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Feeder dykes showing irregular margins: (A) the Chese feeder dyke (Tenerife) shows several apophyses and fingers. (B) The Negrita feeder dyke (Tenerife) displays a ball-and-chain structure in its more superficial part, when intruding pyroclastic deposits of the cinder cone formed during its eruption.
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Most volcanic hazards depend on an injected dyke reaching the surface to form a feeder. Assessing the volcanic hazard in an area is thus related to understanding the condi-tion for the formation of a feeder dyke in that area. For this latter, we need good field data on feeder dykes, their geome-tries, internal structures, and other characteristics...
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Context 1
... (except the dyke in Fig. 4) are sub-vertical. Yet their margins are irregular in shape and many show fingers and apophyses (Figs. 5, 6). The apophy- ses are more common where the host rock is soft (mainly py- roclastics; Figs. 5 and 6). One remarkable dyke dissects the pyroclastic fallout deposits formed during eruption to which it was the feeder (Fig. 6b). This dyke shows a sort of pinch- and-swell structure (a ball-chained structure). In this case, there is no relationship between the changes in thickness and the host-rock properties which are essentially ...
Context 2
... margins are mostly poorly developed and thin along the feeders and only occasionally glassy (obsidian). Where a feeder dyke intrudes a cinder cone, the lapilli de- posits in the vicinity of the dyke are reddish and, closer to the dyke, become welded and stuck to the chilled margin (Fig. 6b). Parallel to the chilled margins in the dyke centre, an elongated hollow is sometimes observed (Fig. 7a). The elongated hollow or cavity is commonly confined to the up- permost 2-3 m of the dyke, that is, just below the surface. The opening (aperture) of the hollow may be several tens of cen- timetres and is lined with solidified magma ...
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Citations
... Alternatively, where bands are defined by groundmass grain size, they appear as a series of discontinuities, involving a drop in grain size moving toward the dyke center (Holness & Humphreys, 2003). Other bands are defined by narrow, highly vesicular or amygdaloidal regions running parallel to the dyke margins (Delcamp et al., 2012;Galindo & Gudmundsson, 2012;Kavanagh et al., 2018;Kile, 1993;Platten, 2000;Thiele et al., 2021;Thordarson & Self, 1996;Walker & Eyre, 1995). Finally, some authors describe platy textures and color variations at dyke margins, the cause of which is uncertain (Coward, 1980;Healy et al., 2018;Roberts & Sanderson, 1971). ...
... We first use the primary field and textural observations to create a conceptual framework for marginal band formation, based on the overall similarities between the two samples, and other examples of marginal bands from This repetitive process is significant because dyke margins comprise the earliest material to solidify within the fracture at the dyke tip, meaning that their textures are intrinsically related to propagation. The cyclical textures of the marginal bands indicate a repetitive dyke-tip process, and their prevalence within dykes across diverse volcanic settings (Brouxel, 1991;Delcamp et al., 2012;Drever & Johnston, 1959;Galindo & Gudmundsson, 2012;Holness & Humphreys, 2003;Kavanagh et al., 2018;Platten, 2000;Thiele et al., 2021;Thordarson & Self, 1996;Walker, 1987;Walker & Eyre, 1995) suggests that this process is common. ...
... There is an accompanying variation in vesicularity, which peaks toward the side of the band closer to the dyke center. In the literature, most occurrences of marginal bands fall into one of three categories: phenocryst-defined (Drever & Johnston, 1959); groundmass-defined (Brouxel, 1991;Holness & Humphreys, 2003); or vesicle-defined (Delcamp et al., 2012;Galindo & Gudmundsson, 2012;Kavanagh et al., 2018;Kile, 1993;Platten, 2000;Thiele et al., 2021;Thordarson & Self, 1996;Walker, 1987;Walker & Eyre, 1995). The Teno sample is therefore rare in that it displays cyclic variations in both phenocryst concentration and vesicularity, and this allows us to infer that they form via the same process. ...
