Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands

The relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented. Our field data, aerial image analysis, and ⁴⁰Ar/³⁹Ar chronology from Anaga basaltic shield volcano on Tenerife, Canary Islands, support a rift zone—flank instability relationship. A single rift zone dominated the early stage of the Anaga edifice (~6–4.5 Ma). Destabilization of the northern sector led to partial seaward collapse at about ~4.5 Ma, resulting in a giant landslide. The remnant highly fractured northern flank is part of the destabilized sector. A curved rift zone developed within and around this unstable sector between 4.5 and 3.5 Ma. Induced by the dilatation of the curved rift, a further rift-arm developed to the south, generating a three-armed rift system. This evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift, and facilitates dike intrusions on the opposite side. Our study demonstrates a feedback relationship between flank instability and intrusive development, a scenario probably common in ocean island volcanoes. We therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability.Editorial responsibility: T. DruittThis revised version was published online in February 2005 with typographical corrections and a changed wording.     

Dykes, faults and palaeostresses in the Teno and Anaga massifs of Tenerife (Canary Islands)

A structural field study was made of 578 sheet intrusions (mostly dykes) and 153 (mostly normal) faults dissecting the Anaga and Teno massifs, where a complex volcanic succession of Tertiary age (the ‘Old Basaltic Series’) representing the shield-building stage of Tenerife (Canary Islands) crops out. Many of the intrusions, mostly sub-vertical mafic dykes, are emplaced by multiple magma injections, with cumulative thicknesses mostly less than 2 m. Dyke tips are exposed and preserved for 12% of the dykes. Three differently oriented sets of dykes exist in the Anaga massif (NNW–SSE, NNE–SSW, E–W), whereas there is only one main set in Teno, trending NNW–SSE. Dyke swarms and other structural features having similar orientations also exist in other Canary Islands. A minimum value of the horizontal component of extension induced by dykes is computed using a step of 5° of azimuth, accounting also for the dip of dykes. The cumulative crustal dilation is at least 300 m (4%) in Anaga and 270 m (6%) in Teno; the maximum extension peaks at N75° in Anaga and N60° in Teno, indicating a general prevailing extension in direction ENE–WSW. Most of the measured faults are normal and strike NNW–SSE. Computation of palaeostresses from inversion of fault-slip data sets suggests the existence of a polyphase brittle deformation due to an extensional stress field with the minimum compressive principal axes trending NE–SW and WNW–ESE.     

Groundwater behaviour in Tenerife,volcanic island (Canary Islands,Spain)

Tenerife is the largest of the seven Canary Islands, encompassing an area of 2,058 km². It is situated in the Atlantic Ocean between 16–17°W longitude and 28–29°N latitude. The topography of the island is characterized by generally steep slopes. The Teide Volcano has an elevation of 3,718 m. Precipitation is caused mainly by invasions of maritime polar air. Maximum mean precipitation recorded for 25-year period (1940–1965) is 1,000 mm.The fractured volcanic aquifer of the Old Basaltic Series is the main supplier of groundwater in Tenerife. Smaller quantities of groundwater are supplied by the Cañadas Series and minor amounts by alluvial sediments. Groundwater compartments develop in areas of dikes and contacts between permeable and impermeable zones. These compartments are irregular in volume, shape, and structure. The groundwater system forms a tortuous chain of compartments. Water circulates from one groundwater compartment to another through secondary fractures and other permeable elements which branch and intersect. Fractures which extend to the surface play an important role in recharge.The hydrologic system at Tenerife is characterized by three zones: the upper vadose, the lower vadose, and the saturated zone. In both the upper and lower vadose zones the dominant direction of flow is vertical, while in the saturated zone flow is generally oblique toward the sea.     

