Shift from magmatic to phreatomagmatic explosions controlled by the lateral evolution of a feeder dike in the Suoana-Kazahaya eruption, Miyakejima Volcano, Japan

doi:10.1016/j.epsl.2019.01.038

Earth and Planetary Science Letters

Volume 511, 1 April 2019, Pages 177-189

https://doi.org/10.1016/j.epsl.2019.01.038 Get rights and content

Highlights

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Downslope migration of magma caused the phreatomagmatic explosions in Miyakejima.
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Buried caldera accommodates groundwater in the volcanic edifice for magma-water interaction.
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Drop of magmatic overpressure caused the invasion of groundwater to the conduit.
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Geology of edifice and dike-propagation pattern show the potential of phreatomagmatic explosion.

Abstract

Explosive phreatomagmatic eruption is one of the most hazardous eruption styles, particularly in basaltic systems, as the instability of the conduit system can result in a sudden and unexpected shift of eruption style from a mild effusion of lavas to violently explosive activity. The geological investigations on the phreatomagmatic activities in the 7th Century, Suoana-Kazahaya eruption (SKE) of Miyakejima, reveal that the drop of magmatic overpressure in conduit and the distribution of groundwater controlled the occurrence of phreatomagmatic explosion. The “dry” magmatic eruption in the initial phase of the SKE indicates that the positive overpressure of magma in the propagating feeder dike prevents the invasion of external groundwater into the conduit. Explosive phreatomagmatic eruption occurred at the later phase of the SKE from the vents in the higher elevation. The drop of magmatic overpressure in the upper part of the feeder dike caused by the downslope propagation of the eruption fissure allowed groundwater inflow to the still hot and molten feeder dikes. The limited distribution of phreatomagmatic activities indicated the development of shallow groundwater, hosted in the porous pyroclastic deposits within a basin of less-permeable older edifice. As shifts of eruption style from initial magmatic to later phreatomagmatic explosive eruption style in the top of mafic volcanoes are globally observed in many mafic volcanic systems, such as Kilauea and Mt. Etna, this is probably a far more common eruption mechanism, than previously thought, and hence it needs to be considered in volcanic hazard scenario descriptions. The spatial distributions of phreatomagmatic activities in the SKE suggest that the detection of buried caldera structure in a volcanic edifice can indicate a potential site for phreatomagmatic explosion. The monitoring of the propagation of eruption fissure and drops of magmatic pressure and flux can indicate the potential of the phreatomagmatic explosion by the invasion of groundwater into the hot conduit.

Introduction

The mingling of magma and external water, in a near-surface section of a volcanic conduit, causes explosive phreatomagmatic activity (Wohletz, 2002 and references therein). It is one of the most hazardous eruption styles, particularly in basaltic systems (Lorenz, 2007 and references therein), as the instability of the conduit system can result in a sudden and unexpected shift of eruption style from a mild effusion of lava to violently explosive activity. Therefore, forecasting of the potential sites of phreatomagmatic explosions based on the geological structures of the edifice and the evolution pattern of eruption can contribute the precise hazard evaluation of the violent explosive phreatomagmatic eruption.

Occurrence of phreatomagmatic eruptions is an interesting problem, because a phreatomagmatic eruption derives from a complex interaction of magma and surrounding water either as standing water bodies and/or ground water tables. Magma-water interaction within a volcanic conduit is controlled both by external factors, such as the surrounding hydrological environment and host-rock geology (e.g., Morrissey et al., 2000, Houghton et al., 2004, Rosseel et al., 2006, Valentine, 2012, Kósik et al., 2016), and internal factors, such as physico-chemical properties of magma, the structural development of the volcanic feeder system, and magma dynamics (e.g., Liu et al., 2017).

Distribution of water in the volcanic edifice (Celico and Summa, 2004) is an external control for the occurrence of phreatomagmatic activity (Kereszturi et al., 2011, Delcamp et al., 2016). Statistically significant link between the location of aquifer-hosting lithologies and the presence of phreatomagmatic vents across volcanic fields (e.g., Kereszturi et al., 2017) implies that understanding groundwater distributions, and its surrounding within a volcanic edifice, are crucial requirements for understanding the potential for phreatomagmatic activities in composite or stratovolcanic systems (e.g., Finizola et al., 2006, Sohn et al., 2012, Agustin-Flores et al., 2015, van den Hove et al., 2017, Reyes-Guzman et al., 2018).

The internal magmatic processes, such as the flow pattern and pressure distribution in a conduit may control the entrainment of external water into the magma within the conduit. Particularly, evolution of a conduit geometry, through propagation of a feeder dike, can be important, as it may change the flow pattern and pressure distribution of magmas inside a conduit (e.g., Agustin-Flores et al., 2015). The temporal evolution of the phreatomagmatic activities observed in several fissure eruptions, such as the 2001 AD eruption of Mt. Etna (Behncke and Neri, 2003), might have been controlled by the change of internal magmatic process.

Recognizing the relative role of the external and internal controlling factors on the influence of eruptions styles from the magmatic to phreatomagmatic explosive spectrum is a fundamental geological problem and their resulting pyroclastic deposits in the geological record need to be precisely identified. To understand the combined effects of internal and external factors on the occurrence of phreatomagmatic eruptions, combined analyses of the eruption sequence, textural characteristics of ejecta, and the underground geometry of the conduit may highlight the control mechanism for phreatomagmatic eruptions. However, there are few examples on Earth where information is available together on the eruption sequence, on the eruptive materials, and on the geological structure of phreatomagmatic explosion sites.

