Invited research article
Large explosive basaltic eruptions at Katla volcano, Iceland: Fragmentation, grain size and eruption dynamics

https://doi.org/10.1016/j.jvolgeores.2018.01.024Get rights and content

Highlights

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    Both vesicular lapilli and blocky ash grains form in explosive basaltic eruptions at Katla volcano.

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    The regularity index model show detailed fragmentation patterns.

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    Granulometric deconvolution modeling is used to assess detailed eruption dynamics.

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    Qualitative observations support interpretation of quantitative tephra data.

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    Subglacial fragmentation is a combination of magmatic and phreatomagmatic processes.

Abstract

Katla volcano in Iceland produces hazardous large explosive basaltic eruptions on a regular basis, but very little quantitative data for future hazard assessments exist. Here details on fragmentation mechanism and eruption dynamics are derived from a study of deposit stratigraphy with detailed granulometry and grain morphology analysis, granulometric modeling, componentry and the new quantitative regularity index model of fragmentation mechanism. We show that magma/water interaction is important in the ash generation process, but to a variable extent.

By investigating the large explosive basaltic eruptions from 1755 and 1625, we document that eruptions of similar size and magma geochemistry can have very different fragmentation dynamics. Our models show that fragmentation in the 1755 eruption was a combination of magmatic degassing and magma/water-interaction with the most magma/water-interaction at the beginning of the eruption. The fragmentation of the 1625 eruption was initially also a combination of both magmatic and phreatomagmatic processes, but magma/water-interaction diminished progressively during the later stages of the eruption. However, intense magma/water interaction was reintroduced during the final stages of the eruption dominating the fine fragmentation at the end. This detailed study of fragmentation changes documents that subglacial eruptions have highly variable interaction with the melt water showing that the amount and access to melt water changes significantly during eruptions.

While it is often difficult to reconstruct the progression of eruptions that have no quantitative observational record, this study shows that integrating field observations and granulometry with the new regularity index can form a coherent model of eruption evolution.

Introduction

Katla volcano in the SE part of Iceland (Fig. 1) is one of the most prominent and hazardous Icelandic volcanoes known to produce major explosive eruptions reaching beyond Iceland (Thordarson and Larsen, 2007; Thordarson and Hoskuldsson, 2008; Biass et al., 2014; Budd et al., 2016). During the last millennium at least two basaltic eruptions from Katla has reached Europe (Det Kongelige Danske Videnskabernes Selskab, 1758; Thorarinsson, 1981) with observed tephra fall in 1625 and 1755, respectively. Even though Katla is a bimodal system (Larsen, 2000, Larsen, 2010; Oladottir et al., 2007, Oladottir et al., 2008), explosive basaltic eruptions by far outnumber silicic ones by 300 to 20 within the Holocene (Larsen, 2000; Oladottir et al., 2007). Katla has not produced an eruption since 1918, although the average repose time since 1500 CE is 47 years (Larsen, 2010), but recent seismic unrest suggests another eruption might be imminent (Budd et al., 2016, Sgattoni et al., 2017). It is therefore very important to understand the mechanisms responsible for creating these potent basaltic explosive eruptions.

The explosivity of the basaltic eruptions in Iceland is usually attributed to interaction of the magma with glacial melt water creating phreatomagmatic explosions (e.g. Oladottir et al., 2007, Oladottir et al., 2008; Larsen, 2010 and references therein). However, the tephra fall deposits of many Katla tephras, including those of the 1625 and 1755 eruptions, consist of vesicular ash and lapilli and they hardly contain any lithics (Oladottir et al., 2007), suggesting a more complex fragmentation history involving magmatic gasses. As fragmentation mechanism has a great influence on the hazard assessment of future eruptions (Németh and Cronin, 2011; Németh et al., 2012) it is of key importance to sort out the details of the fragmentation process driving large explosive basaltic Katla eruptions.

