NE Rift Research Articles

The Thermal Imbalances Recorded at the NE Rift during the 2012 Explosive Activity at the South East Cone (Mt. Etna, Italy)

Mild thermal anomalies are sensitive to change in the advection processes in a volcanic system. A mild thermal anomaly, near the top of the North-East Rift of Mt. Etna (Italy), has been monitored from January 2010 to September 2012 by means of four temperature sensors buried in the shallow ground. The pulses of the convective circulation have been tracked and the diffuse heat flux has been evaluated. The positive pulses of the convective front reflected the local increases of volcanic degassing; conversely, the negative pulses showed the contraction of the convective front emerging through the North-East Rift. The steam condensation depth fluctuated below the monitoring site, from depths of a couple of meters to more than 30 meters, while the New South-East crater was erupting. The data hourly recorded, relative to the 2012 eruptive period, were compared to the radiant energy released by the paroxysms. We registered a dramatic decrease in the diffuse heat flux several hours before the onset of the two most energetic paroxysms (12 and 23 April). Thereafter, the convective front (the steam condensation depth) showed many negative pulses, reaching the deepest recorded levels. Thermal transients could be one of the early signals, possibly heralding transitions in the dynamic equilibrium conditions.

Reconstruction of the subaerial Holocene volcanic activity through paleomagnetic and 14C dating methods: El Hierro (Canary Islands)

Volcanic hazard assessment relies on the accurate knowledge of the eruptive style and recurrence of volcanic eruptions in the past. At El Hierro (Canary Islands) historical and prehistorical records are still poorly defined, and although the island was the location of one of the most recent eruptions (La Restinga, 2011 CE) of the Canarian archipelago, the recent subaerial volcanism is still poorly studied. Information about the age of Holocene volcanic activity as well as the stratigraphy of the deposits is scarce: few eruptions are dated so far, whereas the others are classified as pre-or Holocene events considering lava flow characteristics along the coast.Here, we report on the dating of eleven (Mña Chamuscada, Mña del Tesoro, Orchilla, Las Calcosas, Mña Negra, Lomo Negro, Below Lomo Negro, Cuchillo del Roque, Malpaso Member, and Mña del Guanche) Holocene subaerial eruptions, distributed along the three rift zones, combining paleomagnetic and 14C methods. We also provide geochemical analyses for nine of them. Results indicate that Mña Chamuscada and Mña del Tesoro occurred more recently than previously considered, setting them within the last two thousand years. Conversely, paleomagnetic and 14C ages found for Lomo Negro eruption are consistent with literature data (Villasante-Marcos and Pavón-Carrasco, 2014) and constrain the occurrence of this event in the XVI century CE. Finally, for Malpaso Member deposits, the two 14C datings obtained by charcoals found below and above the trachytic layer set the eruption during the Holocene epoch, between ~7300 BCE and ~4700 BCE. For the other eruptions, in two cases (Orchilla and Las Calcosas) many possible time windows during the last 14 ka have been found, whereas a few possible ages have been obtained for the others. On the whole, the resulting chronological reconstruction of the recent activity of El Hierro indicates that eruptions occurred unevenly along the three main rifts, with nine eruptions in the WNW rift, six in the NE rift, and four in the SSE rift. We document at least two periods characterized by high eruptive frequency: an old one, between 8000 BCE and 1000 BCE, with eight eruptions, three of which characterized by more evolved compositions (phonotephrite and trachyte), and a recent one, between 1000 BCE and present day, with at least seven eruptions, mainly showing basanite compositions. The new data yield a significant improvement of Holocene eruption chronology, thus are instrumental for a correct evaluation of the volcanic hazard at El Hierro.

Modelling and simulation of a lava flow affecting a shore platform: a case study of Montaña de Aguarijo eruption, El Hierro (Canary Islands, Spain)

ABSTRACT Recent subaerial volcanism at El Hierro Island (Canary Islands, Spain) consists of monogenetic volcanic fields. This volcanism generated cinder cones, tephra air-fall deposits, and lava flows. The lava flows reach several kilometres in length extending through shore platforms and, sometimes, penetrating under the sea level. The volcanic landforms of El Hierro convert it into a natural laboratory for topographic and morphometric modelling and lava flow simulations. We perform the modelling and simulation of the Montaña de Aguarijo eruption, a cinder cone at the NE rift. The associated lava flow channelled through a V-shaped ravine until reaching a cliff, where formed cascades. The flow spread at the cliff base over a platform before reaching the sea modifying the coastline. Different maps were designed to show the results, including the geomorphologic reconstruction of the area affected by this eruption and the lava flow simulations obtained with the Q-LavHA plugin.

