Insights into the dynamics and evolution of the 2010 Eyjafjallajökull summit eruption (Iceland) provided by volcanic ash textures

- The dynamics 2010 Eyjafjallajökull eruption is described through ash texture.
- Magma fragmentation was mainly related to decompression-driven gas exsolution.
- Ash records the shift between processes of brittle vs ductile magma fragmentation.
- Fragmentation due to magma-water interaction was confined to the opening phase.
- Ash texture reveals to be a good tracer of changes in eruption and magma dynamics.

The April-May 2010 eruption of the Eyjafjallajökull volcano (Iceland) represents an example of explosive activity dominated by prolonged, low- to mid-intensity emission of ash, as it was characterized by a continuous injection of tephra into the atmosphere that affected various economic sectors in Iceland and caused a world-wide interruption of air traffic. This eruption has become a benchmark for the understanding of the processes that govern the dynamics of ash-dominated eruptions, also representing a unique opportunity for direct ash particle investigation.In this paper, selected ash samples from all phases of the eruption were studied in order to characterize: 1) the morphology, composition and texture of ash fragments; 2) the variability of the products of each phase of the eruption; 3) the progressive changes of these features with time. The large morphological and textural variability of the ash fragments throughout the eruption is unrelated to any important compositional change, and it reflects changes in eruption dynamics and in the mechanisms of magma fragmentation. Textural and morphological features of juveniles suggest that primary magma degassing dominated and modulated the dynamics of the entire eruption, while hydromagmatic fragmentation was particularly effective only in the very initial phase. As a consequence, the large production of fine-grained ash cannot be attributed to processes of magma-water interaction, and mechanisms of brittle to ductile fragmentation related to magma degassing or ash recycling must be invoked. The study demonstrates how the textural and morphological analysis of ash fragments can give important hints to the understanding of the eruption dynamics of complex, long lasting eruptions.

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