Therefore this study aims to frame the fragmentation mechanism(s) of the 1755 and 1625 eruptions through detailed studies of the deposits with detailed stratigraphy, granulometric analysis, contemporary descriptions of eruption dynamics and weather conditions, and the fragmentation index of the ash component. These two eruptions had a major impact in Iceland according to the contemporary sources (Hallgrímsson, 1870) and have left thick and widespread deposits. However, the deposits are not uncharacteristically thick compared to the total Icelandic Holocene tephra record for Katla (Larsen, 2000; Oladottir et al., 2008; Thordarson and Hoskuldsson, 2008; Larsen, 2010), and therefore represent a likely future eruption scenario. With Katla being a very active system and overdue for the next explosive eruption, the information on fragmentation-and eruption dynamics could in turn influence hazard assessments of past and future Katla eruptions.

Section snippets

The Katla volcanic system

The Katla volcanic system is ~80 km long and situated on the Southern part of the Eastern Volcanic Zone (EVZ) in Iceland (Fig. 1). It consists of the 30 km wide Katla central volcano and the 75 km southwest-northeast trending Eldgjá fissure swarm (Fig. 1). Katla central volcano is partly covered by the 600 km2 Myrdalsjökull glacier, which reaches an average thickness of 230 m (Björnsson et al., 2000). The central volcano is situated on the SW part of the Katla volcanic system, while the Eldgjá

Key section

To make a good characterization of the fragmentation pattern for the two large eruptions in this study, we needed a well-preserved deposit. However, due to the presence of the Myrdalsjökull glacier the most proximal undisturbed tephra sections are located >14 km from the suspected vent area outside the glacier itself (Fig. 1). Work on the main axis of historical Katla eruptions of Larsen (2010) show that downwind sections are found in the areas E/NE of Katla (Fig. 1), and we have selected a

The 1755 Rjupnafell field deposit and componentry

The 1755 deposit can be divided into 7 units (Fig. 2a,b), where each unit consists of a poorly consolidated ash layer overlain by loose lapilli. The lack of flow structures and textures in any of the beds support that this is a primary fall deposit. The deposit consists of ash to lapilli sized juvenile grains. Componentry of the 3–4φ (125–63 μm) ash shows that brown sideromelane glass-shards dominates this grain size making up 67–75%, while opaque tachylite represents 23–33% (Fig. 3a). Crystal

An integrated interpretation of eruption dynamics

Since the Rjupnafell section is at a medial distance of 22–23 km from the vent area we assume that only substantial changes in the eruption dynamics or local weather would be recorded in the tephra deposit. We have therefore aimed at relating major changes in the deposit to changes in eruption activity and weather conditions.

Implications for fragmentation modeling of basaltic eruptions

Recent studies have shown that fragmentation in subglacial eruptions is not solely driven by magma/water-interaction (Graettinger et al., 2013; Jude-Eton et al., 2012; Cioni et al., 2014; Liu et al., 2015). Graettinger et al. (2013) reported feedback loops between decoupled gas magma eruption and phreatomagmatic explosions, which are in line with our observations.

Heap et al. (2014) showed that large vesicles in low porosity magma could decrease the brittle strength of the magma more than small

Conclusion

We have studied the eruptive processes of the two large basaltic explosive eruptions of 1755 and 1625 from Katla from 3 different datasets. The field data show that the eruptions are comparable in terms of total thickness, a cyclic eruptive behavior, and componentry. However, there are differences in grading, apparent vesicularities of lapilli, and thicknesses of units. Grain size variations show a similar range of grain sizes and an overall bimodal grain size distribution for both deposits,

Acknowledgements

This project is part of a PhD-project conducted on the 1755 and 1625 eruptions from Katla and part of the ICAO data collection. We would like to extend our grateful thanks to Eirik Gjerløw, Cathrine Gallagher, William Moreland, and Rob Askew for assisting with and making fieldwork in SE Iceland most enjoyable. Our sincere gratitude also goes to Bergrún Óladottír for her helpful input and comments. Finally we would also like to acknowledge the financial support of the Nordic Volcanological Center

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