Submarine Eruption Research Articles

Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography

During the first week of the 2011 El Hierro submarine eruption, abundant light-coloured pumiceous, high-silica volcanic bombs coated in dark basanite were found floating on the sea. The composition of the light-coloured frothy material (‘xeno-pumice’) is akin to that of sedimentary rocks from the region, but the textures resemble felsic magmatic pumice, leaving their exact mode of formation unclear. To help decipher their origin, we investigated representative El Hierro xeno-pumice samples using X-ray computed microtomography for their internal vesicle shapes, volumes, and bulk porosity, as well as for the spatial arrangement and size distributions of vesicles in three dimensions (3D). We find a wide range of vesicle morphologies, which are especially variable around small fragments of rock contained in the xeno-pumice samples. Notably, these rock fragments are almost exclusively of sedimentary origin, and we therefore interpret them as relicts an the original sedimentary ocean crust protolith(s). The irregular vesiculation textures observed probably resulted from pulsatory release of volatiles from multiple sources during xeno-pumice formation, most likely by successive release of pore water and mineral water during incremental heating and decompression of the sedimentary protoliths.

Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)

Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufriere Hills deposits, indicating that the island experienced large-magnitude, subaerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit’s combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly subrounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (<2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water.

Heavy metals concentration variation in marine water over time as a consequence of the submarine eruption in El Hierro, Canary Islands

Heavy metals concentration variation in marine water over time as a consequence of the submarine eruption in El Hierro, Canary Islands

Seaweed community response to a massive CO2 input

Changes in the structure of seaweed communities were examined following a massive CO2 input caused by a submarine eruption near the coast of El Hierro island (Canary Islands, Spain). The event lasted almost five months (October 2011–March 2012) and created a significant pH gradient. Specifically, we compared three different zones: highly affected with extreme low pH (6.7–7.3), affected with low pH (7.6–7.8), and unaffected ambient pH zone (∼8.1) according to the pH gradient generated by the predominate currents and waves in the south of the island. Studies were carried out before, during and after the CO2 input event in each zone. We found community-wide effects on seaweed communities during the eruption; these included changes in species abundance and changes in the diversity. However, changes in all these community traits were only evident in the highly affected zone, where there were major shifts in the seaweed community, with a replacement of Lobophora variegata by ephemeral seaweeds. Lobophora variegata dropped in cover from 87–94 to 27% while ephemeral seaweeds increased 6–10 to 29%. When the impact ended Lobophora variegata began to recover reaching a cover higher than 60%. In the moderate affected area the Lobophora variegata canopies maintained their integrity avoiding phase shifts to turfs. Here the only significant changes were the reduction of the cover of the crustose and geniculate coralline algae.

Exploring the submarine Graham Bank in the Sicily Channel

&lt;p&gt;In the Sicily Channel, volcanic activity has been concentrated mainly on the Pantelleria and Linosa islands, while minor submarine volcanism took place in the Adventure, Graham and Nameless banks. The volcanic activity spanned mostly during Plio-Pleistocene, however, historical submarine eruptions occurred in 1831 on the Graham Bank and in 1891 offshore Pantelleria Island. On the Graham Bank, 25 miles SW of Sciacca, the 1831 eruption formed the short-lived Ferdinandea Island that represents the only Italian volcano active in historical times currently almost completely unknown and not yet monitored. Moreover, most of the Sicily Channel seismicity is concentrated along a broad NS belt extending from the Graham Bank to Lampedusa Island. In 2012, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) carried out a multidisciplinary oceanographic cruise, named “Ferdinandea 2012”, the preliminary results of which represent the aim of this paper. The cruise goal was the mapping of the morpho-structural features of some submarine volcanic centres located in the northwestern side of the Sicily Channel and the temporary recording of their seismic and degassing activity. During the cruise, three OBS/Hs (ocean bottom seismometer with hydrophone) were deployed near the Graham, Nerita and Terribile submarine banks. During the following 9 months they have recorded several seismo-acoustic signals produced by both tectonic and volcanic sources. A high-resolution bathymetric survey was achieved on the Graham Bank and on the surrounding submarine volcanic centres. A widespread and voluminous gas bubbles emission was observed by both multibeam sonar echoes and a ROV (remotely operated vehicle) along the NW side of the Graham Bank, where gas and seafloor samples were also collected.&lt;/p&gt;

Seafloor doming driven by degassing processes unveils sprouting volcanism in coastal areas.