Basaltic fissure eruptions, which are the most common type of eruption on Earth, are fed by dykes which mediate magma transport through the crust. Dyke propagation processes are important because they determine the geometry of the transport pathway and the nature of any geophysical signals associated with magma ascent. Here, we investigate small‐scale (mm–cm wide) banding features at the margins of dykes in the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (CRBP) (USA). Similar marginal bands have been reported for dykes in numerous localities around the world. Dyke margins record valuable information about propagation because they are the first material to solidify against the host rock at the propagating dyke tip. We find that the marginal bands are defined by cyclic variations in phenocryst concentration and vesicularity, and we infer that these cyclic variations in texture are a product of cyclic variations in magma flow rates and pressures within the dyke tip. This indicates that dyke emplacement occurs in pulses, with propagation repeatedly hindered by the rapid cooling and solidification of magma in the narrow dyke tip. Using a 1D conduction model, we estimate the time taken for each band to cool and solidify, which provides a timescale of several minutes to tens of minutes for the pulses. The occurrence of similar bands in various volcanic settings suggests that pulsatory propagation is a common, if not ubiquitous, process associated with dyke emplacement.
... The dyke width has a Gaussian distribution with a mean of 10 m and a standard deviation of 4 m. The model node spacing precludes dykes narrower than 2 m, even though this is wider than those mapped by Pallister ( 1981 ) in the Samail ophiolite in Oman and from studies of dykes on Iceland (Gudmundsson 1984 ;Galindo & Gudmundsson 2012 ), where the mean is less than 1 m. Ho wever , individual intrusions narro wer than 2 m penetrating into Layer 2A would cool rapidly and so be more likely to be fractured, increasing their porosity and lowering their seismic velocity (Wilkens et al. 1991 ), so w e ha ve assumed that they would have similar properties to that of the heterogeneity of Layer 2A. ...
... Ho wever , individual intrusions narro wer than 2 m penetrating into Layer 2A would cool rapidly and so be more likely to be fractured, increasing their porosity and lowering their seismic velocity (Wilkens et al. 1991 ), so w e ha ve assumed that they would have similar properties to that of the heterogeneity of Layer 2A. The assigned dyke velocity has a mean of 6100 m s −1 , which is perturbed by the depth of its top such that dykes that intrude to a shallower subsurface depth have a lower velocity (proxy for lower density and higher porosity; Galindo & Gudmundsson 2012 ). A v ertical v elocity gradient of 0.3 s −1 is imposed (Carlson 2001 ) to match the trend in the sonic log from 504B, to which a further 3 per cent Gaussian perturbation was added to mirror heterogeneity in dyke composition (Becker et al. 1989 ). ...
This study focuses on the 3-D velocity structure and thickness of ∼7 Myr-old oceanic crust surrounding borehole 504B, located ∼235 km from the intermediate-spreading Costa Rica Rift (Panama Basin). It investigates how well seismic structure determined by 3-D tomography compares with actual lithology and, consequently, what the origin and cause might be of an amplitude anomaly, the 2A Event, that is observed in multichannel seismic data. Our P-wave model shows an ∼0.3 km-thick sediment layer of velocity between ∼1.6-1.9 km s−1 (gradient 1.0 s−1), bound at its base by a velocity step to 4.8 km s−1 at the top of oceanic crustal Layer 2. Layer 2 itself is subdivided into two main units (2A and 2B) by a vertical velocity gradient change at 4.5 km depth, with a gradient of 1.7 s−1 above (4.8-5.8 km s−1) and 0.7 s−1 below (5.8-6.5 km s−1). The base of Layer 2, in turn, is defined by a change in gradient at 5.6 km depth. Below this, Layer 3 has a velocity range of 6.5-7.5 km s−1 and a gradient of ∼0.3 s−1. Corresponding S-wave igneous layer velocities and gradients are: Layer 2A, 2.4-3.1 km s−1 and 1.0 s−1; Layer 2B, 3.1-3.7 km s−1 and 0.5 s−1; Layer 3, 3.7-4.0 km s−1 and 0.1 s−1. The 3-D tomographic models, coupled with gravity modelling, indicate that the crust is ∼6 km thick throughout the region, with a generally flat-lying Moho. Although the P- and S-wave models are smooth, their velocities and gradients are remarkably consistent with the main lithological layering subdivisions logged within 504B. Thus, using the change in velocity gradient as a proxy, Layer 2 is interpreted as ∼1.8 km thick and Layer 3 as ∼3.8 km thick, with little vertical variation throughout the 3-D volume. However, the strike of lateral gradient variation is not Costa Rica Rift-parallel, but instead follows the orientation of the present-day adjacent Ecuador Rift, suggesting a reorientation of the Costa Rica Rift spreading ridge axis. Having determined its consistency with lithological ground-truth, the resulting P-wave model is used as the basis of finite difference calculation of wave propagation to find the origin of the 2A Event. Our modelling shows that no distinct interface, or transition, is required to generate this event. Instead, it is caused by averaging of heterogeneous physical properties by the seismic wave as it propagates through Layer 2 and is scattered. Thus, we conclude that the 2A Event originates and propagates exclusively in the lower part of Layer 2A, above the mean depth to the top of the dykes of Layer 2B. From our synthetic data we conclude that using the 2A Event on seismic reflection profiles as a proxy to determine the Layer 2A/2B boundary's depth will result in an overestimate of up to several hundred metres, the degree of which being dependent on the specific velocity chosen for normal moveout correction prior to stacking.
