Eruption dynamics and frequency-magnitude relationships of explosive eruptions at Mt. Ruapehu, New Zealand over the past 1800 years : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

Appendices A-I (listed on pp. 259-261) have not been included in the thesis, but may be consulted by contacting the author at [email protected] to moderate explosive eruptions (VEI ≤ 3) constitute the most frequent eruptions and often involve several phases characterised by different eruption dynamics. Deposits associated with small-scale multi-phase eruptions tend to be underrepresented in geological records and the resulting probabilistic eruption forecast models. This PhD research presents a refined high-resolution tephrostratigraphic framework for the 1800-year Tufa Trig Formation at one of New Zealand’s most active volcanoes, Mt. Ruapehu. This framework is used to characterise short- and long-term changes in eruption behaviour aiming to identify time-variable processes in the volcanic system of a long-lived andesite volcano. Systematic mapping and lithosedimentological characterisation of tephra deposits are combined with geochemical fingerprinting and radiocarbon dating to create a detailed frequency-magnitude record of single- and multi-phase eruptions of the last 1800 years. At least 32 eruptions can be identified, ranging from low to mid-intensity single-phase eruptions (1–10 × 10⁶ m³ deposit volumes) to complex multi-phase eruptions up to two magnitudes larger. The largest eruption is the T13-sequence that comprises at least 5 eruption phases. Multi-lobate dispersal pattern and componentry analyses show that individual eruption phases represent multiple fall events of similar eruption style and magnitude. Major and trace element analyses of juvenile glass display limited syn-sequence variability, while heterogeneous pyroclast and textural characteristics suggest that short-term changes in eruption behaviour are predominantly controlled by shallow conduit processes. The frequency-magnitude record is integrated with geochemistry and statistical modelling, identifying time-variable pattern in Mt. Ruapehu’s eruption behaviour: the time span 1718–1300 cal BP involves low-intensity single-phase eruptions every ~40 years and is followed by a low rate regime (one eruption every 125 years). The largest multi-phase eruptions of the last two millennia occur between 610 and 370 cal BP, while the past 370 years are dominated by smaller multi-phase eruptions every ~40 years, suggesting that long-term changes in Mt. Ruapehu’s eruption behaviour are related to changes in magma supply. This research adds critical complexity to the understanding of the processes and timescales controlling eruption behaviour in the southern Taupo Volcanic Zone and provides insights into the dynamic behaviour of small to moderate multi-phase eruptions. These results will constitute the framework for refining dynamic eruption forecast models at Mt. Ruapehu and other similar volcanoes globally

Physical characterization of long-lasting hybrid eruptions: the 2021 tajogaite eruption of Cumbre Vieja (La Palma, Canary Islands)

Long-lasting, hybrid eruptions can be of complex description and classification, especially when associated with multiple eruptive styles and multiple products. The 2021 Tajogaite eruption of La Palma, Canary Islands, was associated with a magma-gas decoupled system that resulted in the simultaneous emission of lava flows and tephra plumes from various vents. Even though the tephra blanket (∼2 × 107 m3) represents only 7%–16% of the total erupted volume, it provides fundamental insights into the overall eruptive dynamics. Tephra was mostly dispersed NE-SW due to a complex regional and local wind patterns and was subdivided into 3 units and 11 layers that well correlate at different distances from the vent and with both tremor data and lava emission rate. While plume height varied at the temporal scale of a few hours, the average mass eruption rate associated with the tephra blanket of the different units remained relatively constant (∼3–4 × 103 kg s−1). In contrast, the emission rate of lava largely increased after the first week and remained higher than the overall emission of tephra throughout the whole eruption (average value of ∼6 × 104 kg s−1). Based on a detailed characterization of the tephra blanket in combination with atmospheric wind, tremor, and lava emission trend, we demonstrate the need of (a) multidisciplinary strategies for the description of hybrid eruptions that account for both the duration of individual phases and the quantification of the mass of multiple products, and of (b) dedicated ash dispersal forecasting strategies that account for the frequent variations of eruptive and atmospheric conditions.Research activities were supported by Swiss National Science Foundation (Grant 200020_188757) and by the projects (a) VOLRISKMAC (MAC/3.5b/124) and (b) VOLRISKMAC II (MAC2/3.5b/328), financed by the Program INTERREG VA Spain-Portugal MAC 2014–2020 of the European Commission; (c) Cumbre Vieja Emergencia, financed by the Science and Innovation Ministry, Spanish Government; and (d) Tfassistance, financed by the Cabildo Insular de Tenerife. JER fieldwork was partially financed through NSFGEONERC-DisEqm (NERC Reference: NE/N018575/1) and V-PLUS projects (Prof. Mike Burton).Peer reviewe