Mapping and evaluating kinematics and the stress and strain field at active faults and fissures: a comparison between field and drone data at the NE rift, Mt Etna (Italy)

Abstract. We collected drone data to quantify the kinematics at extensional fractures and normal faults, integrated this information with seismological data to reconstruct the stress field, and critically compared the results with previous fieldwork to assess the best practice. As a key site, we analyzed a sector of the northeast rift of Mt Etna, an area affected by continuous ground deformation linked to gravity sliding of the volcano's eastern flank and dike injections. The studied sector is characterized also by the existence of eruptive craters and fissures and lava flows. This work shows that this rift segment is affected by a series of NNE- to NE-striking, parallel extensional fractures characterized by an opening mode along an average N105.7∘ vector. The stress field is characterized by a σHmin trending northwest–southeast. Normal faults strike parallel to the extensional fractures. The extensional strain obtained by cumulating the net offset at extensional fractures with the fault heave gives a stretching ratio of 1.003 in the northeastern part of the study area and 1.005 in the southwestern part. Given a maximum age of 1614 CE for the offset lavas, we obtained an extension rate of 1.9 cm yr−1 for the last 406 years. This value is consistent with the slip along the Pernicana Fault system, confirming that the NE rift structures accommodate the sliding of the eastern flank of the volcano.

Multiple related flank collapses on volcanic oceanic islands: Evidence from the debris avalanche deposits in the Orotava Valley water galleries (Tenerife, Canary Islands)

Catastrophic flank collapses are common on oceanic islands of volcanic origin and often recognizable from the resulting morphology as large U-shaped embayments. However, post-collapse volcanic activity can infill such features, thereby obscuring them. This study takes advantage of a dense network of long sub-horizontal galleries for groundwater extraction in the flank collapse structure of the Orotava Valley (Tenerife, Canary Islands). This impressive landform is located in the overlapping zone of the NE Rift and the Cañadas Edifice of this volcanic island. Three debris avalanche deposits (DADs) have been identified inside the waterworks bored into the valley's western sector. The deeper layer, the Lower-DAD (L-DAD) was previously described under the local name of mortalón. This deposit lies unconformably over older volcanic rocks, where a prominent shear-zone is developed, here interpreted as the detachment plane of a massive rockslide. The L-DAD was therefore produced during the large failure event, the Orotava Landslide (OL), which carved the depression. Two younger DADs, much smaller in volume and not previously described, were also identified in the underground strata: the Intermediate- (I-DAD) and Upper-DAD (U-DAD). Both correlate well with the thick breccia deposits cropping out at the base of the marine cliff along the western coast of the valley. The I-DAD and U-DAD are conformably intercalated between the lava flows and other volcanics infilling the depression, and their bases are erosive/depositional features without structural deformation zones below them. Their stratigraphic setting and geometrical reconstruction indicate that both deposits were emplaced after the valley was formed by the OL event, in two successive failures here called Western Orotava Landslides (WOLs). The younger, called WOL-2, was dated at 494 ± 22 ka using the 40Ar/39Ar method. These two subaerial failures affected the upper eastern flank of the Cañadas III Edifice, previously destabilized when the OL failure event carved a steep wall at its foot, the western lateral escarpment of the Orotava Valley.

Inflation Leading to a Slow Slip Event and Volcanic Unrest at Mount Etna in 2016: Insights From CGPS Data

AbstractGlobal Positioning System (CGPS) data from Mount Etna between May 2015 and September 2016 show intense inflation and a concurrent Slow Slip Event (SSE) from 11 December 2015 to 17 May 2016. In May 2016, an eruptive phase started from the summit craters, temporarily stopping the ongoing inflation. The CGPS data presented here give us the opportunity to determine (1) the source of the inflating body, (2) the strain rate parameters highlighting shear strain rate accumulating along NE Rift and S Rift, (3) the magnitude of the SSE, and (4) possible interaction between modeled sources and other flank structures through stress calculations. By analytical inversion, we find an inflating source 5.5 km under the summit (4.4 km below sea level) and flank slip in a fragmented shallow structure accommodating displacements equivalent to a magnitude Mw6.1 earthquake. These large displacements reflect a complex mechanism of rotations indicated by the inversion of CGPS data for strain rate parameters. At the scale of the volcano, these processes can be considered precursors of seismic activity in the eastern flank of the volcano but concentrated mainly on the northern boundary of the mobile eastern flank along the Pernicana Fault and in the area of the Timpe Fault System.

