S. Meletlidis

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.

Monitoring Giant Landslide Detachment Planes in the Era of Big Data Analytics

Open image in new window A small mesh of sensors which monitor movements across detachment planes of the giant San Andres Landslide on the northeastern lobe of El Hierro in the Canary Islands was established in 2013. In this paper we present the results obtained over a two year period spanning from October 2013 to October 2015. Our results demonstrate that the detachment planes are affected by sinistral strike slip displacements and subsidence of the depleted mass of the landslide. While these general trends are consistent the movements recorded at particular monitoring points differ in detail as one site is characterised by progressive strike slip and dip slip trends while another is characterised by movement pulses and reversals in the sense of movement. These findings contrast markedly with suggestions that the giant landslide is inactive and demonstrate that its reactivation is a possibility which cannot be dismissed categorically. Big data analytics have been used to identify interdependence between the recorded movements and a range of climatic and geophysical variables such as seismic data, tidal data, and geomagnetic data. We have found that the recorded movements correlate only weakly or moderately with climatic and seismic parameters but strongly to the horizontal and vertical intensity of the magnetic field. These findings are rather unexpected and we emphasise that special care must be taken in pushing the conclusions of a purely numerical analysis. The advantages of adopting a big data mindset led us to make significant improvements to the instrumental infrastructure in early 2016. These incremental improvements to the small mesh of sensors are driven partly by our desire to understand the kinematic behaviour of landslide itself and partly by our desire to explore the potential of big data analytics in geoscientific research.

Large landslide stress states calculated during extreme climatic and tectonic events on El Hierro, Canary Islands

Composed volcanic edifices are particularly prone to large-scale failures—these often result from the acceleration of preexisting deep-seated gravitational slope deformations. Consequently, a complete understanding of the kinematic behaviour of such slope deformations would represent an important step towards mitigating against human casualties or fatalities and damage to critical infrastructure. In this manuscript, a 9-month time series of three-dimensional fault displacement measurements has been used to determine the stress states of the San Andrés Landslide on El Hierro in the Canary Islands. These stress states have been calculated on the basis of single-displacement events using a novel approach which only requires information about the magnitude of the movement vector and its orientation. The analysis focused on four specific periods: a reference period in November 2013; an extreme rainfall event at the beginning of December 2013; and two endogenous impulses at the end of December 2013 and during the middle of March 2014. On the basis that the direction of principal stress represents a marker for the direction of landslide mass movement, it has been possible to define six landslide activity modes which correspond to specific stress states. The response of the landslide to the extreme rainfall event was immediate and reflected increasing saturation of the porous landslide mass. The response of the landslide to the endogenous impulses was more complicated as compressional pulses often alternated with gravitational relaxation. In this study, it is demonstrated that the landslide stress state can be determined on the basis of a single-displacement event whenever fault displacements are monitored in three dimensions. This innovative approach may represent a valuable step towards a complete understanding of the kinematic behaviour of potentially catastrophic slope deformations, particularly those which are in a critical stability state.

Multidisciplinary Approach to the Study of Holocene Flank Eruptions in Tenerife (Canarias)

A multidisciplinary approach has been applied in a study of eruptive fissures on the slopes of the main volcanic edifices in Tenerife. Our work concentrated on the youngest portion of these cones, including their ages and distribution. Detailed fieldwork was performed, producing a 1:5000-scale geological map of volcanic deposits and structural features. Thus far, an area of about 10 km2 has been surveyed. Sampling in the field was aimed at obtaining age constraints for the main volcanic record over the last 8000 year by means of the archaeomagnetic method. In addition, petrochemical and rheological analyses are currently underway.