We report evidences of active seabed doming and gas discharge few kilometers offshore from the Naples harbor (Italy). Pockmarks, mounds, and craters characterize the seabed. These morphologies represent the top of shallow crustal structures including pagodas, faults and folds affecting the present-day seabed. They record upraise, pressurization, and release of He and CO2 from mantle melts and decarbonation reactions of crustal rocks. These gases are likely similar to those that feed the hydrothermal systems of the Ischia, Campi Flegrei and Somma-Vesuvius active volcanoes, suggesting the occurrence of a mantle source variously mixed to crustal fluids beneath the Gulf of Naples. The seafloor swelling and breaching by gas upraising and pressurization processes require overpressures in the order of 2–3 MPa. Seabed doming, faulting, and gas discharge are manifestations of non-volcanic unrests potentially preluding submarine eruptions and/or hydrothermal explosions.

Influence of fortnightly tides on earthquake triggering at the East Pacific Rise at 9°50′N

AbstractThe study of tidal forcing provides an opportunity to view the response of the solid Earth to known changes in stress. Earthquakes triggered by diurnal and semidiurnal tidal forces have been well documented for the deep ocean and some terrestrial environments. However, few studies examine the influence of fortnightly tides on earthquake triggering and only in terrestrial environments. We assess triggering of microearthquakes related to true fortnightly tides and fortnightly tidal modulations along the East Pacific Rise at 9°50′N, where strong diurnal and semidiurnal triggering of earthquakes has been observed. An ocean bottom seismograph experiment collected microearthquake data between October 2003 and January 2007, a time period that included a well‐documented submarine eruption culminating in January 2006. We examine how triggering changed through the build up to and aftermath of this seafloor‐spreading event. Results show that earthquakes occur preferentially during times of increasing peak volumetric stress leading up to the eruption, with strong statistical significance (Schuster Test p values up to 2.22e−31). However, the correlation loses its strength immediately prior to and during the eruptive period (p value as high as 0.2837), when competing tectonic and magmatic stresses may occur. Post‐eruption triggering reappears (p value 1.45e−26) but with opposite phase. Statistical significance is also explored using Monte Carlo method, highlighting limitations of swarm‐dominated time series for the Schuster Test. Nevertheless, the observations are consistent with a shift in the dominating stress triggering the earthquakes from extensional tectonics to magmatic processes, followed by a post‐eruption period of cooling, relaxation, and possibly magma chamber refilling.

Real-time infrasonic monitoring of the eruption at a remote island volcano using seismoacoustic cross correlation

Abstract On 2013 November 20, a submarine eruption started close to Nishinoshima island, which lies ∼1000 km south of Tokyo. Real-time monitoring of the eruption is crucial for understanding the formation processes of the new volcano island and related disaster prevention. In situ monitoring, however, is difficult in practice because the closest inhabited island, Chichijima, is 130 km away from Nishinoshima. This study presents an infrasonic monitoring method that uses cross-correlating records at a pair of online stations on Chichijima. One is the horizontal ground velocity recorded at a permanent seismic station operated by the Japan Meteorological Agency (JMA). The seismic records were corrected for atmospheric pressure using an empirical ground response to infrasound. The other is the air pressure recorded at the JMA Meteorological Observatory. For each station, we divided the whole records into 3600-s segments. To suppress outliers, each segment was normalized by the envelope function. We then calculated cross-correlation functions between the pair of stations using the fast Fourier transform. They present clear successive arrivals of infrasound coming from Nishinoshima. We also conducted an offline tripartite-array observation using three low-frequency microphones with a station spacing of ∼50 m installed in 2013 May. The array analysis supports the results obtained from the online stations. The typical root-mean-squared amplitude is on the order of 0.01 Pa, and the typical duration is several days. The amplitudes were primarily controlled by the effective sound velocity structure from Nishinoshima to Chichijima. The infrasonic observations together with the meteorological observation at Chichijima suggest that infrasonic activity was not present in the first two weeks in 2015 January. With the help of a more quantitative estimation of the meteorological effect, we could infer eruptive activity in real time. Now many online seismic stations are available worldwide. This study shows that installation of another infrasonic sensor close to the existing seismic station (∼1 km) is useful for monitoring a remote island volcano in real time.

