NE Rift Zones Research Articles

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.

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.

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.

Age distribution of cinder cones within the Bandas del Sur Formation, southern Tenerife, Canary Islands

AbstractThe Quaternary Bandas del Sur Formation in the south of Tenerife comprises a complex sequence of pyroclastic rocks and lavas. In contrast to the NW- and NE-Rift zone on Tenerife, the S-Rift zone comprises a number of characteristics with respect to the morphological features, eruption cyclicity and the geochemistry of the volcanic deposits. Various flank eruptions of the Las Cañadas volcano associated with basaltic lavas and the formation of cinder cones within the Bandas del Sur are important volcanic units for understanding the explosive volcanic cycles during the Pleistocene on Tenerife. A number of palaeomagnetic studies, as well as major and trace element geochemistry and two radio-isotope dates (K–Ar), have been carried out on prominent cinder cones, in order to discover their stratigraphic position. Combining our results with previous K–Ar data, the cones and lavas can be subdivided into three stratigraphic units. The first unit contains cinder cones with reverse magnetization and Y/Nb ratios between 0.37 and 0.41. Cinder cones which belong to the second unit show normal magnetization and Y/Nb ratios of < 0.35. The third unit comprises cinder cones with normal magnetization and Y/Nb ratios of about 0.47. The first two units were constructed between c. 0.948–0.779 Ma and 0.323–0.300 Ma. These units define volcanic cycles ending in violent Plinian eruptions. The third and youngest unit possibly marks the beginning of a further volcanic cycle that started c. 0.095 Ma ago.

Lengths and hazards from channel-fed lava flows on Mauna Loa, Hawai‘i, determined from thermal and downslope modeling with FLOWGO

Using the FLOWGO thermo-rheological model we have determined cooling-limited lengths of channel-fed (i.e. ‘a‘ā) lava flows from Mauna Loa. We set up the program to run autonomously, starting lava flows from every 4th line and sample in a 30-m spatial-resolution SRTM DEM within regions corresponding to the NE and SW rift zones and the N flank of the volcano. We consider that each model run represents an effective effusion rate, which for an actual flow coincides with it reaching 90% of its total length. We ran the model at effective effusion rates ranging from 1 to 1,000 m3 s−1, and determined the cooling-limited channel length for each. Keeping in mind that most flows extend 1–2 km beyond the end of their well-developed channels and that our results are non-probabilistic in that they give all potential vent sites an equal likelihood to erupt, lava coverage results include the following: SW rift zone flows threaten almost all of Mauna Loa’s SW flanks, even at effective effusion rates as low as 50 m3 s−1 (the average effective effusion rate for SW rift zone eruptions since 1843 is close to 400 m3 s−1). N flank eruptions, although rare in the recent geologic record, have the potential to threaten much of the coastline S of Keauhou with effective effusion rates of 50–100 m3 s−1, and the coast near Anaeho‘omalu if effective effusion rates are 400–500 m3 s−1 (the 1859 ‘a‘ā flow reached this coast with an effective effusion rate of ∼400 m3 s−1). If the NE rift zone continues to be active only at elevations >2,500 m, in order for a channel-fed flow to reach Hilo the effective effusion rate needs to be ≥400 m3 s−1 (the 1984 flow by comparison, had an effective effusion rate of 200 m3 s−1). Hilo could be threatened by NE rift zone channel-fed flows with lower effective effusion rates but only if they issue from vents at ∼2,000 m or lower. Populated areas on Mauna Loa’s SE flanks (e.g. Pāhala), could be threatened by SW rift zone eruptions with effective effusion rates of ∼100 m3 s−1.

Tectonic control on the eruptive dynamics at Mt. Etna Volcano (Sicily) during the 2001 and 2002–2003 eruptions

The eruptive events of the July–August 2001 and October 2002–January 2003 at Mt. Etna provide new insights for reconstructing the complex geometry of the feeding system and their relationship to regional tectonics. The 2001 eruption took place mainly on the upper southern sector of the volcano. The eruption was preceded by a large earthquake swarm for a few days before its onset and accompanied by ground deformation and fracturing. The development of surface cracking along with the seismic pattern has allowed us to recognize three distinct eruptive systems (the SW–NE, NNW–SSE and N–S systems) which have been simultaneously active. Such eruptive systems are only the upper portions of a complex feeding system that was fed at the same time by two distinct magmas. The SW–NE and NNW–SSE systems, connected with the SE crater conduit, were fed by magma coming from depth, whereas the N–S system served instead as an ascending pathway for an amphibole-bearing magma residing in a shallow reservoir. The eruptive activity started again on October 2002 on the NE Rift Zone, where about 20 eruptive vents were aligned between 2500 and 1900 m a.s.l., and on the southern flank, from the central crater to the Montagnola. The onset of eruptive activity was accompanied by a seismic swarm. As in the 2001 eruptive event, two independent feeding systems formed, characterized by distinct magmas. The SW–NE system controlled the feeding of the Northeast Rift and was accommodated by left-lateral displacement along the WNW–ESE trending Pernicana Fault. The N–S system fed the eruptions on the southern flank. Moreover, the associated crustal deformation triggered seismic reactivation of tectonic structures in the eastern flank of the volcano and offshore. These two last eruptions indicate that at Mt. Etna the ascent of magma, as well as the accommodation of deformation, is strongly dominated by local extensional structures that are connected to a regional tectonic regime.