Volcanic evolution of the island of Tenerife (Canary Islands) in the light of new K-Ar data

New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma, and a late cycle around 3.6 Ma. In Teno (NW), after some undated units, the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S), the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion.The large Cañadas volcano, made up of basalts, trachytes and phonolites, was built essentially between 1.9 and 0.2 Ma. To the NE of this central volcano, linking it with Anaga, is a chain of basaltic emission centers, with a peak of activity around 0.8 Ma. The Cañadas Caldera had several collapse phases, associated with large ignimbrite emissions. There were, at least, an older phase more than 1 Ma old, on the western part of the volcano, and a younger one, less than 0.6 Ma old, in the eastern side. The two large “valleys” of Guimar and la Orotava were formed by large landslides less than 0.8 Ma ago, and probably before 0.6 Ma ago. The present Cañadas caldera was formed by another landslide, less than 0.2 Ma ago. This caldera was later filled by the huge Teide volcano, which has been active even in historic times. During the same period a series of small volcanoes erupted at scattered locations throughout the island.The average eruptive rate in Tenerife was 0.3 km³/ka, with relatively small variations for the different eruptive periods. This island and La Gomera represent a model of growth by discontinuous pulses of volcanic activity, separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years. The neighbouring Gran Canaria, on the other hand, had an initial, rapid “shield-building phase” during which more than 90% of the island was built, and a series of smaller pulses at a much later period.A comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation. Several gaps are present on each island, overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the Canaries.     

Causes and mobility of large volcanic landslides: application to Tenerife, Canary Islands

Giant volcanic landslides are one of the most hazardous geological processes due to their volume and velocity. Since the 1980 eruption and associated debris avalanche of Mount St. Helens hundreds of similar events have been recognised worldwide both on continental volcanoes and volcanic oceanic islands. However, the causes and mobility of these enormous mass movements remain unresolved. Tenerife exhibits three voluminous subaerial valleys and a wide offshore apron of landslide debris produced by recurrent flank failures with ages ranging from Upper Pliocene to Middle Pleistocene. We have selected the La Orotava landslide for analysis of its causes and mobility using a variety of simple numerical models. First, the causes of the landslide have been evaluated using Limit Equilibrium Method and 2D Finite Difference techniques. Conventional parameters including hydrostatic pore pressure and material strength properties, together with three external processes, dike intrusion, caldera collapse and seismicity, have been incorporated into the stability models. The results indicate that each of the external mechanism studied is capable of initiating slope failures. However, we propose that a combination of these processes may be the most probable cause for giant volcanic landslides. Second, we have analysed the runout distance of the landslide using a simple model treating both the subaerial and submarine parts of the sliding path. The effect of the friction coefficient, drag forces and hydroplaning has been incorporated into the model. The results indicate that hydroplaning particularly can significantly increase the mobility of the landslide, which may reach runout distances greater than 70 km. The models presented are not considered definite and have mainly a conceptual purpose. However, they provide a physical basis from which to better interpret these complex geologic phenomena and should be taken into account in the prediction of future events and the assessment of landslide related hazards.     

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.     

Cooling dynamics of spatter-fed phonolite obsidian flows on Tenerife,Canary Islands

Relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on Tenerife to determine their thermal history across the glass transition. The phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits. The detailed sampling resolution within the two series enabled an accurate quantification of their thermal history. Cooling rates within the investigated spatter-fed flows vary over more than two orders of magnitude. The highest cooling rates of 0.39 K/min were modeled for the central vesiculated part of one flow. The dense basal obsidian layers of both flows were cooled at substantially lower rates of 0.0042 and 0.0028 K/min, respectively. There appears to be an influence of in-situ vesiculation processes on the thermal budget of the investigated flows. In addition, the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering. Continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with (quasi-) isothermal annealing within this “décollement”. Numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows. The effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data. In addition, viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition.     

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.     

Occurrence of shallow earthquakes following periods of intense rainfall in Tenerife, Canary Islands

The question as to whether there is a seasonality in the occurrence of local seismic activity in the volcanic island of Tenerife, and if it could be associated to intense rainfall events is addressed. Analogue records from the TFMB seismic station and records on the daily precipitation at the Izaña Meteorological Observatory for the period December 1987–October 1992 were used to check this question. Statistical analyses show a non-random component in the temporal distribution of local microearthquakes at greater than 99% confidence, and a relatively strong contemporaneous correlation with intense rainfall periods. If the suggested correlation is confirmed by further investigations, it will allow better identification and discrimination of local seismic events that could be associated with volcanic activity, and thereby increase the performance of surveillance measures.     

