Vicente Araña

Stratigraphy, structure and geochronology of the Las Cañadas caldera (Tenerife, Canary Islands)

After a long period of subaerial fissure-fed extrusions of basaltic magmas (∼ 12 to > 3 Ma) volcanic activity was then concentrated in the central part of Tenerife. Phonolitic magma chambers formed and a central volcanic complex was constructed (the Las Canadas edifice). The formation of a large depression (the Las Canadas caldera) truncated the top of the edifice. The active twin strato-cones Teide—Pico Viejo are sited in this depression. The history of the Las Canadas caldera and edifice are established from stratigraphy, geochronology (K—Ar dates) and volcanological studies. Two different groups are recognized, separated by a major unconformity. The Lower Group is dated at 2 to 3 Ma and includes the products of several volcanic centres, which together represent several cycles. The Upper Group ranges from 1.56 to 0.17 Ma and includes three different formations representing three long-term (∼ 100 to 300 Ka) volcanic cycles. The periods of dormancy between each formation were of ∼ 120 to 250 Ka duration. The Las Canadas caldera is a multicyclic caldera which formed over the period 1.18–0.17 Ma. Each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the Las Canadas edifice. Geological observation and geochronology support an origin by collapse into a magma chamber. The minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume (45 km3), but approaches the inferred volume of the original caldera depression (> 140 km3). After the formation of the caldera, sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall.

Assessment and Modelling of Lava Flow Hazard on Lanzarote (Canary Islands)

This paper presents an evaluation of the lava flowhazard on Lanzarote (Canary Islands) by means of aprobabilistic maximum slope model. This model assumesthat the topography plays the major role indetermining the path that a lava flow will follow. Thearea selected for containing future emission centreshas been chosen taking into account thecharacteristics of the recent eruptive activity andthe present activity of the island. The results of thesimulations constitute hazard maps whose values ateach point represent the probability of being coveredby lava. These results are qualitatively analysed toprovide some indication of the risk to the lifelines(electricity, drinking water etc.) of the island.

Magma mixing in alkaline magmas: An example from Tenerife, Canary Islands

Abstract The central region of Tenerife (Canary Islands) offers a wide variety of alkaline rocks of intermediate composition between basalts and more differentiated magmas. A recent (less than 0.1 Ma old) eruption on the southwestern slopes of Teide volcano produced separate felsic (SiO 2 ∼59%) and mafic (SiO 2 ∼47%) rocks as well as abundant hybrid inclusions (SiO 2 : 50–57%) that show a wide range of geochemical and mineralogical variation between the end members. Mineralogical evidence (reverse-zoning, skeletal and patchy feldspars, incompatible assemblages, mineral disequilibrium) and geochemical data (characteristic trends of major and trace elements, REE ratios, major and trace mass-balance calculations) support the generation of a mixing process. We present a model in which basaltic magma entered a trachyte magma chamber, which resulted in a stratified chamber of trachytic above more mafic (hawaiitic) melts. The formation of hybrid magmas between these two end-members is believed to involve some chemical diffusion across the interface between the two magmas before mechanical mixing. A lowering of temperature due to the cooling effect of the trachytic magma and, consequently, a decrease in density by crystallization of plagioclase permitted the basaltic magma to penetrate through the trachyte, which caused the eruption. This natural example is in good agreement with the results obtained in experimental work on interdiffusion of silicate melts.

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S‐P time and seismogram shape has permitted to establish three groups of events: local seismicity (S‐P time between 3 and 5 s), very local earthquakes (S‐P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag CrossCorrelation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide‐Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task. q 2000 Elsevier Science B.V. All rights reserved.

Physical-chemical conditions of the Teide volcanic system (Tenerife, Canary Islands)

The Teide volcano (3717 m) is the central structure of the island of Tenerife and at present its morphology is that of a stratovolcano which has grown on a large caldera with a collapse 17 km in diameter, which was generated some 0.6 million years ago. The different studies that have been carried out seem to indicate that, in a oversimplified model, there is an intermediate magma chamber with an approximate volume of 30 km3 and located 2–3 km below the actual base of the caldera, i.e., almost at sea level, with a temperature of 430 ± 50°C, and a pressure of 400 ± 100 bar. The summit fumarole emissions are 85°C and are formed mainly of CO2 with small amounts of sulphur species, H2, CH4 and He. The water vapor (68–82%) emitted with the gases comes from the vaporization of a perched aquifer in the upper cone, as shown by the isotopic analyses.

Zonation of the main volcanic hazards (lava flows and ash fall) in Tenerife, Canary Islands. A proposal for a surveillance network

The island of Tenerife is volcanically complex, and its eruptive history predominantly reflects the processes and products of two different eruptive styles: (1) non-explosive effusions of basaltic lavas from fissure vents mostly aligned along two ridges; and (2) less frequent but explosive salic eruptions from central vents associated with the Las Canadas volcanic edifice and associated summit caldera. We have taken into account this fundamental distinction to develop a volcanic-hazards zonation (for lava flows and ash fall only) that includes: definition of the principal hazards; identification of the areas that have higher probability of containing emission centres; and numerical modelling of the vulnerable areas to be affected by volcanic hazards. Not only does the volcanic-hazards zonation map provide emergency-management officials with an updated assessment of the volcanic hazards, but it also represents a starting point for the preparation of a volcanic risk map for Tenerife. Finally, the hazards-zonation map also furnishes the basis for the design of a proposed volcano surveillance network.