Evidence of a shallow magma intrusion beneath the NE Rift system of Mt. Etna during 2013

AbstractContinuous GPS (CGPS) data, collected at Mt. Etna between April 2012 and October 2013, clearly define inflation/deflation processes typically observed before/after an eruption onset. During the inflationary process from May to October 2013, a particular deformation pattern localised in the upper North Eastern sector of the volcano suggests that a magma intrusion had occurred a few km away from the axis of the summit craters, beneath the NE Rift system. This is the first time that this pattern has been recorded by CGPS data at Mt. Etna. We believe that this inflation process might have taken place periodically at Mt. Etna and might be associated with the intrusion of batches of magma that are separate from the main feeding system. We provide a model to explain this unusual behaviour and the eruptive regime of this rift zone, which is characterised by long periods of quiescence followed by often dangerous eruptions in which vents can open at low elevation and thus threaten the villages in this sector of the volcano.

Discerning magmatic flow patterns in shallow-level basaltic dykes from the NE rift zone of Tenerife, Spain, using the Anisotropy of Magnetic Susceptibility (AMS) technique

Abstract The anisotropy of magnetic susceptibility (AMS) technique is a rapid petrophysical method used to infer magma flow directions within dykes as well as other igneous intrusions. Samples for AMS study were collected from dykes along the upper part of the NE Rift Zone (NERZ) of Tenerife, Canary Islands, Spain. Of the analysed dykes, 28 have interpretable normal magnetic fabrics. These 28 dykes are therefore suitable to assess the magma flow direction using the imbrication of the magnetic foliation plane from paired dyke margins and/or the overall trend and plunge of the magnetic lineations. AMS fabrics show downwards and upwards flow that could be related to flank and summit eruptions. Overall, however, the direction and sense of magma flow does not follow a specific trend across the NERZ, suggesting that the dykes are supplied by local shallow-level reservoir(s) underneath the ridge or are responding to variations in the local stress field across the axis of the rift zone. The variability of the AMS fabrics suggests a rather complicated propagation mode of magma within the dykes of the NERZ, contrasting with the common assumption of uniform magma propagation within rift zones. Our data therefore support the notion that magma propagation beneath active volcanic systems is inherently more complex than simple subvertical flow from source to final emplacement level, which bears on volcanic hazards worldwide.

Hydrothermal system of Central Tenerife Volcanic Complex, Canary Islands (Spain), inferred from self-potential measurements

An extensive self-potential survey was carried out in the central volcanic complex of Tenerife Island (Canary Islands, Spain). A total amount of ~237km of profiles with 20m spacing between measurements was completed, including radial profiles extending from the summits of Teide and Pico Viejo, and circular profiles inside and around Las Cañadas caldera and the northern slopes of Teide and Pico Viejo. One of the main results of this mapping is the detection of well-developed hydrothermal systems within the edifices of Teide and Pico Viejo, and also associated with the flank satellite M. Blanca and M. Rajada volcanoes. A strong structural control of the surface manifestation of these hydrothermal systems is deduced from the data, pointing to the subdivision of Teide and Pico Viejo hydrothermal systems in three zones: summit crater, upper and lower hydrothermal systems. Self-potential maxima related to hydrothermal activity are absent from the proximal parts of the NE and NW rift zones as well as from at least two of the mafic historical eruptions (Chinyero and Siete Fuentes), indicating that long-lived hydrothermal systems have developed exclusively over relatively shallow felsic magma reservoirs. Towards Las Cañadas caldera floor and walls, the influence of the central hydrothermal systems disappears and the self-potential signal is controlled by the topography, the distance to the water table of Las Cañadas aquifer and its geometry. Nevertheless, fossil or remanent hydrothermal activity at some points along the Caldera wall, especially around the Roques de García area, is also suggested by the data. Self-potential data indicate the existence of independent groundwater systems in the three calderas of Ucanca, Guajara and Diego Hernández, with a funnel shaped negative anomaly in the Diego Hernández caldera floor related to the subsurface topography of the caldera bottom. Two other important self-potential features are detected: positive values towards the northwestern Santiago rift, possibly due to the relatively high altitude of the water-table in this area; and a linear set of minima to the west of Pico Viejo, aligned with the northwestern rift and related to meteoric water infiltration along its fracture system.

