The application of a ThHfTa diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province
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Determining the volume of the various types of products of a highly frequent active volcano can be very difficult, especially if most of them are deposited on a growing volcanic cone. The New South-East Crater (NSEC) of Mt Etna, Italy, may be considered one of the best case studies because of tens of paroxysmal episodes which it produced in the last few years. On 25-26 October 2013, a lava fountain at the NSEC produced magma jets up to 500 m high, a maximum ~8 km high column, a multilobate lava flow field 1.3 to 1.5 km long, and almost 30 m of growth in height of the NSEC cone. Mapping of explosive and effusive deposits allowed us to calculate the total volume of erupted products, including lava flows, proximal and distal tephra fallout, and the amount of coarse pyroclastics on the cone. The estimation of the latter products was also confirmed subtracting digital elevation models (DEMs) obtained at different stages of the NSEC growth. Results show that the volume of tephra fallout away from the cone was only <5 % of the total erupted magma, while the total volume of pyroclasts (distal plus proximal fallout) is about a third of the lava volume. Our analysis suggests that, at least for the studied event, three fourth of the involved magma was already partially degassed and thus emitted as lava flows. Hence, the main distinctive character of lava fountains at Etna, i.e. formation of eruption column and propagation of tephra-laden volcanic plumes to tens of km away from the volcano, would not contribute significantly to the final budget of erupted magma of the 25-26 October 2013 eruption. We finally propose that the same magma dynamics probably occur also during most of the common lava fountain episodes.
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Abstract Over the last 20 years, new concepts have emerged into understanding the processes that lead to build up to large silicic explosive eruptions based on integration of geophysical, geochemical, petrological, geochronological and dynamical modelling. Silicic melts are generated within magma systems extending throughout the crust by segregation from mushy zones. Segregated melt layers become unstable and can assemble into ephemeral upper crustal magma chambers rapidly prior to eruption. In the next 10 years, we can expect major advances in dynamical models as well as in analytical and geophysical methods, which need to be underpinned in field research.
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Abstract Despite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.
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<p>Large silicic systems can produce devastating eruptions with emitted volumes greater than 100 km&#179; and worldwide impacts. Such eruptions suggest the presence of significant reservoirs of silicic magma at shallow depths. Understanding how these reservoirs form is crucial to understanding how they affect the surrounding rock. But the shape and the organization of magmatic storage are still debated, despite their crucial influence on the results of theoretical predictions. Based on physical considerations of silicic-magma properties and the continental-crust state of active systems; our hypothesis is that the rise of silicic magma is stopped by the Brittle Ductile Transition. As the relaxation time of the ductile part of the crust is very short compared to the lifetime of such systems, magma storage could be considered as a buoyant liquid stored beneath an elastic plate. We thus used a plate model to theoretically predict the stress above those large magma chambers. To test our hypothesis, we computed the general behaviours of large silicic systems and compared them to natural cases. We first calculated the stress field produced in the plate. Results show that stressed values can reach tens of MPa, which is enough to cause plate failure. Then, we compared reservoir dimensions and volumes predicted by our model when failure could occur with documented ones for past eruptions. We showed that the two are consistent with each other. In a broader perspective, we then showed that stresses produced in the plate by the magma chamber can produce circular faults above the storage zone. This result has direct implications for the understanding of caldera formation during large silicic eruptions.</p>
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This chapter contains sections titled: Introduction Previous Studies Geologic Setting Geologic Evidence Favoring a Large Silicic Magma Body Geophysical Data and Interpretations Geologic Constraints on Silicic Magma Bodies Controls on the Medicine Lake Magma System Possible Magmatic Models Conclusions
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