Optimum station distribution to monitor seismic activity of Teide Volcano,Tenerife, Canary Islands

Optimization and location capability methods have been applied to the design of a seismic monitoring network composed of five stations to be installed around Teide-Pico Viejo stratovolcano in Tenerife, Canary Islands.In terms of location errors coming from the relative spatial distribution of seismic stations and hypocenters, the optimum network provides an improvement in epicentral location capability of 60% compared with a preliminary choice of five stations, while its improvement in focal depth determination is between 30% and 60%, depending on the average focal depth of seismic activity. Network enlargement to six stations, adding station CTFE from the Spanish National Geographic Institute (IGN), gives an improvement of 20% with respect to the optimum configuration.The interest and importance of previous planning on optimum distribution of seismic stations have become evident, as well as the need to enlarge the network in the near future, to attain more accurate focal depths.     

3-D Magnetotelluric Exploration of Tenerife Geothermal System (Canary Islands,Spain)

The resistivity structure of the Tenerife geothermal system has been determined by the 3-D inversion of data from different magnetotelluric surveys. In this paper, the ocean and topography effects on the magnetotelluric data were investigated by constructing a 3-D conceptual geoelectrical model of the island. The study showed that these effects should be taken into account in order to obtain a reliable subsurface model of the island. Data from 148 sites were used during three-dimensional inversion. The most interesting feature in the final geoelectrical model of the geothermal system is a low resistivity structure (<10 Ωm) above the resistive core of the system. The low resistivity structure has been interpreted as a hydrothermal clay alteration cap typically generated in the conventional geothermal systems. The resistivity model has been correlated with a recent seismic velocity model, showing that a low resistivity structure surrounds an area with high P wave velocity and medium–high resistivity. This medium–high resistivity area can be associated with a slowly solidified magma and, therefore, with a hotter part of the geothermal system.     

Underground Temperature Measurements as a Tool for Volcanic Activity Monitoring in the Island of Tenerife, Canary Islands

The spatial distribution of groundwater temperatures in the volcanic island of Tenerife, Canary Islands, has been inferred through measurements of water temperatures collected in the vast network of wells and subhorizontal tunnels, locally called “galleries,” which constitutes the main water supply of the island. The spatial coverage of the network of galleries allows us to reach from depth almost any geological feature of the island. The complex spatial distribution of temperatures in the interior of Tenerife is the result of the complex geological evolution of the island. Groundwater temperatures are greatly affected by groundwater flow and are considerably warmer in those galleries located in areas where water circulation is reduced due to the low permeability of materials and/or to the low infiltration rate of cooling meteoric water. In this sense, groundwater temperature should be characterized in quiescent conditions (background level), in order to facilitate monitoring changes in heat flow, such as those induced by ascending gases expected with an increase in volcanic activity.     

New sonar evidence for recent catastrophic collapses of the north flank of Tenerife, Canary Islands

Previous sonar surveys show that the north flank of Tenerife has been subject to at least four major landslides during the past 1 Ma. The youngest, Icod, affected the region to the north of the Teide-Pico Viejo complex, the world's third highest oceanic volcano. Recently, we obtained the first detailed acoustic images of Icod using a deep-tow side-scan sonar. The images suggest that Tenerife's north flank has experienced at least two types of flow deposit in the recent past. The older flow deposit, Icod I, is characterised by a 15- to 20-km-wide, >65-km-long, chaotic debris avalanche deposit which includes several very large blocks. We believe the deposit to be ~170 ka, and that it represents the mass-wasting products of the Cañadas edifice, remnants of which are now found in the Las Cañadas caldera wall. The younger flow deposit, Icod II, associated with a shute in its proximal part, appears to have produced a less chaotic deposit in its distal part which clearly preserves flow structures such as latitudinal boulder ridges and longitudinal shear structures. The sonar images cannot determine how much younger Icod II is than Icod I, although it is likely that they are a consequence of the same lateral collapse event. There is evidence from the shute area for erosional scour and sediment deposition since the Icod landslide. If this is correct, then it suggests that mass wasting is an ongoing process that has already started to modify the Teide-Pico Viejo complex itself.     

