Spreading and potential instability of Teide volcano, Tenerife, Canary Islands

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L05305, doi: /2007gl032625, 2008 Spreading and potential instability of Teide volcano, Tenerife, Canary Islands Alvaro Márquez, 1 Iván López, 1 Raquel Herrera, 1 Fidel Martín-González, 1 Tatiana Izquierdo, 1 and Francisco Carreño 1 Received 16 November 2007; revised 13 January 2008; accepted 7 February 2008; published 11 March [1] We analyze the role of low-strength materials in the potential structural instability related to volcano spreading at the active Teide stratovolcano (Tenerife, Canary Islands). To study the low-strength materials we took advantage of a network of large tunnels within the volcano, excavated for water supply, which allows the in situ inspection and reconstruction of both volcano and substratum structure. We identified two factors which are potentially important to Teide s instability: 1) a dipping low-strength substratum breccia layer; and 2) a hydrothermally-altered weak core inside the volcano. Despite we do not find clear structural evidence for volcano substratum spreading, both Teide topographic shape and the summit faulting are features similar to those accepted as examples of volcano flank spreading over a weak core. The detected asymmetric deformation of the edifice makes the north flank of Teide a strong candidate for a potential failure. Citation: Márquez, A., I. López, R. Herrera, F. Martín-González, T. Izquierdo, and F. Carreño (2008), Spreading and potential instability of Teide volcano, Tenerife, Canary Islands, Geophys. Res. Lett., 35, L05305, doi: /2007gl Introduction [2] Teide is an active stratovolcano on Tenerife Island (Canary Islands, Spain), an area that has suffered several north-directed lateral collapses of previous volcanic edifices [e.g., Watts and Masson, 2001], but no evaluation of the structural stability of Teide has yet been performed. Volcano spreading [see Borgia et al., 2000] is a key process in the structural stability of large stratovolcanoes. Volcanoes exert a load upon their substrata, and if the substrata contain lowstrength materials, they can deform and spread outward. In a number of different volcanoes it has been recognized that substratum deformation, in turn, induces deformation of the volcano above (e.g., Etna, Kilauea) [Borgia et al., 2000]. Where spreading is restricted to sectors of the volcano it can induce lateral sector collapse [van Wyk de Vries and Francis, 1997]. Two scenarios can produce the volcano sector spreading: a) asymmetric flank spreading over a hydrothermal weak core [e.g., van Wyk de Vries et al., 2000]; and b) asymmetric spreading of a low-strength volcano substratum [e.g., Wooller et al., 2004]. In both cases distinctive deformation structures appear related to volcano instability well before catastrophic collapse [e.g., van Wyk de Vries et al., 2000]. Flank spreading 1 Área de Geología, Universidad Rey Juan Carlos, Móstoles, Madrid, Spain. Copyright 2008 by the American Geophysical Union /08/2007GL can produce a summit rift and characteristic concave-convex profiles on volcano flanks [e.g., Cecchi et al., 2005]. Asymmetric substratum spreading also produces summit graben, and compression at the volcano foot reflected by arcuate strikeslip faults or by thrust faults and folds [Wooller et al., 2004]. We present a two-step methodology to analyze the potential instability of Teide volcano: 1) studying low-strength materials in the volcanic edifice and 2) conducting a topographical and structural study to identify the typical deformation features of volcanic spreading processes. 2. Geological Setting [3] Teide volcano (3718 m a.s.l.) is located in Las Cañadas Caldera in the center of Tenerife Island (Figure 1a). Teide is a large volcanic complex with two overlapping vents (Teide and Pico Viejo) forming an edifice 8 5 km wide and 1300 meters high. Several scoria cones and lava domes and flows form the base of the main construct and cover the entire Las Cañadas Caldera and the Icod Valley [Ablay and Martí, 2000; Carracedo et al., 2007]. The last eruptive episodes in the Teide volcanic complex were a flank eruption on the western slope of Pico Viejo in 1798 and a lava flow field from Teide summit ( AD) [Carracedo et al., 2007]. Teide has had continuous fumarolic activity from the first reports in the 15th century to the present day. [4] Las Cañadas Caldera is an asymmetrical depression open to the north that resulted from the destruction of Las Cañadas volcano, a former complex stratovolcano that covered the center of Tenerife. Offshore geophysical surveys carried out north of Tenerife confirmed the existence of several huge landslides deposits, the youngest located directly offshore from the Icod Valley [Watts and Masson, 2001]. Two main hypotheses have been proposed to explain the origin of Las Cañadas Caldera and the Icod Valley. The first hypothesis considers Las Cañadas as a nested collapse caldera formed by three different vertical collapses. The most recent collapse formed the eastern sector simultaneously triggering the lateral landslide of the northern flank of Las Cañadas volcano that formed the Icod Valley [Martí et al., 1997]. The second hypothesis neglects the role of vertical caldera collapse and relates Las Cañadas Caldera to the occurrence of one [Navarro and Coello, 1989] or several [Cantagrel et al., 1999] lateral landslides of Las Cañadas volcano to the north. [5] Hundreds of horizontal tunnels (called galerías in local terminology) have been constructed in Tenerife in search for groundwater. These nearly horizontal tunnels are about 2 meters high and up to several kilometers long (see and consequently penetrate a considerable way into the volcanic constructs. 