63) 95–270 (144) 26–28 (27) 27–29 (28) the distance from the axis. The structural axis is 3–6kilometerwide

GSI = Geological Strength Index, σci = uniaxial compressive strength, γdry = dry unit and is characterized by a dense network of dikes, associated with in- weight, γsat = saturated unit weight. tense fracturing and alteration. There is a significant presence of pyro- clastic deposits and lava flows with scoria layers and dikes. The submarine edifice the slopes range from 3° to 19°, with a maximum esti- number of dikes and the intrusion density observed in the galleries mated depth of 2500 m, according to the present seabed depth. The lith- have been considered to characterize the different units, according to ological units forming the emerged edifice have been defined from the classification previously established (very intense intrusion, more outcrop data and observations inside the galleries, while the lithological than 10 dikes per 100 m of gallery length; intense, 4–10 dikes per units of the submarine edifice were assessed from boreholes and pub- 100 m; low, less than 4 dikes per 100 m). Some strength properties lished data. The hydrogeological conditions of the pre-landslide edifice and alteration characteristics of the rock masses have been influenced

Fig. 19. Geological model representing the pre-landslide volcanic edifice across the Dorsal edifice of Tenerife (Fig. 3). WT = water table. SL = sea level. SA = structural axis. 106 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 by hydrological processes, therefore the water table depth has been analysis and geomechanical modeling. Strength properties, such as taken into account to assess different degrees of alteration, making pos- cohesion (c′) and friction (Φ′), are necessary since most geotechnical sible the distinction of different lithological units. Table 6 shows the ap- software is developed in terms of the Mohr–Coulomb failure criteri- proximate percentages, by volume, of the different rock masses forming on. However c′ and Φ′ for rock masses cannot be directly measured the lithological units of the emerged pre-landslide edifice, flanks and or determined in-situ, and “equivalent” parameters must be derived structural axis (Fig. 19). These units are described below. from intact rock properties (e.g. σci) and rock mass quality indexes (e.g. GSI). In this sense, the application of Hoek–Brown criterion • Unit 1: Recent lava flows with scoria layers, showing a low degree (Hoek et al., 2002, and references therein) has been widely used to of alteration and layers of loose scoria and cavities. determine c′ and Φ′ values of volcanic rock masses (Watters et al., • Unit 2: Slightly altered lava flows with scoria layers, with a lower 2000; Concha-Dimas, 2004; Okubo, 2004; Thomas et al., 2004; Apuani presence of cavities than Unit 1. et al., 2005a, 2005b; Moon et al., 2005; Thompson et al., 2008). • Unit 3: Altered lava flows and highly compacted pyroclastic de- Here, the Hoek–Brown criterion (Hoek et al., 2002) has been applied posits with intense dike intrusion. Depending on the dike intrusion to estimate the strength parameters of the rock masses of the geological density two sub-units have been differentiated (3a and 3b), with model, using the RocLab software (www.rockscience.com). Equivalent Unit 3b showing higher density of dike intrusion. cohesion (c′) and friction angle (Φ′) were calculated from the input pa- • Unit 4: Pyroclastic deposits and lava flows with scoria, and intense rameters: uniaxial compressive strength of intact rock constituting the dike intrusion. This unit presents lower alteration, compaction and rock mass (σ ), the Geological Strength Index (GSI), the material con- dike intrusion density than Unit 5. ci stant m depending on the textural and petrographical characteristics • Unit 5: Pyroclastic deposits and altered lava flows, showing very intense i of the intact rock, and the assumed confining pressure (σ ). The gen- dike intrusion. These materials are highly compacted and fractured. 3max eralized Hoek–Brown criterion is expressed as: fi ! 5.2. Submarine edi ce volcanic materials ′ a ′ ′ σ σ ¼ σ þ σ m 3 þ s 1 3 ci b σ ′ Most of the information about submarine materials has been ci obtained from boreholes drilled in the area and a review of the rele- vant literature (Staudigel and Schmincke, 1984; Nielson and Stinger, 1996; Schmincke and Sumita, 1998, 2010; DePaolo et al., 2001; σ′ σ′ García and Davis, 2001). Table 7 shows the percentages of the differ- where 1 and 3 are the major and minor effective principal ent types of materials forming the submarine lithological units stress at failure σ (Fig. 19) which are described below. ci is the uniaxial compressive strength of the intact rock material mb is a reduced value of the material constant mi and is given by: • Unit 6: Hyaloclastite rocks, originated from subaerial flows which formed ‘lava deltas’ entering the sea, as well as originated from sub- marine eruptions. This unit is also composed of pillow lavas. The − thickness of this unit has been estimated in 800 m (Staudigel and ¼ GSI 100 mb mi exp − D Schmincke, 1984; DePaolo et al., 2001; García and Davis, 2001). 28 14 Two subunits have been differentiated depending on the dike intru- s and a are constants for the rock mass given by the following rela- sion and the distribution of these materials in the volcanic edifice. tionships: • Unit 7: Pillow-lavas formed by submarine eruptions, representing the main submarine growth phase of the island. In the same way GSI−100 s ¼ exp than Unit 6, two subunits have been differentiated. 9−3D • Unit 8: Gravitational deposits formed by gravitational and sedimen- fl tary processes on the submarine anks. This unit includes slope fa- 1 1 − = − = a ¼ þ e GSI 15−e 20 3 : cies, formed by the slides occurred throughout the growth of the 2 6 island, and basin facies, composed of interbedded distal turbidites, fallout tephra layers and hemipelagic sediments. D is a factor which depends upon the degree of disturbance to • Unit 9: Dikes and plutonic complex. These volcanic complexes, known which rock mass has been subjected. In this case the lithological as the Basal Complexes, crop out in other Canaries (La Palma, La units of the model were regarded as undisturbed (D=0).