... Dikes supply magma to most eruptions on Earth (e.g., Galindo and Gudmundsson, 2012;Browning et al., 2015;Rivalta et al., 2015;Elshaafi and Gudmundsson, 2016;Townsend et al., 2017;Drymoni et al., 2020;Gudmundsson et al., 2022). In the past decades there has been very great progress in monitoring dike propagation during unrest periods in volcanoes. ...
The paper seeks to explain why multiple dikes - dikes formed when many magma injections follow the same path (with anywhere from days, weeks and years to decades, centuries, and millennia or more between successive injections) - are normally more likely to result in one or more magma injections reaching the surface to become a feeder-dike than single-injection dikes. I provide a theoretical framework as to how multiple dikes form, and support the theoretical conclusions with with many field examples of multiple (and composite) dikes. Then I discuss some recent eruptions that have been fed by multiple dikes, including the 2021-2023 eruptions in Fagradalsfjall on the Reykjanes Peninsula in Southwest Iceland.
The paper is also availabe on EarthArXiv (https://doi.org/10.31223/X5M67Q)
... Regarding the estimates of the thickness or maximum aperture for each element of this model, the maximum opening displacement of the feeder dike originating at a depth of~12 km and feeding the volcanic fissure associated with this eruption would be 6.7 ± 1.5 m. This value is of the same order of magnitude as that found in previous studies of feeder dikes in similar volcanic areas [50,56,57]. Regarding the different sills, we obtained maximum vertical thicknesses of 11 ± 2.2, 5.9 ± 1.1 and 4.4 ± 0.9 m for the 12, 6 and 2 km deep sills, respectively. ...
... Histogram of the final residuals of the covariance analysis); Section S5: Energy spectrum analysis of the gravity data ( Figure S4: Logarithm of the power spectrum of the Bouguer anomaly); Section S6: Gravitational effect of the new topography on the gravity data (Table S5: Bulk and grain density values at Cumbre Vieja; Figure S5: Gravity effect induced by the new topography); Section S7: Gravity Inversion model ( Figure S6: Histogram of the fit between the local gravity data and the calculated gravity from the inversion model); Section S8: Estimation of the width of the feeder dyke and related sills from aspect ratio analysis ( Figure S7: Location of the VT events); Section S9: Model of the magma plumbing system for the 2021 eruption of Cumbre Vieja volcano ( Figure S8: Geometric characteristics of the simple magma plumbing system model; Figure S9: Conceptual model of the inferred magma system). References [7,12,16,19,30,[39][40][41]44,45,47,49,50,56,57,75,[84][85][86][87][88][89][90][91][92][93][94] Data Availability Statement: The land gravity anomaly values used in this study were provided as supplementary data for the peer-review purpose. Upon acceptance, this dataset will be available from the Zenodo repository (https://doi.org/10.5281/zenodo.7539248). ...
This study used spatiotemporal land gravity data to investigate the 2021 eruption that occurred in the Cumbre Vieja volcano (La Palma, Canary Islands). First, we produced a density model by inverting the local gravity field using data collected in July 2005 and July 2021. This model revealed a low-density body beneath the western flank of the volcano that explains a highly fractured and altered structure related to the active hydrothermal system. Then, we retrieved changes in gravity and GNSS vertical displacements from repeated measurements made in a local network before (July 2021) and after (January 2022) the eruption. After correcting the vertical surface displacements, the gravity changes produced by mass variation during the eruptive process were used to build a forward model of the magmatic feeding system consisting of dikes and sills based on an initial model defined by the paths of the earthquake hypocenters preceding the eruption. Our study provides a final model of the magma plumbing system, which establishes a spatiotemporal framework tracing the path of magma ascent from the crust–mantle boundary to the surface from 11–19 September 2021, where the shallowest magma path was strongly influenced by the low-density body identified in the inversion process.