Graben structure in the Las Cañadas edifice (Tenerife, Canary Islands): implications for active degassing and insights on the caldera formation

A graben structure has been identified at the western area of the Las Cañadas caldera wall, here referred as the Los Azulejos Graben. This graben is 1 km wide and is bounded by two major normal faults trending NE–SW, the Los Azulejos Fault and the Ucanca Fault. The graben was active for at least 0.5 Ma, from the end of the Ucanca Fm to the end of the Guajara Fm, and before the collapse of the Las Cañadas edifice that formed the western caldera. A right-lateral transtension regime operated in the graben as suggested by small fault orientations and kinematics. The prolongation of the NE rift zone of Tenerife to the Cañadas edifice is the most likely volcano-tectonic scenario for the graben. In this context, inflation of phonolitic shallow magma chambers may have produced reverse faults and reactivation of normal faults. An intense and widespread hydrothermal alteration, here called Azulejos-type, occurred mainly before the graben formation, while a fault-related hydrothermal alteration occurred during and after the graben. Diffuse carbon dioxide and hydrogen degassing in and around the Las Cañadas caldera show relatively enriched values along a NE–SW trend suggesting that faults in the Los Azulejos Graben act as a pathway for deep-seated gases to the surface. Diffuse degassing and hydrothermalism indicate that the graben area has been a zone of intense fluid circulation during the evolution of the Las Cañadas edifice.

Geological and geomorphological evolution of the Etna volcano NE flank and relationships between lava flow invasions and erosional processes in the Alcantara Valley (Italy)

In this paper, the interrelationships between volcanic activity and fluvial events in the Alcantara Valley are investigated. Based on the correlation between the stratigraphy of the NE flank of Mt Etna and subsurface data, the geological and geomorphological evolutions of the valley are reconstructed. New 1:10 000 scale geological mapping shows that the bulk of this sector of the volcano is made up of the Ellittico volcano lava flows, though they are widely covered by the products of the eruptive activity of the last 15 ka. The present-day morphological setting of the Alcantara Valley is the result of two main evolutionary phases initiated during the activity of the Ellittico volcano. Only one lava flow invasion of the valley floor occurred in the first phase. This phenomenon was followed by a long period of erosional processes leading to the entrenchment of the drainage pattern and the erosion of the Ellittico lava flow. About 20–25 ka ago, an important change in the frequency of the lava flow invasions into the valley occurred associated with the final stage of the Ellittico volcano activity marking the beginning of the second phase. During this phase, volcanic processes became predominant with respect to other morphogenetic processes in the Alcantara Valley. Lava flows coming from the NE flank of the Ellittico volcano caused a radical modification of the morphological setting of this area, even though only one lava flow emitted by an eruptive fissure located within the valley partially filled the riverbed. During the eruptive activity of the last 15 ka, the complete filling of the Alcantara Valley floor occurred. In particular, between 15 and 7 ka, a lava flow originated from the Mt Moio scoria cone filled the valley floor for a distance of about 9 km. Following a short period of erosion, an eruptive fissure located within the valley generated a 20–21-km-long lava flow that was channelled along the full extent of the Alcantara Valley and stretches for about 3 km offshore in the Ionian sea. In the last 7 ka, lava flows originating from the NE-Rift zone produced only temporary damming of the riverbed without any important contribution to the filling of the Alcantara Valley.

Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: volcanic rift zone reconfiguration as a precursor to volcano flank instability?

The Cumbre Vieja volcano is the youngest component of the island of La Palma. It is a very steep-sided oceanic island volcano, of a type which may undergo large-scale lateral collapse with little precursory deformation. Reconfiguration of the volcanic rift zones and underlying dyke swarms of the volcano is used to determine the present degree of instability of the volcano. For most of its history, from before 125 ka ago to around 20 ka, the Cumbre Vieja volcano was characterised by a triple (“Mercedes Star”) volcanic rift zone geometry. The three rift zones were unequally developed, with a highly productive south rift zone and weaker NE and NW rift zones: the disparity in activity was probably due to topographic-gravitational stresses associated with the west facing Cumbre Nueva collapse structure underneath the western flank of the Cumbre Vieja. From 20 ka to about 7 ka, activity on the NW volcanic rift zone diminished and the intersection of the rift zones migrated slightly to the north. More recently, the triple rift geometry has been replaced at the surface by a N–S-trending rift zone which transects the volcano, and by E–W-trending en echelon fissure arrays on the western flank of the volcano. The NE rift zone has become completely inactive. This structural reconfiguration indicates weakening of the western flank of the volcano. The most recent eruption near the summit of the Cumbre Vieja, that of 1949, was accompanied by development of a west facing normal fault system along the crest of the volcano. The geometry of this fault system and the timing of its formation in relation to episodes of vent opening during the eruption indicate that it is not the surface expression of a dyke. Instead, it is interpreted as being the first surface rupture along a developing zone of deformation and seaward movement within the western flank of the Cumbre Vieja: the volcano is therefore considered to be at an incipient stage of flank instability. Climatic factors or strain weakening along the Cumbre Nueva collapse structure may account for the recent development of this instability.