37 of these tunnels L of5

2 Figure 1. (a) Topographic map of central Tenerife showing the location of water tunnels (black lines) and boreholes/wells (black dots). Grey dots: debris avalanche breccia (mortalón) outcrops inside tunnels and wells. Contours (dashed lines): inferred altitude (meters above sea level) of the upper limit of the breccia deposit. Black box: location of Figure 1 in auxiliary material. (b) Cross section across Teide volcano (line I-I in a) showing the inferred geometry of breccia deposits (numbers of tunnelswells in Table 1 of auxiliary material). Light grey area in Teide volcano represents the proposed hydrothermal core. penetrate Teide volcano materials at several depths mainly in the northern flank (Figure 1a and auxiliary material 1 ). We took advantage of this unique situation to collect and integrate all existing data on tunnels and wells (see Table 1 in auxiliary material), both from published literature and from the archived data of the Tenerife Water Council (Consejo Insular de Aguas de Tenerife), into a Geographic Information System. We also made direct observations in two tunnels in the Icod Valley ( Salto del Frontón and Vergara-2 ). 3. Factors Affecting Volcano Stability: The Case of Teide 3.1. Hydrothermal System [6] Fumarolic activity on Teide summit is related to an important hydrothermal system that has resulted in the pervasive replacement of the original volcanic rocks by clay minerals and sulphates. The main hydrothermal alteration zone (around 1000 m m from aerial photographs measurements) affects trachytic lava flows extending below the youngest phonolitic lava flows to an undetermined extent. [7] Self-potential data indicate the presence of a hydrothermally-altered core beneath the lower SE flank of Teide volcano [Aubert and Kieffer, 1998], and gravimetric data support the existence of a low-density core within the volcano with low magnetic values [Araña et al., 2000]. Both are typical of the hydrothermal replacement of volcanic rocks by clay-rich materials. The strongly altered rocks on the surface, the geophysical data and the evidence of long-lived shallow magma chambers in the area [e.g., Triebold et al., 2006] suggest the existence of an important volume of altered rocks, although the geometry of the weak core cannot be fully constrained with present data (see Figure 1b) Volcano Substratum [8] Data from tunnels, wells and boreholes in central Tenerife (Figure 1a and auxiliary material) show a thick (>200 m) breccia layer (the so called mortalón in local terminology) underlying the sequence of lava flows from Teide volcano [e.g., Bravo, 1962; Navarro and Coello, 1989]. This polymictic breccia is composed of heterogeneous angular fragments of volcanic rocks in a clay-rich matrix, and shows characteristic features of a debris avalanche deposit [Navarro and Coello, 1989]. None of the tunnels reached the breccia base because tunnels walls rapidly converge because the breccia deposit undergoes plastic deformation, which has continued even after the installation of support steel arcs or concrete blocks, which usually bulge and even break [Bravo, 1962]. This behavior, known as squeezing in rock engineering, provides strong evidence of the low-strength (ductile) character of the breccia substratum. [9] In order to reconstruct the geometry of this ductile substratum, we plotted data from 37 tunnels and 12 wells/ boreholes (see Table 1 in auxiliary material) regarding the depth of the breccia deposit or the maximum depth of lava flows infill (Figure 1a), in order to construct intersecting geological cross sections (see Figure 1 in auxiliary material), which in turn, allow us to map the depth of upper limit of the avalanche deposit (Figure 1; see methodology in auxiliary material). We obtained a single coherent surface of the upper limit of the landslide breccia deposit, with good correlation between the off- and on-shore breccia distributions, showing a U-shape body covered by several hundreds of meters of lava flows (Figure 1b) that extends inside Las Cañadas Caldera directly below Teide volcano (Figure 1a and 1b). [10] Thus, our interpretation of the geological structure from tunnels and wells data supports the proposal that Teide volcano is build upon a thick clay-rich breccia layer (Figure 1b) that would form a low-strength layer [e.g., Oehler et al., 2005]. This low-strength layer is buttressed on its south side by the older Las Cañadas volcano and dips approximately 5 to the NNW (Figure 1b). Ductile substratum, even with such low inclination, can lead to 1 Auxiliary material data sets are available at ftp://ftp.agu.org/apend/gl/ 2007gl Other auxiliary material files are in the HTML. 2of5