Deep intrusions, lateral magma transport and related uplift at ocean island volcanoes

Oceanic intraplate volcanoes grow by accumulation of erupted material as well as by coeval or discrete magmatic intrusions. Dykes and other intrusive bodies within volcanic edifices are comparatively well studied, but intrusive processes deep beneath the volcanoes remain elusive. Although there is geological evidence for deep magmatic intrusions contributing to volcano growth through uplift, this has rarely been demonstrated by real-time monitoring. Here we use geophysical and petrological data from El Hierro, Canary Islands, to show that intrusions from the mantle and subhorizontal transport of magma within the oceanic crust result in rapid endogenous island growth. Seismicity and ground deformation associated with a submarine eruption in 2011–2012 reveal deep subhorizontal intrusive sheets (sills), which have caused island-scale uplift of tens of centimetres. The pre-eruptive intrusions migrated 15–20 km laterally within the lower oceanic crust, opening pathways that were subsequently used by the erupted magmas to ascend from the mantle to the surface. During six post-eruptive episodes between 2012 and 2014, further sill intrusions into the lower crust and upper mantle have caused magma to migrate up to 20 km laterally, resulting in magma accumulation exceeding that of the pre-eruptive phase. A comparison of geobarometric data for the 2011–2012 El Hierro eruption with data for other Atlantic intraplate volcanoes shows similar bimodal pressure distributions, suggesting that eruptive phases are commonly accompanied by deep intrusions of sills and lateral magma transport. These processes add significant material to the oceanic crust, cause uplift, and are thus fundamentally important for the growth and evolution of volcanic islands. We suggest that the development of such a magma accumulation zone in the lower oceanic crust begins early during volcano evolution, and is a consequence of increasing size and complexity of the mantle reservoir system, and potentially the lithospheric stresses imposed by increasing edifice load.

Historical seismicity in oceans from sailors’ testimonies

Before seismological catalogs were routinely produced, seafarers experienced major seismic events at sea that were documented in their logs. This article analyzes some of these old records—mostly from eighteenth and nineteenth centuries—in the context of plate tectonics. Large shocks that were felt on ships are related either to earthquakes, sub-marine volcanic eruptions, or to sub-marine sliding of rocks and/or sediments. We analyze various related parameters such as the location and size of the shaking, the duration of the shock, and the associated noise. A total of 396 observations have been retained for this study, mostly located in the Atlantic Ocean, reflecting its intense maritime traffic during the period of interest. Some of the detailed accounts allow us to clearly identify the nature of the shocks, including a possible interpretation in terms of focal mechanism. Our results, when compared to historical catalogs, reveal many previously undetected large events. Macroseismic results for a few large historical events occurring near the coasts confirm the validity of our approach, but also reveal its limitations. The good locations of most of the events allow us to relate them to plate boundaries. The Romanche transform zone has deserved particular interest due to the large number of related testimonies. This study particularly illustrates that historical seismicity may be applied to oceans. The collected testimonies also show how impressive and dangerous these large earthquakes at sea are, despite the absence of S-waves.

Chapter 4 Earthquakes and volcanic eruptions in the Azores region: geodynamic implications from major historical events and instrumental seismicity

Abstract Since the settlement of the archipelago, in the fifteenth century, 31 destructive earthquakes and 28 volcanic eruptions have been registered in the Azores. Major earthquakes occurring in historical times have reached magnitudes &gt;7, often triggering landslides and even small tsunamis. In the same period, subaerial volcanic eruptions have ranged from Hawaiian to sub-Plinian, sometimes with a hydromagmatic character, while submarine eruptions have been Azorean to Surtseyan in style. The temporal and spatial distributions of major historical events are presented and their impacts summarized. The instrumental seismic activity registered since 1980 is discussed taking into account the main volcano-tectonic structures. These seismological data allow us to improve the characterization of the present-day boundary between the Eurasia and Nubia lithospheric plates, herein defined as the East Azores Volcano-Tectonic System. The seismological data also suggest that the location of the Azores Triple Junction is to the west of Faial Island, at about 38° 50′ N, 30° 25′ W, in agreement with proposals made by other authors using aeromagnetic data. A natural seismic gap, centred in the São Jorge structural alignment, is recognized and is interpreted as a zone of stress accumulation with the potential to generate a high-magnitude earthquake similar to that of 1757.