... Dike intrusions are widespread on the Canary Islands, an archipelago with seven main islands located in the Atlantic Ocean. Dikes in the Canaries have mainly been studied in deeply eroded valleys and excavated road cuts and tunnels, indicative of ancient principal stress fields, rift zones and flank movements [Carracedo 1994;Day et al. 1999;Gudmundsson * twalter@gfz-potsdam.de et al. 1999;Ablay and Martı 2000;Walter and Schmincke 2002;Ancochea et al. 2003;Fernández et al. 2006;Ancochea et al. 2008; Galindo and Gudmundsson 2012;Thiele et al. 2020]. ...
Volcanic eruptions are often preceded by episodes of in ation and emplacement of magma along tensile fractures. Here we study the 2021 Tajogaite-Cumbre Vieja eruption on La Palma, Canary Islands, and present evidence for tensile fractures dissecting the new cone during the terminal stage of the eruption. We use synthetic aperture radar (SAR) observations, together with drone images and time-lapse camera data, to determine the timing, scale and complexities associated with a fracturing event, which is diverging at a topographic ridge. By comparing the field dataset with analogue models, we further explore the details of lens-shaped fractures that are characteristic for faults diverging at topographic highs and converging at topographic lows. The observations made at Cumbre Vieja and in our models are transferrable to other volcanoes and add further evidence that topography is substantially affecting the geometry and complexity of fractures and magma pathways, and the locations of eruptions.
... In this paper, I explore and explain the physics of fluid-driven fracture (hydrofracture) paths. The focus is on magma-driven fractures, particularly on dykes, for the simple reason that numerous well-exposed dykes have been studied in the field where they are seen as having propagated through heterogeneous and layered (anisotropic) crustal segments (Geshi et al. 2010(Geshi et al. , 2012Galindo & Gudmundsson, 2012;Geshi & Neri, 2014;Drymoni et al. 2020Drymoni et al. , 2021. Many dyke paths have been studied in detail vertically over hundreds of metres and laterally for kilometres, and traced for tens and, occasionally, hundreds of kilometres. ...
... This section provides a short overview of these aspects of fluid-driven fractures, with a focus on the natural fractures. Much more detailed field descriptions of natural hydrofractures are provided by Segall (1984), Pollard & Aydin (1988), Hillis (2003), Cobbold & Rodrigues (2007), Philipp (2008Philipp ( , 2012, Gudmundsson (2011, Geshi et al. (2010Geshi et al. ( , 2012, Bons et al. (2012), Galindo & Gudmundsson (2012), Kusumoto et al. (2013), Kusumoto & Gudmundsson (2014), Fall et al. (2015, Tibaldi (2015), Gale et al. (2019) and Laubach et al. (2019). ...
... Some dykes have continuous vertical exposures of hundreds of metres (Geshi et al. 2010(Geshi et al. , 2012 and can be traced laterally for many kilometres (Anderson, 1942;Gudmundsson, 1983Pollard & Fletcher, 2005;Geshi & Neri, 2014) although the lateral exposures may not be continuous; the same applies to sills (Fig. 1). By contrast, the exposures of inclined (cone) sheets, both in vertical and in horizontal sections, are normally much more limited (Fig. 2) for the simple reason that these are much smaller structures than regional dykes (Galindo & Gudmundsson, 2012;Kusumoto et al. 2013;Tibaldi, 2015;. ...
... In this paper, I explore and explain the physics of fluid-driven fracture (hydrofracture) paths. The focus is on magma-driven fractures, particularly on dykes, for the simple reason that numerous well-exposed dykes have been studied in the field where they are seen as having propagated through heterogeneous and layered (anisotropic) crustal segments (Geshi et al. 2010(Geshi et al. , 2012Galindo & Gudmundsson, 2012;Geshi & Neri, 2014;Drymoni et al. 2020Drymoni et al. , 2021. Many dyke paths have been studied in detail vertically over hundreds of metres and laterally for kilometres, and traced for tens and, occasionally, hundreds of kilometres. ...