Triggering mechanisms of static stress on Mount Etna volcano. An application of the boundary element method

In the last 30years, numerous eruptions and associated deformation episodes have occurred at Mt. Etna volcano. Datasets recorded by continuous monitoring of these episodes provide a unique opportunity to study the relationships between volcanism, flank instability and faulting activity. We have investigated the stress triggering mechanism between magmatic reservoir inflation, intrusive episodes and flank dynamics. Using three-dimensional numerical Boundary Elements Models we simulated volcano-tectonic events and calculated Coulomb stress changes. Using this modeling approach, we analyzed four realistic scenarios that are representative of recent kinematics occurring at Mt. Etna. The main results obtained highlight how (1) the inflation of a deep spherical magma source transfers elastic stress to a sliding plane and faults, (2) the opening of the NE Rift and S Rift (to a less efficient extent) favors movements of the instable sector and may encourage seismicity on the eastern flank faults, and (3) flank instability may trigger the uprising of magma. Defining the effects of the elastic stress transfer and relationships among the main forces acting on volcano, may help to forecast possible eruption scenarios during future episodes of unrest at Mount Etna and provide an important tool for decision makers during volcanic emergencies involving the highly populated areas of the volcano.

Understanding Etna flank instability through numerical models

As many active volcanoes, Mount Etna shows clear evidence of flank instability, and different mechanisms were suggested to explain this flank dynamics, based on the recorded deformation pattern and character. Shallow and deep deformations, mainly associated with both eruptive and seismic events, are concentrated along recognised fracture and fault systems, mobilising the eastern and south-eastern flank of the volcano. Several interacting causes were postulated to control the phenomenon, including gravity force, magma ascent along the feeding system, and a very complex local and/or regional tectonic activity. Nevertheless, the complexity of such dynamics is still an open subject of research and being the volcano flanks heavily urbanised, the comprehension of the gravitative dynamics is a major issue for public safety and civil protection. The present research explores the effects of the main geological features (in particular the role of the subetnean clays, interposed between the Apennine–Maghrebian flysch and the volcanic products) and the role of weakness zones, identified by fracture and fault systems, on the slope instability process. The effects of magma intrusions are also investigated. The problem is addressed by integrating field data, laboratory tests and numerical modelling. A bi- and tri-dimensional stress–strain analysis was performed by a finite difference numerical code (FLAC and FLAC3D), mainly aimed at evaluating the relationship among geological features, volcano-tectonic structures and magmatic activity in controlling the deformation processes. The analyses are well supported by dedicated structural–mechanical field surveys, which allowed to estimate the rock mass strength and deformability parameters. To take into account the uncertainties which inevitably occur in a so complicated model, many efforts were done in performing a sensitivity analysis along a WNW–ESE section crossing the volcano summit and the Valle del Bove depression. This was mainly devoted to evaluate the effect of topography, geometry and rheological behaviour of the structural units. The 3D numerical model, extended 40×60km, was implemented to simulate the volcano deformation pattern. First, the role of the Pleistocene subetnean clays was investigated, then, two “structural weakness zones” – the Pernicana Fault system and the NE rift – were introduced and their effects on the flank instability evaluated. Two extreme hydrogeological conditions, drained and undrained, were analysed. The results are expressed in terms of stress–strain field, displacement pattern, plasticity states and shear strain increments. Two main instability mechanisms were identified: one at shallow depth, with the sliding surface located inside the subetnean Quaternary clay, and another deep-seated mechanism with a not continuous and less evident sliding surface, developed inside the Apennine–Maghrebian Chain flysch, bordered by active structures. Both mechanisms contribute to explain the present deformation pattern and some of the main structures of the Etna flank. The effect of magma pressure exerted on the active dyke walls during eruptions was then simulated and relations between magmatic activity and flank instability were preliminarily investigated.

Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

Many oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North–East Rift Zone (NERZ) of Tenerife Island, Spain of ...

Flank instability structure of Mt. Etna inferred by a magnetotelluric survey

This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt. Etna. We take thirty soundings along two profiles oriented in the N‐S and NW‐SE directions, and from these data recover two 2D resistivity models of the subsurface. Both models reveal three major layers in a resistive‐conductive‐resistive sequence, the deepest extending to 14 km bsl. The shallow layer corresponds to the volcanic cover, and the intermediate conductive layer corresponds to underlying sediments segmented by faults. These two electrical units are cut by ∼ E‐W‐striking faults. The third layer (basement) is interpreted as mainly pertinent to the Apennine‐Maghrebian Chain associated with ∼ SW‐NE‐striking regional faults. The detailed shapes of the resistivity profiles clearly show that the NE Rift is shallow‐rooted (∼0–1 km bsl), thus presumably fed by lateral dikes from the central volcano conduit. The NW‐SE profile suggests by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Toward the SE, the resistive basement dramatically dips (from ∼3 km to ∼10 km bsl), in correspondence with the Timpe Fault System. Several high‐conductivity zones close to the main faults suggest the presence of hydrothermal activity and fluid circulation that could enhance flank instability. Our results provide new findings about the geometry of the unstable Etna flank and its relation to faults and subsurface structures.

Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada

The Archean Blake River Group of the Abitibi greenstone belt represents a megacaldera complex which evolved over 8–11M.y. from approximately 2704 to 2696Ma. The early Misema Caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems. These remnants are found in the Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill and Colnet regions, where summit calderas are delineated by circular ring dyke structures. The secondary 330°-trending New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. Finally, the felsic-dominated, 070°-striking Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex.Deformation has overprinted the calderas, but structural patterns can be used in order to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic dome-flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator calderas. Felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution. The Misema Caldera formed between 2704 and 2702Ma via the amalgamation of shield volcanoes that were probably active prior to 2704Ma. The New Senator Caldera was generated between 2702 and 2700Ma during paroxysmal felsic volcanism, followed by the collapse of the Noranda Caldera culminating between 2700–2696Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture, whereas the SE-trending New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stages of megacaldera evolution.

Evolution of ocean-island rifts: The northeast rift zone of Tenerife, Canary Islands

The northeast rift zone of Tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone. Field geology, isotopic dating, and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the NE rift zone, which includes a period of very fast growth toward instability (between ca. 1.1 and 0.83 Ma) followed by three successive large landslides: the Micheque and Guimar collapses, which occurred approximately contemporaneously at ca. 830 ka and on either side of the rift, and the La Orotava landslide (between 690 +/- 10 and 566 +/- 13 ka). Our observations suggest that Canarian rift zones show similar patterns of development, which often includes overgrowth, instability, and lateral collapses. Collapses of the rift flanks disrupt established fissural feeding systems, favoring magma ascent and shallow emplacement, which in turn leads to magma differentiation and intermediate to felsic nested eruptions. Rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes. Conversely, the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars, even if concealed by postcollapse volcanism. Comparative analysis of the main rifts in the Canary Islands outlines this general evolutionary pattern: (1) growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions; (2) flank collapse and catastrophic disruption of the established feeder system of the rift; (3) postcollapse centralized nested volcanism, commonly evolving from initially ultramafic-mafic to terminal felsic compositions (trachytes, phonolites); and (4) progressive decline of nested eruptive activity.

Detachment depth revealed by rollover deformation: An integrated approach at Mount Etna