Compositional studies of the Baia-Fondi di Baia eruption, Campi Flegrei, Italy: Insights into the magmatic system

The Baia-Fondi di Baia eruption is a small-scale eruptive event which occurred in the western part of the Campi Flegrei caldera (CFc). The CFc is an active volcanic system situated in the western part of the densely populated area of Naples Bay, Italy. In its past it has been characterised by various epochs of volcanic activity. The eruptions of the Campanian Ignimbrite (CI, 39 ka BP) and of the Neapolitan Yellow Tuff (NYT, 15 ka BP) have determined the Campi Flegrei’s present morphology and its dominant, partly submerged, nested caldera structure. In the time span following the NYT eruption, about 70 known eruptive events have taken place within the caldera; they can be mainly subdivided in 3 eruptive epochs, separated by periods of quiescence of variable duration: Epoch 1 (15.0–9.5 ka), Epoch 2 (8.6–8.2 ka), and Epoch 3 (4.8–3.7 ka). The most recent eruption is represented by the event of Monte Nuovo at 1538 AD. The Baia-Fondi di Baia eruption marks the onset of Epoch 2; it occurred after a 1000 years-long quiescence and is dated back to 9525–9696 cal BP (Di Vito et al., 1999; Smith et al., 2011). The aim of this study is the understanding of the magmatic system and the magmatic processes that took place prior to the eruption and which likely led to the reactivation of the Campi Flegrei plumbing system after a period of quiescence. The reconstruction is based on field investigations, compositional analyses of the collected samples and further investigations on the geochemical and mineralogical characteristics of the deposits. The obtained results reveal that the Baia-Fondi di Baia eruption can be considered as a complex series of events which can be grouped in two main eruptive sequences (Baia and Fondi di Baia). Two magmatic systems have been identified at a pressure range between ~20 and 170 MPa. They involve a shallow, isolated reservoir of trachytic magma and a more complex, interconnected system at greater depths with compositions ranging from phonolite to trachyte. Both systems were tapped during the Baia as well as during the Fondi di Baia event, though variations in relative proportions of the two involved magmas are evident between the two eruptive sequences

Explosive eruptions at Stromboli volcano (Italy): a comprehensive geochemical view on magma sources and intensity range

International audienceA comprehensive understanding of the processes that occur during magmatic storage and pre-eruptive ascent—and of their associated timescales—is critical to identifying potential precursory signals, and to developing robust volcano early-warning systems. Stromboli's persistent activity comprises continuous degassing and explosive activity that ranges from hourly, low-intensity "normal" activity to occasional, more violent, paroxysmal activity. While the magma source processes that drive normal and paroxysmal activity are reasonably constrained, eruptive activity intermediate in magnitude and intensity (i.e., major explosions) remains elusive in terms of classification, source region, and pre-eruptive timescales. Here, we investigate the 19 July 2020 major explosion that geophysical parameters place at the upper limit of the major explosions field, close to small-scale paroxysms such as the 2003 and 2007 events. The geochemical signatures of matrix glass, olivine, melt inclusions, and embayments—integrated with gas measurements—highlight important differences in eruption source, ascent behaviour, and pre-eruptive timescales of the studied event when compared to paroxysms. Melt inclusion volatile contents identify that magma rise begins from a slightly shallower source (~9.5 km below sea level, b.s.l.) than for paroxysms (11.4 km b.s.l.), with the activation of a shallower ponding zone at 5-6 km b.s.l.. This, in combination with intermediate matrix glass compositions, suggests complex ascent behaviour, characterised by CO2 buffering in the deep ponding region and magma self-mixing in the shallower zone. Fe-Mg-diffusion modelling in olivine indicates a system perturbation starting ~20-25 days before eruption onset, in agreement with the timescale of volcanic gas CO2/SO2 ratio changes observed in the plume, and significantly shorter than that observed prior to paroxysms (~4 months). The geochemical dataset provides insights into the processes controlling the steady-state conditions and the broad spectra in eruption magnitude and intensity at Stromboli and bears important implications for eruption forecasting