Internal structure of Tenerife (Canary Islands) based on gravity, aeromagnetic and volcanological data

Gravity and magnetic methods have been applied to the Tenerife Island, to provide new information about its internal structure. For this study, 365 gravity stations covering the central part of the island have been selected. The anomalous density maps at different depths were obtained by means of an inversion global adjustment, on fixed density contrast, to describe the three-dimensional (3D) geometry of the anomalous bodies. On the other hand, several analysis techniques, such as reduction to the pole, spectral analysis, low-pass filtering, terrain correction and forward modelling, were applied to process the high-resolution data obtained in an aeromagnetic survey, completed with marine and terrestrial data.The joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island. A strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of Tenerife. The sources of the observed gravity highs from 8 km b.s.l. may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic.The long-wavelength magnetic anomalies reveal highly magnetic sources, interpreted as gabbro-ultramafic cumulates associated with the root zone of a large dyke swarm. This intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5.7±0.8 km depth. Rooted in this highly magnetic zone, two dike–like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes. A shallow magnetic horizon (1.4 km a.s.l.) can be correlated with the bottom phonolites of the Las Cañadas Edifice.In the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low-density and low-magnetic materials, that infill a collapsed caldera system. The structures close to the surface are characterised by low-density areas connected with the recent volcanism, in particular the minimum over the Teide volcano. Hydrothermal alteration is assumed to be the cause of a short-wavelength magnetic low over the Teide volcano.     

Phreatomagmatic to Strombolian eruptive activity of basaltic cinder cones: Montaña Los Erales, Tenerife, Canary Islands

Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.     

Ar–Ar ages and geochemistry of the basaltic shield stage of Tenerife, Canary Islands, Spain

We report the first ³⁹Ar–⁴⁰Ar ages from the three early basic shield-like massifs of Tenerife, Canary islands, and couple these with detailed major and trace element chemistry to constrain the nature and timing of the mantle melting processes. The massifs have chemically different sources, and independent evolutionary histories. The Teno and Roque del Conde massifs appear chemically to represent the products of single mantle melting cycles, with progressive decrease in mean melt fraction and increase in mean melting depth in younger rocks. The Teno massif (NW) was erupted in a short time period around 6.0–6.4 Ma, while at least the lower half of the Roque del Conde massif (SW) is older than 11 Ma. In contrast, the Anaga massif (NE) is polygenetic, with ³⁹Ar–⁴⁰Ar ages ranging from 8.0–4.2 Ma, and no simple stratigraphic chemical progression. These ages run counter to published suggestions of progressive younging of Canary shield stages to the southwest. Basic rocks in all three massifs are the result of much deeper melting and smaller melt fractions than equivalent units in Gran Canaria, but nevertheless the melting column must have extended significantly into the spinel facies, requiring substantial disruption of the local lithosphere. The age and melting relationships broadly support the mantle blob model for Canary magmatism proposed by Hoernle and Schmincke (Hoernle, K., Schminke, H.-U., 1993. The role of partial melting in the 15-Ma geochemical evolution of Gran Canaria: a blob model for the Canary hotspot. J. Petrol. 34, 599–626). In all three massifs, extensive fractional crystallisation has taken place at crustal levels so that mean MgO contents are only some 6–7%. The fractionation sequence is olivine–clinopyroxene–magnetite in basaltic compositions, with the involvement of plagioclase, amphibole and apatite only to generate the infrequent more evolved hawaiites to benmoreites. Despite the abundance of basanitic magmas in the Tenerife older massifs, these follow a differentiation trend towards weakly undersaturated benmoreite rather than to phonolite. This probably reflects early crystallisation of magnetite, perhaps resulting from somewhat high oxygen fugacity. The chemical evidence for replenished magma chambers in Tenerife described by Neumann et al. (Neumann, E.R., Wulff-Oedersen, E., Simonsen, S.L., Pearson, N.J., Martí, J., Mitjavila, J., 1999. Evidence for fractional crystallisation of periodically refilled magma chambers in Tenerife, Canary Islands. J. Petrol. 40, 1089–1123) is a consequence of treating as a single cogenetic suite the products of several magmatic systems that differ in parental melt fraction.     