Flank instability is common at volcanoes, even though the subsurface structures, including the depth to a detachment fault, remain poorly constrained. Here, we use a multidisciplinary approach, applicable to most volcanoes, to evaluate the detachment depth of the unstable NE flank of Mt. Etna. InSAR observations of Mount Etna during 1995–2008 show a trapdoor subsidence of the upper NE flank, with a maximum deformation against the NE Rift. The trapdoor tilt was highest in magnitude in 2002–2004, contemporaneous with the maximum rates of eastward slip along the east flank. We explain this deformation as due to a general eastward displacement of the flank, activating a rotational detachment and forming a rollover anticline, the head of which is against the NE Rift. Established 2D rollover construction models, constrained by morphological and structural data, suggest that the east‐dipping detachment below the upper NE flank lies at around 4 km below the surface. This depth is consistent with seismicity that clusters above 2–3 km below sea level. Therefore, the episodically unstable NE flank lies above an east‐dipping rotational detachment confined by the NE Rift and Pernicana Fault. Our approach, which combines short‐term (InSAR) and long‐term (geological) observations, constrains the 3D geometry and kinematics of part of the unstable flank of Etna and may be applicable and effective to understand the deeper structure of volcanoes undergoing flank instability or unrest.

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

In this paper we investigate ground motion properties in the western part of the Pernicana fault. This is the major fault of Mount Etna and drives the dynamic evolution of the area. In a previous work, Rigano et al. (2008) showed that a significant horizontal polarization characterizes ground motion in fault zones of Mount Etna, both during earthquakes and ambient vibrations. We have performed denser microtremor measurements in the NE rift segment and in intensely deformed zones of the Pernicana fault at Piano Pernicana. This study includes mapping of azimuth‐dependent horizontal‐to‐vertical spectral ratios along and across the fault, frequency–wave number techniques applied to array data to investigate the nature of ambient vibrations, and polarization analysis through the conventional covariance matrix method. Our results indicate that microtremors are likely composed of volcanic tremor. Spectral ratios show strong directional resonances of horizontal components around 1 Hz when measurements enter the most damaged part of the fault zone. Their polarization directions show an abrupt change, by 20° to 40°, at close measurements between the northern and southern part of the fault zone. Recordings of local earthquakes at one site in the fault zone confirm the occurrence of polarization with the same angle found using volcanic tremor. We have also found that the directional effect is not time‐dependent, at least at a seasonal scale. This observation and the similar behavior of volcanic tremors and earthquake‐induced ground motions suggest that horizontal polarization is the effect of local fault properties. However, the 1‐Hz resonant frequency cannot be reproduced using the 1‐D vertically varying model inferred from the array data analysis, suggesting a role of lateral variations of the fault zone. Although the actual cause of polarization is unknown, a role of stress‐induced anisotropy and microfracture orientation in the near‐surface lavas of the Pernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone.

La dorsal NE de Tenerife: hacia un modelo del origen y evolución de los rifts de islas oceánicas

The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evi-dence of the origin and dynamics of these major volcanic features. The rift developed in three successi-ve, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quies-cence or reduced activity: A Miocene stage (7266 ± 156 ka), apparently extending the central Mioceneshield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710 ± 58 ka) and the lateststage, with the main eruptive phase in the Pleistocene.Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, andradioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of riftactivity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis ofthe rift and show reverse polarity (corresponding to the Matuyama chron). Dykes are of normal andreverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes areconsistently of normal polarity (Brunhes chron). Volcanic units of normal polarity crossed by dykes ofnormal and reverse polarities yield ages apparently compatible with normal subchrons (M-B Precursorand Jaramillo) in the Upper Matuyama chron. Three lateral collapses successively mass-wasted the rift:The Micheque collapse, completely concealed by subsequent nested volcanism, and the Guimar and LaOrotava collapses, that are only partially filled. Time occurrence of collapses in the NE rift apparentlycoincides with glacial stages, suggesting that giant landslides may be finally triggered by sea level chan-ges during glaciations. Pre-collapse and nested volcanism is predominantly basaltic, except in theMicheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions.Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial, plume-rela-ted fractures acting throughout the entire growth of the islands. Basaltic volcanism forms the bulk of theislands and rift zones. However, collapses of the flanks of the rifts disrupt their established fissural feedingsystem, frequently favouring magma accumulation and residence at shallow emplacements, leading to diffe-rentiation of magmas, and intermediate to felsic nested eruptions. Rifts and their collapse may therefore act asan important factor in providing petrological variability to oceanic volcanoes. Conversely, the possibility existsthat the presence of important felsic volcanism may indicate lateral collapses in oceanic shields and ridge-likevolcanoes, even if they are concealed by post-collapse volcanism or partially mass-wasted by erosion.