Magmatic reactivation of the Campi Flegrei volcanic system: insights from the Baia–Fondi di Baia eruption

The Baia–Fondi di Baia was a multi-stage, small-scale eruption which occurred in the western part of the Campi Flegrei caldera at 9525–9696 BP, marking the onset of Epoch 2 of post-Neapolitan Yellow Tuff volcanism. The eruption was characterized by a complex series of events related to two distinct eruptive episodes (Baia and Fondi di Baia) separated by a short time interval, and each characterized by several eruptive phases. Mineralogical, geochemical (major, and trace elements on whole rocks, major and volatile elements on matrix glasses, and melt inclusions), and Sr isotope characterization of the tephra material sampled along the entire sequence was carried out in order to constrain magmatic evolution and dynamics of the feeding system. Three main compositional groups were identified in matrix glasses and interpreted as representative of different magma bodies: (i) a trachyte (SiO260.3–64.7 wt.%), which is volumetrically predominant; (ii) a tephriphonolite-latite (SiO2: 55.1–57.9 wt.%); and (iii) an intermediate magma group between phonolite and trachyte compositions. This wide compositional heterogeneity contrasts with the narrow variability recognized in the bulk-rock compositions, which are all trachytic. Mineral, melt inclusions, and Sr isotope data suggest that the trachytic magma possibly derived from the Campanian Ignimbrite reservoir located at 6–9 km depth. Volatile content in matrix glass indicates a storage depth of at least 6 km for the tephriphonolite-latitic magma. The intermediate magma is interpreted as being derived from a remelting and assimilation process of a partially crystallized trachytic body (crystal mush) by the hotter tephriphonolite-latitic magma. As the tephriphonolite-latite was erupted together with the trachyte from the beginning of the eruption, we suggest that the ascent of this magma played a fundamental role in triggering the eruption. Upwards through the tephra sequence, we observed a progressive increase of the tephriphonolite-latitic and intermediate phonolite-trachytic components. The presence of banded clasts characterized by different compositions is also indicative of syn-eruptive mingling during the final phases of the eruption

Tephra sedimentation and grainsize associated with pulsatory activity: the 2021 Tajogaite eruption of Cumbre Vieja (La Palma, Canary Islands, Spain)

Long-lasting eruptions are of complex characterization and are typically associated with challenging risk assessment and crisis management due to the usual occurrence of multiple interacting hazards evolving at different temporal and spatial scales (e.g., lava, tephra, and gas). The 2021 Tajogaite eruption of Cumbre Vieja (La Palma) demonstrated how even hybrid events that are mostly effusive can be associated with widespread and impacting tephra deposits as a result of a complex interplay among gas flux, conduit geometry, and magma feeding rate. In this novel study, direct observations, syn-eruptive and post-eruptive sampling, and statistical analysis of pulsatory activity have been combined to provide new insights into eruption dynamics. They show how rapid gas segregation and high magma ascent rate modulated the gas flux at multiple vents, resulting in short-time fluctuations among the different explosive styles (ash-poor gas puffing, Strombolian, violent Strombolian, and lava fountaining) and unsteady tephra ground accumulation. Various size-selective sedimentation processes were also observed, including particle aggregation and ash fingers, which have impacted the overall tephra dispersal. In fact, even though both local and total grainsize distributions of selected layers, units, and of the whole tephra blanket are unimodal with a low fine-ash content, grainsize analysis of 154 samples suggests no correlation of particles &amp;lt;63 μm with distance from vents. Our analyses demonstrate the need to include a detailed characterization of all products of hybrid eruptions for a comprehensive interpretation of eruptive dynamics and to use multiple classification strategies that can capture eruptive styles at different temporal scales.</p