New insights into the characteristics and dynamics of rhyolite long-lasting volcanic eruptions
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Abstract
Recent rhyolite eruptions on Earth have demonstrated their capacity to produce a multitude of hazards, including ash formation lasting months and impacting the large reaches of the southern hemisphere. The eruptions of Chaitén volcano in 2008 and Cordón Caulle in 2011, both in Chile, ended a period of almost 100 years without major silicic events. Since then, significant effort has been invested in the characterization of these long-lasting eruptions and their products. However, still many aspects about the dynamics and underlying mechanisms driving rhyolite eruptions remain unravelled.
These rhyolite eruptions provided not only vast amounts of fresh volcanic glass and hence a physicochemical window into the eruption dynamics, but also key temporal aspects of explosive and effusive phases. Here, almost 500 glass shards from the different eruptive phases of 2008 Chaitén eruption were analysed for their H2O content. Results show that eruptive products preserve a wide range of residual H2O contents (0.1 to 3.4 wt.%), with an overall trend of H2O depletion from explosive to effusive. In addition, the presence of “H2O content windows” within stratigraphic horizons of the pyroclastic sequences, defined by upper and lower H2O content values, was detected. Interestingly, these windows shift with stratigraphic position and demarcate clear H2O gaps with respect to effusive obsidians.
Guided by the H2O distributions observed in the different deposits, 94 heating experiments were carried out at 0.1 MPa, temperatures between 740–1030°C on cylindrical obsidian cores (4X10mm) with water content between 0.13–1.4 wt.%. Results reveal different degassing behaviors and deformation timescales of the glassy material as a function of the initial H2O content of the sample. The most striking finding is the occurrence of explosive fragmentation at T >874°C in samples with H2O = 1.4 wt.%. This provides the first observations on explosive fragmentation due to experimental vesiculation of hydrous rhyolite melt and demonstrate that starting H2O concentration and temperature control eruptive behaviour. A comparison with a separate set of high-P experiments performed in a fragmentation bomb —which simulates fragmentation by rapid decompression— suggest that both fragmentation mechanisms might have operated during different stages of 2008 Chaitén eruption.
Finally, the characteristics and impacts of ash-related hazards associated with these long-lasting events were explored. In addition to the impact of primary tephra fallout, a vast area of the Argentinian Patagonia was also significantly affected by wind-remobilization of ash. Results of this study show the primary importance of wind, rainfall and ash availability in controlling the occurrence and persistence of these events. Regarding the impacts, although in the short-term they were highly negative, the environment and the community start showing signs of recovery after five years.