Phreatomagmatic to Strombolian eruptive activity of basaltic cinder cones: Montaña Los Erales, Tenerife, Canary Islands

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Abstract

Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.

A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.

Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.

Introduction

Hydrovolcanism results from the interaction between magma/lava and external water, including groundwater, surface water, seawater, meteoric water, hydrothermal water, ice-melt water or lake water (Morrissey et al., 2000). Phreatomagmatic eruptions are driven primarily by the volumetric expansion of water as it is heated through contact with magma, causing the magma to fragment explosively.

Phreatomagmatic or hydrovolcanic activity is a frequent phenomenon worldwide, can occur in a wide variety of environments, and can exhibit a considerable range of eruptive phenomena (e.g. Lorenz, 1987). This style of activity is not always readily recognised in the eruptive record of a particular volcano and therefore not automatically considered a major factor for risk assessment. Phreatomagmatism is, however, quite distinct from pure magmatic fragmentation in both eruptive phenomena and types of fragments, producing unique juvenile grain populations that reflect the relative amounts of water and magma involved in the process, as the generated particle sizes reflect the kinetic energy available (Heiken, 1972, Heiken and Wohletz, 1985, Marshall, 1987, Büttner et al., 1999, Zimanowski et al., 2003). Analysis of tephra and tephra morphologies, therefore, allows to a) interpret the relative contribution of magma vesiculation versus water interaction in the eruption of certain pyroclasts and b) to reconstruct eruptive regimes on the basis of the degree of fragmentation and alteration to infer a qualitative measure of the kinetic energy released during explosive volcanic eruptions. This is a key parameter for volcanic hazard assessment and civil defence (Büttner et al., 2006).

The present paper focuses on Los Erales cinder cone in the Southeast of Tenerife in the Canarian Archipelago as a case study for phreatomagmatism in an ocean island setting. Los Erales is of particular interest as this cinder cone is recognised as having undergone a sequential eruptive regime from phreatomagmatic, through transitional to Strombolian within a lineament of almost exclusively Strombolian cinder cones, thus representing a classic example of variability of eruptive processes due to external factors. The role of phreatomagmatic activity on the Canary Islands as a whole is also reviewed and several examples of such activity are discussed.

Section snippets

Geological setting

The Canarian archipelago lies along the Northwest passive margin of the African plate (Fig. 1) and the island of Tenerife has an area of 2057 km2 and a central volcanic complex with a summit height of 3718 m. It is thus the largest of the Canary Islands and indeed the third highest oceanic-island volcanic structure in the world (Carracedo and Day, 2002). In 1990, Pico de Teide, Tenerife's central volcano, was selected for study in the International Decade for Natural Disaster Reduction and in

Tephra analysis

Eruptive history can be deduced by studying the deposits of a volcano, as tephra morphologies are thought to reflect the fragmental regime at the time of eruption (e.g. Heiken and Wohletz, 1985, Büttner et al., 1999). Thus, explosivity, or kinetic energy of an eruption, can be inferred from its deposits and thus elucidate dynamic eruptive behaviour (e.g. Büttner et al., 2006 and references therein).

Constraining the eruptive nature of past eruptions is imperative for the understanding of

Eruptive history of Los Erales and water source for the phreatomagmatic activity

Styles of eruptions and types of products at cinder cones may change on short time scales, within minutes or hours depending on a number of factors such as vent conditions, type of magma erupted, volatile content, confinement pressures, and chamber and conduit morphology at the time of eruption. Different eruptive phases may be identified within deposits that allow us to reconstruct the conditions that prevailed at the time of eruption. Los Erales can be considered composite in terms of

Conclusion

Quaternary vent alignment in Southern Tenerife shows Strombolian to phreatomagmatic activity with both seawater (Amarilla) and ground or surface water influence (Erales), within the close proximity of only a few km. Los Erales vent began erupting explosively due to the interaction with a fresh water source and the initial phreatomagmatic events produced abundant “muddy” surge deposits interlayered with highly fragmented fallout deposits. With proceeding eruptive activity the water source became

Acknowledgements

We would like to acknowledge the help of Kevin Byrne and Rebecca Gould during data acquisition, and the technical support from Neal Leddy of the Centre for Microscopy and Analysis Trinity College. Furthermore we thank Chris Stillman, Andreas Klügel and Ben van Wyk de Vries for discussions, François Xavier Devuyst for help with binocular photography and Manel Leira of the Botany Department, Trinity College for inspecting diatom fragments. We are also grateful to Pauline Agnew, Declan Burke, Neil

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    Present address: Uppsala University, Dept. of Earth Sciences, Villavägen 16, SE-752 36, Uppsala, Sweden.

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