3 Figure 2. (a) Three dimensional shaded-relief view of Teide volcano showing the two prominent bulges along the NW and NE flanks, and the E-W trending summit flat area delimited by two inward-dipping scarps (labeled A and D) that enclosed two smaller north-dipping scarps (labeled B and C). (b) Left: Topographic profiles from SRTM-derived topographic data of recognized stable volcanoes (dashed lines) and deformed flanks (solid lines) caused by spreading over a hydrothermally-altered weak core [van Wyk de Vries et al., 2000; Cecchi et al., 2005]. Right: Topographic profiles of Teide volcano (SRTM data) showing the characteristic concave-convex profiles typical of deformed flanks. (c) Photograph of Teide volcano illustrating the difference between the stable SE flank and the deformed NE flank, and the two southernmost summit north-dipping scarps C and D. asymmetrical gravitational spreading of a volcano [Wooller et al., 2004]. 4. Evidence of Possible Spreading at Teide Volcano 4.1. Flank Spreading [11] To determinate whether the hydrothermally-altered zone of Teide volcano induces volcano spreading, we analyzed the topography of the volcano using 3D shaded views (Figure 2a) and topographic profiles of the volcano flanks (Figure 2b), comparing these data with flank profiles from both recognized stable and deformed volcanoes, each with the documented presence of a weak core (Figure 2b). The shaded view shows a flat summit area delimited by several inward dipping topographic scarps and two bulges on the NW and NE volcano flanks (Figure 2a). Profiles across these bulges (Figure 2b) show the characteristic convex-concave topography associated with flank spreading in other volcanoes [van Wyk de Vries et al., 2000; Cecchi et al., 2005]. The ENE flank shows the strongest deformation, whereas the SE flank is the least deformed (Figure 2b). This pronounced difference in the amount of spreading in different sectors may be produced by the buttressing effect of Pico Viejo edifice on the WSW flank of Teide volcano (Figure 2a) and the N-S topographic asymmetry of the base of the edifice (Figure 1a). [12] Scarps of the Teide summit area (Figure 2a) can be interpreted as extensional faults produced as a result of the volcano spreading over its weak core. The most relevant fault, located NE of the summit cone and marked by a south-dipping E-W scarp (labeled A in Figures 2a and 3a) clearly cuts some lava flows (Figure 3). A second parallel north-dipping scarp (labeled B in Figures 2a and 3a) appears around 300 m southward. Two additional north-dipping scarps occur SSW of the summit cone (labeled C and D in Figures 2a and 2c) Spreading of the Substratum [13] Analogue models show that an inclined ductile volcanic substratum can potentially produce asymmetric volcano spreading, resulting in distinctive thrust anticlines or strike-slips faults at the base [Wooller et al., 2004]. Any contractional tectonic structures produced at the foot of Teide volcano by substratum spreading would be expected to be located offshore, beyond the lava flows from the Icod Valley (Figure 1b). Published bathymetric data of the area [e.g., Watts and Masson, 2001] does not show any morphologic feature that might provide indisputable evidence of faulting or anticlines at the lava fronts. 5. Discussion: Potential Teide Instability [14] Analysis of surface and subsurface geological data from Teide volcano shows the presence of key recognized prerequisites for volcano instability: 1) the existence of a hydrothermally-altered weak core; 2) the presence of a lowstrength layer in the substratum; 3) inclination of this substratum to the NNW; and 4) the buttressing of volcano and substratum on the south side. [15] In addition, we find topographic and geological features (convex-concave flanks and summit faults; Figures 2 and 3) consistent with the development of asymmetric flank spreading of Teide volcano over its weak core. Previously the scarps of the Teide summit area have been interpreted as two collapse calderas [Ablay and Martí, 2000; Carracedo et al., 2007] whereas the bulge on Teide NW flank has been interpreted as the reflection of the head of the Icod valley landslide [Ablay and Martí, 2000] or as an older flank explosive vent covered by young phonolitic lava flows [Carracedo et al., 2007]. In our opinion, the topo- 3of5