... This section provides a short overview of these aspects of fluid-driven fractures, with a focus on the natural fractures. Much more detailed field descriptions of natural hydrofractures are provided by Segall (1984), Pollard & Aydin (1988), Hillis (2003), Cobbold & Rodrigues (2007), Philipp (2008Philipp ( , 2012, Gudmundsson (2011, Geshi et al. (2010Geshi et al. ( , 2012, Bons et al. (2012), Galindo & Gudmundsson (2012), Kusumoto et al. (2013), Kusumoto & Gudmundsson (2014), Fall et al. (2015, Tibaldi (2015), Gale et al. (2019) and Laubach et al. (2019). ...
... Some dykes have continuous vertical exposures of hundreds of metres (Geshi et al. 2010(Geshi et al. , 2012 and can be traced laterally for many kilometres (Anderson, 1942;Gudmundsson, 1983Pollard & Fletcher, 2005;Geshi & Neri, 2014) although the lateral exposures may not be continuous; the same applies to sills (Fig. 1). By contrast, the exposures of inclined (cone) sheets, both in vertical and in horizontal sections, are normally much more limited (Fig. 2) for the simple reason that these are much smaller structures than regional dykes (Galindo & Gudmundsson, 2012;Kusumoto et al. 2013;Tibaldi, 2015;. ...
Fractures that form when fluid pressure ruptures the rock are referred to as fluid-driven fractures or hydrofractures. These include most dikes, inclined sheets, and sills, but also many mineral veins and joints, as well as human-made hydraulic fractures. While considerable field and theoretical work has focused on the geometry and arrest of hydrofractures, how they select their propagation paths, particularly in layered and faulted rocks, has received less attention. Here I propose that of all the possible paths that a given hydrofracture may follow, it selects the path of least (minimum) action as determined by Hamilton's principle. This means that the selected path is the one along which the energy transformed (released) multiplied by the time taken for the propagation is a minimum. Hydrofractures advance their tips/fronts in steps, with a time lag between the fracture front and the fluid front. In the present framework, each step is then controlled by Hamilton's principle. The results suggest that when the hosting rock body is regarded as homogeneous, isotropic and non-fractured, hydrofracture paths are everywhere perpendicular to the trajectories of the minimum compressive (maximum tensile) principal stress σ3 and follow the trajectories of the maximum principal compressive stress σ1. When applied to layered and faulted rock body, the results indicate that hydrofracture paths may follow existing faults for a while, depending primarily on (1) the dip of the fault (steep faults are the most likely to be used by vertically propagating hydrofractures), and (2) the tensile strength across the fault as compared with the tensile strength of the host rock along a path following the direction of σ1. The results suggest that hydrofractures may use faults as parts of their paths primarily if the fault is steeply dipping and with close to zero tensile strength.
... Geyer and Martí (2010) propose the DRZ and SRZ reflect contemporary volcanism that originated from crustal fractures that acted as fissure vents. Geyer and Martí (2010) and Galindo and Gudmundsson (2012) demonstrated that basalt dykes trending parallel to the Santiago and Dorsal ridges were fissure vents for eruptions from the beginning of shield forming volcanism that has built Tenerife as an oceanic shield volcanic system. Geyer and Martí (2010) undertook numerical modelling of structural trends (e. g. alignment of eruption centres and trends of fissure dykes parallel to the trends of those rifts) and stress fields and indicated that the SRZ and DRZ are true magmatic rift zones that have been long existing structures on Tenerife, even during OBS shield edifice time. ...