Stratigraphy, structure, and volcanic evolution of the Pico Teide–Pico Viejo formation, Tenerife, Canary Islands

The structure and volcanic stratigraphy of the Pico Teide–Pico Viejo (PT–PV) formation, deriving from the basanite–phonolite stratovolcanoes PT and PV, and numerous flank vent systems, are documented in detail based on new field and photogeologic mapping, geomorphologic analysis, borehole data, and petrological and geochemical findings. Results provide insight into the structure and evolution of the PT–PV magma system, and the long-term, cyclic evolution of Tenerife's post-shield volcanic complex. The PT–PV formation comprises products of central volcanism, mainly emplaced into the Las Cañadas caldera (LCC), and contemporaneous products from adjacent rifts. PT–PV central volcanic products become more differentiated up-section with felsic lavas dominating the recent output of the system. This is attributed to the evolution of a shallow magma reservoir beneath PT that was emplaced early in the PT–PV cycle on the intra-caldera segment of Tenerife's post-shield rift system. The rift axis has been the focus of PT–PV intrusive and eruptive activity, and has controlled the location of the stratocones. The current geometry of the rifts reflects a major structural reorganisation defining the start of the PT–PV cycle at 0.18 Ma, namely the truncation of the north side of the LCC/LCE by the giant Icod landslide. The internal stratigraphy of the PT–PV formation suggests that PT developed early, with PV developing as a satellite vent. Activity has since alternated between PT and PV due to episodes of vent blockage or chamber sealing. These processes have allowed significant volumes of phonolitic magmas to develop and accumulate within the PT chamber, which have vented through radial dike systems during tumescence episodes and from the rift system, which has permitted lateral magma transport. The PT–PV magma system is a potentially hazardous source of future, felsic eruptive activity on Tenerife.     

Instantaneous dynamic pressure effects on the behaviour of lithic boulders in pyroclastic flows: the Abrigo Ignimbrite,Tenerife, Canary Islands

A method for estimating the instantaneous dynamic pressure near the base of ancient pyroclastic flows, using large lithic boulders from the late Pleistocene Abrigo Ignimbrite, is proposed here. The minimum instantaneous dynamic pressure is obtained by determining the minimum aerodynamic drag force exerted by a pyroclastic flow onto a stationary boulder that will allow the boulder to overcome static friction with the underlying substrate, and move within the flow. Consideration is given to the properties of the boulder (shape, roughness, size, density and orientation relative to the flow), substrate (type and hill slope angle), boulder-substrate interface (looseness of boulder, coefficient of static friction) and flow (coefficient of aerodynamic drag). Nineteen boulders from massive, lithic-rich ignimbrite deposits at two localities on Tenerife were assessed in this study. Minimum dynamic pressures required for Abrigo pyroclastic flows to move these boulders ranged from 5 to 38 kPa, which are comparable to dynamic pressures previously calculated from observations of the damage caused by recent pyroclastic flows. Considering the maximum possible range in flow density, the derived minimum velocity range for the Abrigo pyroclastic flows is 1.3 to 87 m s⁻¹.     

Precursory Subsurface 222Rn and 220Rn Degassing Signatures of the 2004 Seismic Crisis at Tenerife, Canary Islands

Precursory geochemical signatures of radon degassing in the subsurface of the Tenerife Island were observed several months prior to the recent 2004 seismic-volcanic crisis. These premonitory signatures were detected by means of a continuous monitoring of ²²²Rn and ²²⁰Rn activity from a bubbling CO₂-rich gas spot located at 2.850 m depth inside a horizontal gallery for groundwater exploitation at Tenerife. Multivariate Regression Analysis (MRA) on time series of the radon activity was applied to eliminate the radon activity fluctuation due to external variables such as barometric pressure, temperature and relative humidity as well as power supply. Material Failure Forecast Method (FFM) was successfully applied to forecast the anomalous seismicity registered in Tenerife Island in 2004. The changes in the ²²²Rn/²²⁰Rn ratio observed after the period of anomalous seismicity might suggest a higher gas flow rate and/or changes in the vertical permeability induced by seismic activity.