Here we present an evidence of a significant change of eruption style from magmatic to phreatomagmatic during the propagation of the feeder dike of the Suoana-Kazahaya eruption (SKE) of the Miyakejima volcano, Japan. The SKE is a unique example with the detailed geological evidences for the eruption sequence including the propagation of the eruption fissure, timing and spatial distribution of the explosive phreatomagmatic activity, as well as the geological structure of the volcanic edifice with buried caldera structures. Using the SKE, we examine the timing of the phreatomagmatic activities to evaluate the contributions of the change of magma flows within the conduit, as the internal factor, and the distribution of the phreatomagmatic activities to evaluate the effect of ground water distribution, as the external environmental factor. The case study of SKE reveals the general potential of sudden phreatomagmatic explosion during the downslope propagation of fissure eruption in a composite volcano with the development of groundwater.

Section snippets

Geological background

Miyakejima is an active, basaltic-andesitic volcano, sitting on the northernmost part of the Izu-Mariana volcanic arc (Fig. 1A). It has a gently sloping, conical volcanic edifice, is ∼12 km in diameter at its submarine base, and has a height of ∼1.2 km, including its submarine part (e.g., Tsukui et al., 2005). The subaerial part of the edifice forms a volcanic island, which has a semi-circular shape, ∼8 km in diameter and ∼750 m in height above sea level (ASL). The subaerial part of Miyakejima

Suoana-Kazahaya eruption fissure

The SKE was one of the largest eruptions, in terms of the volume of magma output, during the post-Hatchodaira caldera period (from 2.5 ka to present). The eruption occurred from a ∼3 km-long fissure, running N-S along the northern slope of the volcano (Fig. 1C). The total volume of the tephra was estimated to be $\sim 5 \times 10^{- 2} {km}^{3}$ , using the method of Legros (2000). Radiocarbon ages obtained from the carbonized plant fragments in the deposit indicated that the eruption occurred in the 7th Century AD

Stratigraphy of the fall-out deposit

Fall-out tephra from the eruption fissure was mainly distributed along the eastern side of the eruption fissure (Fig. 1C), though a thin deposit can be detected all over the island. Based on lithofacies, the deposits were divided into four fall units, from Unit I at the base, to Unit IV at the top (Fig. 3).

Here, the lithofacies of the deposits, and the petrological characteristics of the juvenile materials are described. The procedures used to analyze the whole rock chemical compositions, the

Reconstruction of the eruption sequence

Fig. 9 illustrates the sequential and spatial evolution of the SKE reconstructed from the stratigraphy (Fig. 3), and distribution (Fig. 4) of the tephra and texture of the juvenile clasts (Fig. 7). The distribution of Unit I and II deposits shows that the eruption started from the craters No. 1 and 2 in the uppermost part of the eruption fissure and then propagated down the slope to the crater No. 8. The distribution of Unit III deposits indicates that the explosive activities occurred in the

Conclusions

The distribution and sequence of phreatomagmatic activity during the SKE of Miyakejima highlights internal and external factors influencing occurrence of phreatomagmatic activity.

1) The spatial distribution of the phreatomagmatic activities in the SKE fissure reflects how the local distribution of shallow groundwater was controlled by the buried caldera structure. Porous pyroclastic deposits filling the less-permeable caldera structure accommodated groundwater perched inside the volcanic

Acknowledgments

The field surveys in Miyakejima were supported by the Japan Meteorological Agency. A field survey and petrological analysis of the tephra were supported by JSPS KAKENHI Grant Number 16H06348. This paper is a direct result of a grant from the Catalyst Seeding JSPS – Royal Society of New Zealand Project [CSG-MAU1603/2018–2019]. The review comments by Alison Graettinger and an anonymous referee and the comments by Tamsin Mather, the editor in charge, improved the original manuscript.

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This allowed the water from the aquifer to interact explosively with the rising magma, causing the beginning of the phreatomagmatic activity (phase I) and then, the progressive widening of the crater (during phase II). Similar fissure aligned vent shifting and associated eruption style changes as a reflection of the substrate water saturation has recently been observed in Miyakejima Island in Japan (Geshi et al., 2019). Several other examples are known from the literature documenting lateral migration of the explosion loci towards water-saturated parts of the substrate causing variations in the eruption styles (Ort and Carrasco-Núñez, 2009; Sohn and Park, 2005).
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Shift from magmatic to phreatomagmatic explosions controlled by the lateral evolution of a feeder dike in the Suoana-Kazahaya eruption, Miyakejima Volcano, Japan

Highlights

Abstract

Introduction

Section snippets

Geological background

Suoana-Kazahaya eruption fissure

Stratigraphy of the fall-out deposit

Reconstruction of the eruption sequence

Conclusions

Acknowledgments

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Tectonophysics

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

Earth-Sci. Rev.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

What makes flank eruptions? The 2001 Etna eruption and its possible triggering mechanisms

Bull. Volcanol.

Evolution of deeper basaltic and shallower andesitic magmas during the ad 1469–1983 Eruptions of Miyake-Jima Volcano, Izu–Mariana Arc: inferences from temporal variations of mineral compositions in crystal-clots

J. Petrol.