Chapter 1 Volcanic geology of São Miguel Island (Azores Archipelago): introduction

The Azores is an archipelago in the Atlantic Ocean composed of nine volcanic islands. The islands are dispersed along a general WNW–ESE trend crossing the Mid-Atlantic Ridge in the area where the Eurasian, African and North American lithospheric plates meet. While Corvo and Flores lie to the west of the Mid-Atlantic Ridge and emerge from a present-day relatively stable geological setting, the other islands are located in an important seismic and volcanically active zone corresponding to the boundary between the Eurasian and African tectonic plates. Volcanic activity has occurred in historic times on the islands of Faial, Pico, Sao Jorge, Sao Miguel and Terceira, and there have also been several recorded submarine eruptions (Weston 1964). Sao Miguel is the largest and most volcanically active island of the archipelago. In the last 5 ka several eruptions took place on the three active central volcanoes – Sete Cidades, Fogo and Furnas – and in the basaltic fissure systems of Picos and Congro. Although details of the first settlement of the island in the fifteenth century are uncertain, there is evidence that Furnas was in eruption at the time the first settlers arrived, some time between 1439 and 1443 (Queiroz et al. 1995). The historian Gaspar Frutuoso (1522–1591?) provided the first geological descriptions of the Azores in his work ‘Saudades da Terra’, giving important details about the earliest volcanic and seismic events. In the sixteenth and seventeenth centuries there were two explosive trachytic eruptions of sub-Plinian scale on Sao Miguel: Fogo 1563 and Furnas 1630. The last eruption on land occurred in the Picos Fissural Volcanic System, in 1652, and involved the extrusion of lava domes. The …

The proximal marine record of the Marsili Seamount in the last 7 ka (Southern Tyrrhenian Sea, Italy): Implications for the active processes in the Tyrrhenian Sea back-arc

The volcanism of back-arc basins resembles that of oceanic spreading centers, rifts, and, in vanishing stages, extensional arcs, depending on the amount and rate of the dynamic processes associated to the subduction. Marsili Seamount (MS) represents the axial ridge of the Southern Tyrrhenian Sea back-arc basin, which is connected to the slab roll-backing processes affecting the Calabrian Arc (Italy). The Southern Tyrrhenian Sea back-arc is characterized by a significant decline in the spreading rate with time (2.8–3.1mm/a to less than 1.8mm/a in the last 0.78–1Ma). MS develops between about 1Ma and 3ka and mainly consists of lava flows erupted from central and fissural vents. The MS products belong to the calcalkaline association and range in composition from basalts to trachytes. We present new stratigraphic, geochronological, and geochemical data (glass shards and minerals) of tephra from a 2.35m long gravity core (Marsili1 core) recovered on MS at 943m b.s.l. We recognize five tephras [M1 (top of the core) to M5 (bottom)] represented by poorly to highly vesiculated ashes. The lowermost tephra M5 emplaced between ca. 7 and 26ka B.P.; it represents the less evolved distal counterpart of the Unit D related to the Biancavilla–Montalto products of Mount Etna (Sicily). The M1 to M4 tephras emplaced between 2.1 and 7.2ka B.P. and are related to strombolian-like submarine eruptions of NNE–SSW aligned MS vents. The composition of the M1–M4 glasses ranges from basaltic trachyandesites to andesites and trachytes. The M1 to M4 magmas mainly originated by crystal fractionation from a heterogeneous mantle source with varying LILE enrichments by subduction-related fluids. The degree of evolution of the MS magmas increases with decreasing time. The formation of vertically stacked magma storage zones at the crust/mantle interface and within MS is related to the vanishing Southern Tyrrhenian Sea opening, which implies the rapid (<1Ma) evolution from a slow spreading back-arc setting to an arc system.

Nannofossils: the smoking gun for the Canarian hotspot

The origin of volcanism in the Canary Islands has been a matter of controversy for several decades. Discussions have hinged on whether the Canaries owe their origin to seafloor fractures associated with the Atlas Mountain range or to an underlying plume or hotspot of superheated mantle material. However, the debate has recently come to a conclusion following the discovery of nannofossils preserved in the products of the 2011–2012 submarine eruption at El Hierro, which tell us about the age and growth history of the western‐most island of the archipelago. Light coloured, pumice‐like ‘floating rocks’ were found on the sea surface during the first days of the eruption and have been shown to contain fragments of pre‐island sedimentary strata. These sedimentary rock fragments were picked up by ascending magma and transported to the surface during the eruption, and remarkably retained specimens of pre‐island Upper Cretaceous to Pliocene calcareous nannofossils (e.g. coccolithophores). These marine microorganisms are well known biostratigraphical markers and now provide crucial evidence that the westernmost and youngest island in the Canaries is underlain by the youngest sediment relative to the other islands in the archipelago. This finding supports an age progression for the onset of volcanism at the individual islands of the archipeligo. Importantly, as fracture‐related volcanism is known to produce non‐systematic age‐distributions within volcanic alignments, the now‐confirmed age progression corroberates to the relative motion of the African plate over an underlying mantle plume or hotspot as the cause for the present‐day Canary volcanism.