Gomera and Fuerteventura) are composed of a set of submarine and The state of stress at failure is then plotted on a σ′1–σ′3 stress di- plutonic rocks intensely transversed by a dyke swarm, representing agram and the equivalent Mohr–Coulomb strength parameters, fric- the submarine growing stage of the island and the roots (plutons tion angle Φ′ and cohesive strength c′, calculated for each rock mass and dikes) of the successive subaerial growing episodes. and confining stress range. This is done by fitting an average linear re- lationship to the curve generated by solving the generalized Hoek– 6. Geomechanical model of the pre-landslide volcanic edifice Brown equation for a range of minor principal stress values defined

by σt bσ′3 bσ′3max. The value of σ′3max, the upper limit of the confining Representative geomechanical parameters of the lithological units stress over which the relationship between the Hoek–Brown and the of the geological model are necessary to carry out any stability Morh–Coulomb criteria is considered, has to be determined for each individual case. In this study, it has been considered an elastic state

Table 7 of stress in which σ3 =0,33σ1.Soσ′3max was obtained considering Rock materials forming the lithological units in the submarine edifice of Tenerife the maximum vertical stress due to the overlying material loading (Fig. 19). for each lithological unit: σ1 =γh, where γ is the unit weight and h Lithological units Hyaloclastites Pillow-lavas Dikes the depth or thickness of the materials (González de Vallejo and

6a 70% 30% – Ferrer, 2011). 6b 65% 25% 10% The input data for each lithological unit were determined based on 7a 10% 90% – the proportions of the different types of materials (Tables 6 and 7) 7b 5% 85% 10% and their properties (Table 5). To estimate a representative value of 9 – 5% 95% σ′ci for each lithological unit, taking into account their different J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110 107

Table 8 Input data for the application of Hoek–Brown criterion and strength equivalent properties obtained for each lithotechnical unit (Fig. 19) with geomechanical modeling purposes (means in brackets).