4 Figure 3. (a) Aerial photograph of Teide summit area showing the two E-W scarps (labeled A and B) located NE of the summit cone (Google Earth image). (b) Structural interpretation showing how the scarps cut some lava flows. graphic (Figure 2a) and structural (Figure 3) analysis of volcano summit area do not support the double caldera interpretation, mainly due to the actual dip to the north of scarp B in opposition to the south-dipping geometry previously alleged [see Ablay and Martí, 2000; Carracedo et al., 2007]. Otherwise, none of the previous theories concerning the NW flank bulge offer an explanation for the existence of the prominent NE bulge, whereas flank spreading offers a unifying explanation for both the flank bulges and the summit faults. [16] Bulging flanks and a summit graben in a volcano can also be produced by a cryptodome intrusion [Donnadieu et al., 2003]. However, we consider that in the case of Teide volcano the summit faulted flat area away from the bulges (Figure 2a) is clearly different from the sharp transition zone between the bulge and graben that characterize cryptodome intrusions [Donnadieu et al., 2003]. [17] Our interpretation of the flank spreading of Teide is particularly significant because once a sector of the volcano has become destabilized it becomes more susceptible to failure, in response to edifice deformation or both internal and external triggers (e.g., intrusions, earthquakes, or intense rainfall). Asymmetric deformation of the edifice (Figure 2a and 2b), together with its asymmetric topographic setting and buttressing of the southern flank by the older Las Cañadas Volcano (Figure 1b), makes the north flank of Teide a strong candidate for potential failure, despite the claim by Carracedo et al. [2007] that lava domes at the base of the volcano serve to buttress its north flank progressively increasing volcano stability. Therefore, even though GPS and InSAR studies show no evidence of deformation of Teide between 1993 and 2000 [Fernández et al., 2003], we consider that our results imply the need to continue monitoring deformation of Teide volcano. [18] The possibility of active deformation of the breccia deposits underlying Teide volcano has far-reaching implications for volcano stability. The low-strength behavior of the breccia deposits is clearly documented by the deformation problems in the tunnels, but the expected distinctive tectonic structures related to the spreading of such a substratum are not clear in the Teide case. In particular, the apparent absence of tectonic contractional structures at the northern foot of the volcano suggests the substratum has not begun to spread. If this is the case, then Teide volcano has to reach the spreading phase [e.g., Borgia and van Wyk de Vries, 2003] in which the volcano mass is sufficient to initiate ductile spreading of the substratum. [19] Data from Teide tunnels provide clear evidence that volcano debris avalanche deposits can serve as a lowstrength layer in volcanic structures. The growth of a volcanic structure inside a previous landslide scar, and therefore loading upon their debris avalanche deposits, is a common process in the evolution of stratovolcanoes (e.g., Colima, Popocatepetl). Therefore data from Teide tunnels can open new research avenues to understand deformation and instability processes in other active volcanoes worldwide. 6. Conclusions [20] Our study points out that Teide volcano shows two important recognized prerequisites of structural instability caused by asymmetric spreading. Data from tunnels show that the volcano is built upon a breccia deposit related to a huge early-formed debris avalanche. This low-strength clayrich deposit dips approximately 5 to the NNW and is buttressed on its southern side. In addition, Teide shows surface and geophysical evidence of a hydrothermallyaltered weak core. [21] Apparent absence of basal tectonic structures in the off-shore bathymetric data at the base of Teide suggests that the volcano has not reached the spreading phase despite being built on an inclined and buttressed ductile layer. Conversely, topographic and structural studies shows faults at the Teide summit area and typical concave-convex profiles on the north flanks, evidence of edifice spreading by deformation of a weak core. Asymmetric deformation of the edifice suggests that the north flank of Teide is a strong candidate for potential failures. This new working hypothesis require future geophysical studies to better constraint the geometry of the volcano weak-core and to monitor surface deformation, which are required for a complete hazard assessment of Teide volcano. [22] Acknowledgments. This work would not have been possible without the tunnel data and comments by J. M. Navarro, I. Farrujia and J. J. Coello. We thank A. Hernández and R. Fenol who kindly allowed us to visit the tunnels, A. Watts and J. Acosta for the bathymetric data provided, and R. Oyarzun, J. B. Murray, B. van Wyk de Vries and V. Hansen for their reviews of previous versions, which greatly improved the manuscript. Teide National Park provided permission to carry out work in restricted areas. This study has been partly funded by projects REN E and GLC from MEC and PPR from URJC. 4of5

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