Dynamics of deep submarine silicic explosive eruptions in the Kermadec arc, as reflected in pumice vesicularity textures

Despite increasing recognition of silicic pumice-bearing deposits in the deep marine environment, the processes involved in explosive silicic submarine eruptions remain in question. Here we present data on bubble sizes and number densities (number of bubbles per unit of melt matrix) for deep submarine-erupted pumices from three volcanoes (Healy, Raoul SW and Havre) along the Kermadec arc (SW Pacific) to investigate the effects of a significant (>~1km) overlying water column and the associated increased hydrostatic pressure on magma vesiculation and fragmentation. We compare these textural data with those from chemically similar, subaerially-erupted pyroclasts from nearby Raoul volcano as well as submarine-erupted ‘Tangaroan’ fragments derived by non-explosive, buoyant detachment of foaming magma from Macauley volcano, also along the Kermadec arc. Deep submarine-erupted pumices are macroscopically similar (colour, density, texture) to subaerial or shallow submarine-erupted pumices, but show contrasting microscopic bubble textures. Deep submarine-erupted pyroclasts have fewer small (<10μm diameter) bubbles and narrower bubble size distributions (BSDs) when compared to subaerially erupted pyroclasts from Raoul (35–55μm vs. 20–70μm range in volume based median bubble size, respectively). Bubble number density (BND) values are consistently lower than subaerial-erupted pyroclasts and do not display the same trends of decreasing BND with increasing vesicularity. We interpret these textural differences to result from deep submarine eruptions entering the water column at higher pressures than subaerial eruptions entering the atmosphere (~10MPa vs. 0.1MPa for a vent at 1000mbsl). The presence of an overlying water column acts to suppress rapid acceleration of magma, as occurs in the upper conduit of subaerial eruptions, therefore suppressing coalescence, permeability development and gas loss, amounting to closed-system degassing conditions. The higher confining pressure environment of deep submarine settings hinders extensive post-fragmentation clast expansion, coalescence of bubbles, and thinning of bubble walls, causing clasts to have similar BND values regardless of their vesicularity. Although deep submarine-erupted pyroclasts are closely similar to their subaerial counterparts on the basis of bulk vesicularities and macroscopic appearance, they differ markedly in their microscopic textures, allowing them to be fingerprinted in modern and ancient pumiceous marine sediments.

Xenopumice erupted on 15 October 2011 offshore of El Hierro (Canary Islands): a subvolcanic snapshot of magmatic, hydrothermal and pyrometamorphic processes

On 15 October 2011, a submarine eruption offshore of El Hierro Island gave rise to floating volcanic products, known as xenopumices, i.e., pumiceous xenoliths partly mingled and coated with the juvenile basanitic magma. Over the last few years, no consensus in the scientific community in explaining the origin of these products has been reached. In order to better understand the formation of xenopumice, we present a textural, mineralogical, and geochemical study of the possible magmatic, hydrothermal, and pyrometamorphic processes, which usually operate in the plumbing systems of active volcanoes. We carried out a comprehensive SEM investigation and Sr-Nd-Pb isotope analyses on some samples representative of three different xenopumice facies. All the data were compared with previous studies, new data for El Hierro extrusives and a literature dataset of Canary Islands igneous and sedimentary rocks. In the investigated xenopumices, we emphasize the presence of restitic magmatic phases as well as crystallization of minerals (mainly olivine + pyroxene + magnetite aggregates) as pseudomorphs after pre-existing mafic phenocrysts, providing evidence of pyrometamorphism induced by the high-T juvenile basanitic magma. In addition, we identify veins consisting of zircon + REE-oxides + mullite associated with Si-rich glass and hydrothermal quartz, which indicate the fundamental role played by hydrothermal fluid circulation in the xenopumice protolith. The petrological data agree with a pre-syneruptive formation of the xenopumice, when El Hierro basanite magma intruded hydrothermally altered trachyandesite to trachyte rocks and triggered local partial melting. Therefore, the El Hierro xenopumice represents a snapshot of the transient processes at the magma-wall rock interface, which normally occurs in the feeding system of active volcanoes.