Litho-technical units σ′ci GSI′ m′i σ′3max c′ Φ′ (°) (MPa) (MPa) (MPa)

135–153 (59) 41–55 (48) 13–23 (18) 1.6–2.2 (2.1) 0.5–1.8 (0.9) 44–61 (51) 243–161 (66) 44–59 (52) 13–23 (18) 3.6–5.0 (4.6) 1.0–3.1 (1.7) 40–57 (47) 3a 23–71 (38) 32–48 (42) 14–24 (19) 8.5–11.5 (10.5) 1.2–3.3 (2.2) 25–41 (33) 3b 15–43 (28) 28–43 (37) 14–24 (19) 16.3–23.8 (21.0) 1.5–4.3 (2.9) 17–29 (24) 415–45 (18) 32–45 (38) 12–22 (17) 3.4–4.6 (4.2) 0.5–1.4 (0.9) 27–43 (32) 515–40 (25) 27–40 (33) 13– 23 (18) 19.1–23.4 (21.6) 1.6–3.9 (2.6) 15–28 (21) 6a 9–43 (19) 16–32 (24) 11–21 (16) 7.4–8.4 (8.0) 0.5–1.7 (1.0) 14–33 (23) 6b 12–42 (22) 18–35 (26) 12–22 (17) 25.5–30.9 (27.8) 1.3–4.4 (2.5) 11–24 (17) 7a 69–203 (107) 36–55 (53) 14–24 (19) 31.5–34.5 (33.5) 4.5–10.8 (7.8) 25–43 (35) 7b 78–218 (123) 38–57 (55) 15–25 (20) 47.5–54.3 (50.7) 6.6–15.7 (11.3) 24–41 (34) 982–218 (144) 46–62 (50) 15–25 (20) 57.4–64.2 (60.4) 8.6–18.8 (12.5) 25–41 (32)