Tenerife, one of the active oceanic island volcanoes in the Canary Islands, located in the eastern Atlantic Ocean off northwest Africa, is the second largest intraplate oceanic island volcanic system after Hawai'i but is more complex and represents a different and more evolved end-member to Hawai'i in the spectrum of oceanic island volcanic systems. Tenerife began life as a mafic oceanic shield volcano at ~12 Ma, erupting (picro)basalt, basanite, hawaiite, mugearite, benmoreite series lavas of the Older Basaltic Series. These were derived from a spatially variable mantle plume source with varying degrees of magma-lithosphere interaction and fractional crystallisation. At ~3.05 Ma an evolved phonolitic magma system began to develop under the centre of the island. That led to the building of an initial summit effusive and explosive stratovolcano system (the Las Cañadas edifice) represented by the poorly understood Lower Group, followed by development of 3 cycles of explosive phonolitic caldera forming activity (Ucanca, Guajara, Diego Hernández) of the Upper Group, from 1.66 Ma to 0.19 Ma, each cycle separated by ~180 kyr (the recharge interval?). Phonolite genesis is complex, involving fractional crystallisation, partial melting of island crust including syenitic plutons, and recycling of crystal mushes. Three coalesced explosive calderas are preserved at the summit of Tenerife, constituting the Las Cañadas Caldera Complex. Since 0.19 Ma two stratovolcanoes (Teide, Pico Viejo) have been growing along the northern rim of the caldera complex, becoming more phonolitic from basaltic beginnings and more explosive, perhaps heralding the beginning of a new explosive cycle. Simultaneously shield building basaltic volcanism has continued on the flanks to historic times through multiple monogenetic eruptions along the linear northwestern and northeastern rift zones, and a more diffuse southern volcanic zone. Volcanic eruption styles have included fissure fed basaltic shield sheet lava eruptions, monogenetic basaltic cone lava and scoria eruptions, highly explosive plinian phonolitic pumice and ash fallout, and pyroclastic flow forming eruptions. The volumes of the largest explosive eruptions likely caused a component of magma chamber roof block subsidence, with multiple, spaced eruptions during each caldera forming cycle producing incremental caldera collapse and polygenetic calderas. Major landslides coincide with some of the large explosive eruptions, raising the question of cause and effect.
... The most evident morphological characteristic of El Hierro island ( Fig. 1) is its truncated trihedral configuration, given by three main convergent ridges (Carracedo, 1994(Carracedo, , 1996bGalindo and Gudmundsson, 2012), and a horseshoe-shaped embayment caused by large gravitational landslides (Carracedo, 1996a, b;Masson, 1996;Urgelés et al., 1996Urgelés et al., , 1997Carracedo et al., 1999Carracedo et al., , 2001Masson et al., 2002;Walter and Troll, 2003;Manconi et al., 2009;Longpré et al., 2011). This configuration is the result of subsequent constructive and destructive phases, that according to radiometric ages and paleomagnetic data (Abdel Monem et al., 1971;Guillou et al., 1996; more than 10,000 earthquakes were registered along with a vertical ground deformation up to 6 cm on the whole island (Domínguez Cerdeña et al., 2014;Meletlidis et al., 2015). ...
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.
... When compared with the Icelandic eruptions of Cood basalts through a series of vents aligned along Table 2. Criteria for recognition of feeder dykes (after Gudmundsson 1984;Galindo and Gudmundsson 2012;Geshi and Neri 2014) and characters of the BML dyke. ...
The Bhetkheda-Mohana Lineament is traced as a continuous lineament across nearly 100 km in the Central Narmada valley across the Deccan Trap basalts and their basement of Proterozoic sediments. While a major length of this lineament is occupied by a basaltic dyke, there are segments where the dyke is completely absent, and the lineament is represented by a regional fracture /shear / fault zone. At its eastern extremity this dyke is exposed intruding along the axis of a synclinorium of the Vindhyan Supergroup sediments, as a 4 km long string of hillocks of picturesque columnar jointed basalt. It has the presence of ignimbrites and a thin basaltic flow (resting on the sediments) surrounding it, suggesting the presence of an eruptive vent. This dyke intrudes the Mandleshwar Formation lava flows dated at 67-66 Ma and is associated with the Narmada dyke swarm. It has given 40 Ar/ 39 Ar age of 66.6±0.5 Ma. Its chemical characters conform to those of the basaltic flows of the Malwa Traps, indicating a common source and emplacement history. This is a unique example of a dyke that was emplaced along a pre-existing fracture zone cutting through the Proterozoic basement as well as the Deccan Trap lavas, with a distinct petrological identity with the host lava flows, indicating its feeder relation. It endorses the comparison of the Icelandic mode of fissure-fed flood basalts with the eruptive history of the Deccan Volcanic Province.
