A. Delcamp

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.

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Réunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en échelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Réunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active.

Craters of elevation revisited: forced-folds, bulging and uplift of volcanoes

The eighteenth/nineteenth century ‘craters of elevation’ theory required magma to uplift strata, doming the surface and creating a central down-fallen ‘crater’ or graben. Exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes, and rapidly the contemporary competing ‘craters of eruption’ theory replaced it as the paradigm for volcano construction. Several historic examples have shown that intrusions can cause uplift, termed bulges and can create features like those proposed for craters of elevation (e.g. at Usu 1944, Bezymianny 1955 and Mt. St. Helens 1980). Work on sedimentary basins that have had igneous activity has shown that intrusions create ‘forced folds’ that uplift and deform strata in a similar way to that originally proposed for craters of elevation. In view of the above, we investigate large-scale intrusion-related topographic changes at two sites where the craters of elevation theory was developed: the monogenetic volcanoes of the Chaîne des Puys, France and the Teide stratovolcano, Tenerife. We combine observations of such features with examples of forced folding to integrate the two fields of research. Our observations in the Chaîne des Puys show that: (1) the Petit Puy de Dôme has a bulge of up to 150-m uplift. The uplift has a central depressed area (a graben), a dense network of normal faults, basal thrusts and an aborted landslide. (2) The Grosmanaux volcano is a forced fold created by uplift of a previously flat-lying area, and has dense faulting and a graben on the resultant topographic bulge. It was the site also of a major vulcanian eruption from the associated Kilian crater. (3) The Gouttes volcano was uplifted by an intrusion like the Petit Puy de Dôme, but then collapsed to generate a landslide and lateral blast. (4) Excavation in the Lemptégy Volcano exposes intra-eruption intrusions with associated uplift, providing examples in cross-section of the internal deformation likely to be found inside other Chaîne des Puys uplifted bulges. On Teide, a bulge near the summit shows similar structures and surface tilting as seen on the Petit Puy de Dôme and this bulging may have formed during the eruption of the Lavas Negras, the most recent activity on the summit area. Fault scarps on Teide also expose small cryptodomes, like those seen at Lemptégy. These examples, coupled with field studies on eroded intrusions, data on forced folds in basins and analogue models, show how large-scale topographic remodelling and structural change can be created by intrusions. These can rapidly and significantly change the volcanic edifice. A crater of elevation bulge, or forced fold that is stabilised by the cooling of the intrusion, will remain an important structural element in a volcano. This process starts even at the small scale of monogenetic volcanoes, and could occur through the lifetime of any growing stratovolcano. Such activity may be commonplace, but may be masked by concomitant eruption or removed by subsequent collapse. Monitoring and hazard strategies should be ready to deal with such large-scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks.

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 offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth, the intrusive complex development, the flank creep, the speed of flank deformation and the associated changes in topography. Using different approaches, a similar rift evolution has been proposed in volcanic oceanic islands elsewhere, showing that this model likely reflects a general and widespread process. This study, however, shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple; instead, a dynamic interplay between external factors (e.g. collapse, erosion) and internal forces (e.g. intrusions) is envisaged. This model thus provides a geological framework to understand the evolution of the NERZ and may help to predict developments in similar oceanic volcanoes elsewhere.

Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

The monogenetic Lemptegy volcano in the Chaine des Puys (Auvergne, France) was quarried from 1946 to 2007 and offers the possibility to study scoria cone architecture and evolution. This volcano was originally 50–80 m high, but scoria excavation has resulted in a 50-m-deep hole. Beginning in the 1980s, extraction was carried out with the advice of volcanologists so that Lemptegy’s shallow plumbing system and three-dimensional stratigraphy have been preserved. Detailed mapping enabled key stratigraphic units to be distinguished and the constructional phases to be reconstructed. The emplacement and evolution of the shallow plumbing system have also been unraveled. The growth of this monogenetic scoria cone included two temporally well-separated eruptions from closely spaced vents. The activity included Hawaiian, Strombolian and Vulcanian explosions, lava effusion, cryptodome and dome formation, partial collapse, satellite vent formation, eruptive pauses, and intrusion emplacement with consequent uplift. The cone shape, structure, and hence the local stress field, plumbing system, and thermal state were continuously changing, which in turn influenced the eruptive style and location. The plumbing system morphology and microtectonic structures both record local stress field and magmatic flow direction changes. Lemptegy volcano’s internal architecture, stratigraphy, and evolution show how complex a monogenetic volcano can be.