Birth of two volcanic islands in the southern Red Sea.

Submarine eruptions that lead to the formation of new volcanic islands are rare and far from being fully understood; only a few such eruptions have been witnessed since Surtsey Island emerged to the south of Iceland in the 1960s. Here we report on two new volcanic islands that were formed in the Zubair archipelago of the southern Red Sea in 2011–2013. Using high-resolution optical satellite images, we find that the new islands grew rapidly during their initial eruptive phases and that coastal erosion significantly modified their shapes within months. Satellite radar data indicate that two north–south-oriented dykes, much longer than the small islands might suggest, fed the eruptions. These events occurred contemporaneously with several local earthquake swarms of the type that typically accompany magma intrusions. Earthquake activity has been affecting the southern Red Sea for decades, suggesting the presence of a magmatically active zone that has previously escaped notice.

Emplacement of submarine lava flow fields: A geomorphological model from the Niños eruption at the Galápagos Spreading Center

In the absence of any direct observations of an active submarine eruption at a mid-ocean ridge (MOR), our understanding of volcanic processes there is based on the interpretation of eruptive products. Submarine lava flow morphology serves as a primary indicator of eruption and emplacement processes; however, there is typically a lack of visual observations and bathymetric data at a scale and extent relevant to submarine lava flows, which display meter to submeter-scale morphological variability. In this paper, we merge submersible-based visual observations with high-resolution multibeam bathymetry collected by an autonomous underwater vehicle (AUV) and examine the fine-scale geomorphology of Ninos, a submarine lava flow field at the Galapagos Spreading Center (GSC).We identify separate morphological facies (i.e., morphofacies) within the lava flow field, each having distinct patterns of lava flow morphology and volcanic structures. The spatial and stratigraphic arrangement of morphofacies suggests that they were emplaced sequentially as the eruption progressed, implying that the Ninos eruption consisted of at least three eruptive phases. We estimate eruption parameters and develop a chronological model that describes the construction of the Ninos lava flow field. An initial phase with high effusion rates emplaced sheet flows, then an intermediate phase emplaced a platform of lobate lavas, and then an extended final phase with low effusion rates emplaced a discontinuous row of pillow lava domes. We then compare this model to mapped lava flow fields at other MORs. Despite disparities in scale, the morphological similarities of volcanic features at MORs with different spreading rates suggest common emplacement processes that are primarily controlled by local magma supply.

The 2011-2012 El Hierro submarine eruption: a challenge of geochemical and thermal imaging for volcano monitoring

A shallow submarine eruption occurred between October 12, 2011 and March 5, 2012 about 2 km off the southern coast of El Hierro, the youngest and smallest of the seven major islands of the Canarian archipelago. This submarine eruption rose from a depth of 363 meters and finally reached 89 meters bsl and provided us a unique chance to study a variety of precursory signals before the eruption as well as to evaluate for the very first time magma emission rates from a shallow submarine eruption using airborne thermal imaging.

The 8 October 2011 Earthquake at El Hierro (Mw 4.0): Focal Mechanisms of the Mainshock and Its Foreshocks

Abstract We have studied the focal mechanism of an M w 4.0 earthquake that occurred on 8 October 2011 in the southwest of El Hierro (Canary Islands), the largest shock of the swarm that preceded the submarine eruption of El Hierro 2011–2012. The joint focal mechanism solution of 34 foreshocks has also been obtained. The results for the mainshock are consistent with a pure double‐couple mechanism of a strike‐slip motion with a reverse component and a focal depth of 12–13 km. The stress pattern obtained from the focal mechanism indicates horizontal compression in a north‐northwest–south‐southeast direction, parallel to the southern ridge of the island, and a quasi‐horizontal extension in an east–west direction. Similar stress pattern is derived from the joint solution of the foreshocks. The occurrence of this family of earthquakes at the moment of the maximum strain rate of the pre‐eruptive swarm suggests that their rupture process is related to tectonic stress, which led to the eruption only two days later, 5 km away from the mainshock epicenter.