lithological proportions, Laubscher's (1984) criterion was applied. This (the fact that the present rock mass properties might represent the criterion states σ′ci according to an average of % of the strong rock σci, pre-landslide properties), the reliability of the geomechanical in- given by the % of weak rock and the ratio of weak rock σci and strong situ surveys is often limited by the geological complexity and the lo- rock σci. gistical difficulties. In addition to that, the application of empirical Table 8 presents the input parameters and the representative rock criteria of rock mechanics (e.g. Hoek–Brown criterion) to obtain geo- mass strength values, c′ and Φ′, obtained for the geomechanical mechanical parameters, which cannot be obtained otherwise, in- model applying the Hoek–Brown criterion. volves uncertainties and constitutes a limitation. Moreover, when For the island flanks, where lava flows predominate and both the the models include the submarine flanks of volcanic edifices the un- degree of alteration and the dike intrusion intensity increase with certainties are even more. Research on these materials is very limit- depth, the characteristic range of the strength values obtained for ed and little is known about their distribution in depth and their the lithological units is: c′=0.9–2.9 MPa and Φ′=24–51°. For the physical and mechanical properties. structural axis, with a significant presence of pyroclastic deposits However, despite these uncertainties and limitations, the geologi- and very heavy alteration and intrusion below the water table, the cal and geomechanical models of Tenerife presented in this study are characteristic strength values are c′=0.9–2.6 MPa and Φ′=21–32°. the most complete and detailed presented up to now. Many geologi- For the hyaloclastite rocks in the outermost of the submerged flank cal, hydrogeological and geomechanical data from outcrops, bore- the strength values are: c′=0.5–1.7 MPa and Φ′=14–33° (mean holes and galleries have been taken into account. These galleries, values: c′=1.0 MPa and Φ′=23°), and for those in the interior of not existing in most of the world's volcanoes, constitute a unique the volcanic edifice: c′=1.3–4.4 MPa and Φ′=11–24° (mean values: source of observation and data. Moreover we described data also c′=2.5 MPa and Φ′=17°). For the units that form the core of the vol- based on an extensive and comprehensive literature review. In partic- canic edifice, constituted by pillow-lavas and dikes: c′>4.5 MPa and ular, the properties of volcanic materials presented here (and partic- Φ′>25°. In the particular case of Unit 8, formed by gravitational de- ularly the most representative values) can be useful for other posits containing hyaloclastite breccias and sediments, the strength researchers studying volcano flank collapses. 3 parameters obtained are: c=0.5–1 MPa, Φ=20° and γsat ≈20 kN/m . The proposed models have been elaborated with geomechanical modeling purposes in order to evaluate the influence of the factors in- volved in the instability processes and understand the causes and fail- 7. Discussion ure mechanisms of the landslides that generated the Güímar and La Orotava valleys, in the same way that it has been done in other volca- Several limitations and uncertainties are associated with the noes around the world (Okubo, 2004; Apuani et al., 2005b; Moon elaboration of representative geological and geomechanical models et al., 2005; Thompson et al., 2008). Based on the established models, of volcanic edifices that have been affected by instability processes. the influence of the triggering factors such as eruptions or earthquakes It should be taken into account that such research is based on the re- could be evaluated, considering different scenarios in the stability anal- construction of the volcanic edifices starting from the recognition of ysis. In this case, as in Hawaiian landslides (DePaolo et al., 2001; what remains of them after suffering these processes. In most cases, Schiffman et al., 2006; Thompson et al., 2008), hyaloclastites seem to the analyzed instability processes have taken place a long time ago be a key factor in favoring instability processes because of their charac- (several hundreds of thousands of years in the case of the Güímar teristics and low strength properties (Seisdedos, 2008; Ferrer et al., and La Orotava landslides); however it can be assumed that it is pos- 2010, 2011a). We infer that a failure mechanism characterized by the sible to extrapolate data from current conditions. First, it is necessary occurrence of successive landslides (which may have started in the sub- to reconstruct the geometry of the pre-landslide edifice, and in some marine flank, conditioned by the presence of weak and poorly consoli- cases, this requires important assumptions. Moreover, for the geo- dated hyaloclastites) could have taken place in the generation of the logical reconstruction, it is necessary to group volcanic materials Güímar and La Orotava valleys (Seisdedos, 2008). into general lithological units making simplifications and assump- tions, mainly according to their geomechanical properties and be- havior. It is important to know that the contacts between these 8. Conclusions lithological units are not net lines; they are lithological gradual tran- sitions (hardly representable at the scale of investigation). On the This paper presents a geological model representing the island other hand, the definition of the hydrogeological conditions is not volcanic edifice before the pre-historic Güímar and La Orotava land- simple; groundwater distribution in oceanic and continental volca- slides took place. The model has been defined from the analysis of ex- noes is very poorly understood. The geomechanical characterization tensive field data particularly those obtained inside the galleries that is also subject to many uncertainties. As well as those related above run through the interior of the island. 108 J. Seisdedos et al. / Journal of Volcanology and Geothermal Research 239–240 (2012) 92–110

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Deep drilling into a Hawaiian volcano. Eos Acknowledgments 82 (13), 149 (154–155). Deplus, C., Le Friant, A., Boudon, G., Komorowski, J.-C., Villemant, B., Harford, C., Ségoufin, J., Cheminée, J.-L., 2001. Submarine evidence for large scale debris ava- This research has been supported by the Spanish Science Ministry, lanches in the Lesser Antilles Arc. Journal of Volcanology and Geothermal Research through the National Research Plan I+D, and the Geological Survey 192, 145–157. “ ” Duffield, W.A., Stieltjes, L., Varet, J., 1982. Huge landslides blocks in the growth of Piton of Spain (IGME) by means of the research project GRANDETEN de la Fournaise, La Réunion, and Kilauea volcano, Hawaii. Journal of Volcanology (CGL2004-00899). The authors are very grateful to the participants and Geothermal Research 12, 147–160. of this project who helped and contributed to this research. The con- Elsworth, D., Day, S.J., 1999. 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