Gravity structure of Piton de la Fournaise volcano and inferred mass transfer during the 2007 crisis

The structure and mass transfers at Piton de la Fournaise volcano are investigated using new on land and offshore measurements. The structures are defined using 2 and 3D modelling. Short-wavelength positive anomalies to the W and NW of the central area are interpreted as shallow piles of thick, dense lava flows which have filled volcano-tectonic depressions, several hundreds of meters deep. This finding provides a new insight into the evolution of the volcano. Conversely, the currently active central zone is associated with a negative short-wavelength anomaly, explained by the presence of thin, highly vesiculated and fractured lava flows building the Central Cone. Low density formations underlying the Rivières des Remparts and Langevin areas are tentatively interpreted as breccias related to erosion and/or to southward directed mass-wasting. Topographic offshore highs in the continuation of the SE and NE rift zones of Piton de la Fournaise are characterized by negative gravity anomalies and reversed magnetic anomalies. Their interpretation as hyaloclastites and pillow lavas provides a coherent explanation for both types of signals if we admit that they were primarily constructed during the Alizés stage. Two main deeper, dense structures are also found: the Grand Brûlé complex, and a complex beneath the Plaine des Sables and the Enclos Fouqué. Both are identified as hypovolcanic intrusive complexes. The Grand Brûlé complex (10 × 18 km in plan view) shows a marked NS-elongated geometry, unlike the other comparable complexes on the island. The Plaine des Sables-Enclos Fouqué complex, a more or less circular structure 5 km in diameter, is attributed to the Ancient Shield stage. It is centered on the Enclos fault, suggesting that the paleo-center of the Ancient Shield was located about 1.5 km eastward of the previously proposed location. An apparent contradiction was noted between the seismic and gravity images of the internal structure. In particular, a high velocity cylinder beneath the summit was not detected in the gravity models. This contradiction was resolved by considering the relative sensitivity of each method. Finally, the gravity dataset has allowed us to explore the gravity changes that accompanied the major eruptive and volcano-tectonic crisis of April 2007 (350 m collapse of the summit and emission of ∼ 120 × 10 6 m 3 of lava). The change in the mass distribution of the edifice was large enough to be detected in the survey data collected in the central area before and after the crisis. However, the accuracy was too low for the precise assessment of the mass transfers.

The contemporaneous emission of low-K and high-K trachybasalts and the role of the NE Rift during the 2002 eruptive event, Mt. Etna, Italy

Mount Etna volcano erupted almost simultaneously on its northeastern and southern flanks between October 27 and November 3, 2002. The eruption on the northeastern flank lasted for 8 days, while on the southern flank it continued for 3 months. The northeastern flank eruption was characterized by the opening of a long eruptive fracture system between 2,900 and 1,900 m.a.s.l. A detailed survey indicates that the fractures’ direction shifted during the opening from N10W (at the NE Crater, 2,900 m) to N45E (at its lowest portion, 1,900 m) and that distinct magma groups were erupted at distinct fracture segments. Based on their petrological features, three distinct groups of rocks have been identified. The first group, high-potassium porphyritic (HKP), is made up of porphyritic lavas with a Porphyritic Index (P.I.) of 20–32 and K2O content higher than 2 wt%. The second group is represented by lavas and tephra with low modal phenocryst abundance (P.I. < 20) named here oligo-phyric (low-phyric), and K2O content higher than 2 wt% (HKO, high-potassium oligophyric). The third group, low-potassium oligophyric (LKO), consists of tephra with oligophyric texture (P.I. < 20) but K2O content < 2 wt%. K-rich magmas (HKP and HKO) are similar to the magma erupted on the southern flank, and geochemical variations within these groups can be accounted for by a variable degree of fractionation from a single parent magma. The K-poor magma (LKO), erupted only in the upper segment of the fracture, cannot be placed on the same liquid line of descent of the HK groups, and it is similar to the magmas that fed the activity of Etna volcano prior to the eruption of 1971. This is the first time since then that a magma of this composition has been documented at Mt. Etna, thus providing a strong indication for the existence of distinct batches of magma whose rise and differentiation are independent from the main conduit system. The evolution of this eruption provides evidence that the NE Rift plays a very active role in the activity of Mt. Etna volcano, and that its extensional tectonics allows the intrusion and residence of magma bodies at various depths, which can therefore differentiate independently from the main open conduit system.