Magnetotelluric study of the Teide (Tenerife) and Timanfaya (Tanzarote) volcanic areas

The application of a magnetotelluric surveying method, developed specifically for detection of strong conductivity contrasts, has permitted interpretations of the deep structure of two important volcanic formations in the Canary Archipelago: the Timanfaya (Lanzarote) and Teide (Tenerife) volcanoes. Beneath the Timanfaya volcano, a highly conductive body (p ≤ 1 Ω m) has been detected, which is interpreted as a shallow magma chamber associated with the eruption which took place between the years 1730 and 1736. The core of this intrusion, still largely molten, would be located at approximately 4 km depth and would still maintain temperatures in excess of 900 °C. In the Telde caldera the asymmetry of the apparent resistivity components indicates a double depression; furthermore, a highly conductive level encountered in the centre of the eastern part of the volcanic complex has been attributed to the persistance of relatively high temperatures in the root of the last salic dome, extruded inside the caldera. In both zones, the deep limiting horizon is a highly conductive layer corresponding to the mantle (15 km in Lanzarote and 13 km in Tenerife), whereas other, more superficial, discontinuities have been related to local structures.

Analysis and modelling of the aeromagnetic anomalies of Gran Canaria (Canary Islands)

Abstract In this paper we present a crustal magnetic model of the volcanic island of Gran Canaria based on aeromagnetic data. This magnetic study has made it possible to constrain the location and geometry of: (1) the mafic core of Gran Canaria, built as the result of the crystallization of magmas which rose from the mantle during the submarine and shield basaltic stages of its evolution. The most intense magnetic high, displayed over the NW part of the island likely shows the main feeding system of the shield volcano; (2) the residual syenitic shallow magma chamber which fed the salic volcanic activity in the center of the island, linked to the formation of the Tejeda caldera and to the intrusion of felsic rocks which made up a cone sheet; (3) a reversely magnetized linear intrusion located in the marine area to the NW of the island, which could be related to the magmatic source of the submarine volcanism between Gran Canaria and Tenerife. Most of these magnetic sources show a linear pattern with trends that are in close agreement with the orientations of previously identified fractures. The magmatic intrusion to the NW of the island could be related with a WSW–ENE active fault between Gran Canaria and Tenerife, while the main trend and location of the mafic core agree with a NW–SE fault suggested by geological studies. This means that these magnetic sources are the result of the ascent and intrusion of magma along regional fractures. Therefore, this study provides fresh data which demonstrate the influence of regional tectonics on the growth of Gran Canaria during its entire evolution. Finally, the spectral analysis of the magnetic anomaly map suggests that it is possible that rocks located at mantle-like depths (from the Moho to about 23 km) behave as magnetic sources, a fact that can be related with the magmatic underplating detected by other geophysical techniques.

A large volcanic debris avalanche in the Pliocene Roque Nublo Stratovolcano, Gran Canaria, Canary Islands

Abstract During the second magmatic cycle of Gran Canaria, between 5 and 3 Ma, the Roque Nublo Stratovolcano (RNS) achieved its almost total development, reaching a height of 2500–2600 m and spreading about 100 km 3 of products over a surface area of at least 250 km 2 . In the last growth stage of the RNS, and probably in connection with the paroxysmic genesis of the Roque Nublo explosive Caldera, a sector of the volcanic edifice suffered a gravitational collapse of significant importance. The data reported here show that the collapse was accompanied by a great avalanche: the Roque Nublo Debris Avalanche Deposit (RNDAD) composed mainly of block facies. The last explosive emissions of the RNS were restricted to ignimbrites and some explosive breccias that locally overlap the avalanche deposits. The features of the outcrops along the course of the avalanche make it possible to distinguish between the proximal, intermediate and distal deposits. The debris avalanche (3–4 km 3 ) moved SW then S, and finally SSE. Part of the deposit, mainly formed by large megablocks, remained very near the source area. Halfway along the course, the avalanche was channelled by broad and steep paleovalleys. In the most distal areas, the RNDAD opens into a fan-shape, reaching a distance of 28–30 km. A simplified kinematical physical model is used to understand the movement of the RNDAD. The model is based on the speculative reconstruction of the RNS edifice, based in turn on its scarce peripheral remains. An arbitrary avalanche path was selected, with a total travel distance from the source area (top of the stratovolcano) of 28 km. With these assumptions it is possible to calculate the speed at each point ( V m = 90 m/s) and other parameters that show that the RNDAD is very similar to other well-known volcanic avalanches.

InSAR volcano and seismic monitoring in Spain. Results for the period 1992–2000 and possible interpretations

Abstract In September 1999, we started a demonstration of the operational use of InSAR technology in the routine volcano monitoring procedures in Spain for the European Space Agency. This project was complemented with national funding to face the seismic activity. The effort focuses on the Canary Islands and the southern part of the Iberian Peninsula (volcanic and seismic cases, respectively) to supplement the traditional geodetic techniques used to date. We present the results obtained and the first interpretation of the obtained displacements. The research in volcanism is mainly focused on Tenerife Island, where a historical analysis with more than 20 images, from 1992 to 2000 has been done. The observed deformations (unknown to date) in the different interferograms are associated to the zones of the most recent activity in the island. In connection with seismic hazards we have studied displacements associated with the December 23, 1993 earthquake (magnitude Mb =5.0) produced between Adra and Berja cities. This earthquake was extensively felt in Spain, producing some damages on the epicentral zone with maximum intensity of VII (MSK). Our results clearly show the need for InSAR techniques to be included systematically as an operational component whenever possible. It would be a powerful and cost-effective way of supplementing the techniques normally used in geodetic monitoring.