Water in volcanoes: evolution, storage and rapid release during landslides.

Volcanoes can store and drain water that is used as a valuable resource by populations living on their slopes. The water drainage and storage pattern depend on the volcano lithologies and structure, as well as the geological and hydrometric settings. The drainage and storage pattern will change according to the hydrometric conditions, the vegetation cover, the eruptive activity and the long- and short-term volcano deformation. Inspired by our field observations and based on geology and structure of volcanic edifices, on hydrogeological studies, and modelling of water flow in opening fractures, we develop a model of water storage and drainage linked with volcano evolution. This paper offers a first-order general model of water evolution in volcanoes.The volcano’s water plays an important role in volcano stability and instability. Nevertheless, the migration and storage of volcanic water prior and during landslide have not been extensively studied in regard to volcano evolution. We further explore this role and its impact on debris avalanche emplacement behaviour. Isolated water-saturated domains will favour ductile deformation, and unequal distribution of water within the debris avalanche partly explains the coeval occurrence of brittle and ductile deformation, indicating complex rheologies, and varied emplacement mechanisms. If the volcano prior to landslide is storing large amounts of water, this water will quickly flow in the landslide and will form a basal slurry upon which the avalanche will spread further.

Sector collapse events at volcanoes in the North Tanzanian divergence zone and their implications for regional tectonics

The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being—to our knowledge—previously unknown. Our field survey around Mount Meru allowed previous “lahar” deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.

Volcanic Debris Avalanches

Volcanoes are growing mountains with hydrothermal and magmatic systems, which have strong controls on volcanic landslides and debris avalanches. Such landslides are conditioned by the nature of volcanic rock, which is highly fractured, usually in granular form, often clay-rich and water-saturated. In consequence, volcanic landslides are generally more fractured, have more fine material, are more variably saturated than non-volcanic landslides, and they have a tendency to transform into large debris flows. Volcanic landslides vary in size from small failures of a valley side (<million m3) to a large portion of the edifice (tens of km3). The larger landslides are generally more deep-seated, because weak hydrothermal and magmatic systems in the volcano core are involved. Volcanoes undergo significant gravitational and tectonic deformation, creating faulting and fracturing in the edifice. The structures form the framework for landslides, and the resulting debris avalanches tend to form hummocky horst and graben topography that reflects the initial structure. As many volcanic landslides descend onto flat plains, this type of topography is often well preserved. Volcanoes of all types in all geological settings suffer landslides, and even extinct volcanoes are landslide prone. About four volcanic landslides occur world wide per century, meaning they are a significant hazard, specially as they are associated with secondary tsunami, volcanic eruptions, debris flows and lahars.

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.

Pre-Teide Volcanic Activity on the Northeast Volcanic Rift Zone

The northeast rift zone of Tenerife (NERZ) presents a partially eroded volcanic rift that offers a superb opportunity to study the structure and evolution of oceanic rift zones. Field data, structural observations, isotopic dating, magnetic stratigraphy, and isotope geochemistry have recently become available for this rift and provide a reliable temporal framework for understanding the structural and petrological evolution of the entire rift zone. The NERZ appears to have formed in several major pulses of activity with a particularly high production rate in the Pleistocene (ca. 0.99 and 0.56 Ma). The rift underwent several episodes of flank creep and eventual catastrophic collapses driven by intense intrusive activity and gravitational adjustment. Petrologically, a variety of mafic rock types, including crystal-rich ankaramites, have been documented, with most samples isotopically typical of the “Tenerife signal”. Some of the NERZ magmas also bear witness to contamination by hydrothermally altered components of the island edifice and/or sediments. Isotope geochemistry furthermore points to the generation of the NERZ magmas from an upwelling column of mantle plume material mixed with upper asthenospheric mantle. Finally, persistent isotopic similarity through time between the NERZ and the older central edifices on Tenerife provides strong evidence for a genetic link between Tenerife’s principal volcanic episodes.