Phreatic eruptions at crater lakes: occurrence statistics and probabilistic hazard forecast

Phreatic eruptions, although posing a serious threat to people in crater proximity, are often underestimated and have been comparatively understudied. The detailed eruption catalogue for Ruapehu Volcano (New Zealand) provides an exceptional opportunity to study the statistics of recurring phreatic explosions at a crater lake volcano. We performed a statistical analysis on this phreatic eruption database, which suggests that phreatic events at Ruapehu do not follow a Poisson process. Instead they tend to cluster, which is possibly linked to an increased heat flow during periods of a more shallow-seated magma column. Larger explosions are more likely to follow shortly after smaller events, as opposed to longer periods of quiescence. The absolute probability for a phreatic explosion to occur at Ruapehu within the next month is about 10%, when averaging over the last 70 years of recording. However, the frequency of phreatic explosions is significantly higher than the background level in years prior to magmatic episodes. Combining clast ejection simulations with a Bayesian event tree tool (PyBetVH) we perform a probabilistic assessment of the hazard due to ballistic ejecta in the summit area of Ruapehu, which is frequently visited by hikers. Resulting hazard maps show that the absolute probability for the summit to be affected by ballistics within the next month is up to 6%. The hazard is especially high on the northern lakeshore, where there is a mountain refuge. Our results contribute to the local hazard assessment as well as the general perception of hazards due to steam-driven explosions

Discovery of active hydrothermal venting in Lake Taupo, New Zealand

The Horomatangi geothermal system of Lake Taupo, New Zealand, is a sub-lacustrine equivalent of subaerial geothermal activity nearby in the Taupo Volcanic Zone (TVZ). The setting of this system is rare within the TVZ as it is directly associated with an individual volcanic feature, that of the 1.8 ka Taupo eruption vent. Two distinct hydrothermal vent areas, named Te Hoata and Te Pupu, have been discovered during dives with the submersible Jago. Venting of gases was seen at both sites and hot water (up to 45‡C) discharges at the Te Pupu site. Dilute water samples have concentrations of SO4, Cl, Na and SiO2 above ambient lake water values. Gas samples have compositions similar to other TVZ geothermal systems. Gas geothermometers indicate the existence of a hightemperature hydrothermal environment beneath the lake with reservoir temperatures in excess of 300‡C. Chimney structures were found at the Te Pupu site. They are up to 30 cm tall and mineralized by an ‘epithermal’ suite of elements, including S, Hg, As, Sb and Tl. The walls of the chimneys are largely composed of diatoms and strands of silicified filamentous bacteria embedded in an amorphous silica groundmass. Bacterial mats are commonly associated with the gas vents and also occur at two hot springs. Close to the vents, commonly perched on top of dead chimneys and/or exposed outcrops, are dense assemblages of what are probably a new species of sponge of the genus Heterorotula. The sponges host a notably diversified, associated invertebrate fauna and represent a previously unseen biomass on the lake floor. The sponges appear to have bored into the mineralized chimneys

39 Years of Geochemical Monitoring of Laguna Caliente Crater Lake, Poás: Patterns from the Past as Keys for the Future

Since 1978 water chemistry of the Laguna Caliente crater lake has been used to monitor volcanic activity at Poás, Costa Rica, making it arguably the best studied hyper-acidic crater lake on Earth. During these 39 years, three of water of Laguna Caliente, independent on previous deterministic research and resulting conceptual models. Common patterns of chemical parameters in relation with phreatic eruptive activity for the period 1978–Septem- ber 2014 are sought, applying the objective statistical method of Pattern Recognition. This resulted in the definition of the strongest precursory signals and their respective thresh- olds. Numerical outcomes often confirm find- ings based on geochemical models (e.g. SO4, SO4/Cl and pH are strong monitoring param- eters). However, some surprising parameters (opposite behavior of Mg/Cl ratios, decreases in Ca and Mg concentrations, increasing Al/Mg ratios) still need a geochemical expla- nation and should be a focus for future research strategies. The obtained parameters and thresholds were retrospectively applied for the “test period” of the Pattern Recognition method (November 2014–February 2016). This test provided hints that suggested that eruptive activity at Poás was not yet over, despite apparent quiescence in early 2016. Indeed, after new phreatic eruptions since May 2016, the 2006–2016 phreatic eruptive cycle culminated in phreatomagmatic activity in April 2017. We conclude that evaluating time series of chemical composition of crater lakes framed in the Pattern Recognition method can be a useful monitoring approach. Moreover, increased sampling frequency can provide more details and more adequate phases of unrest occurred, manifested through frequent phreatic eruptions, with each a dura- tion of several years to over a decade (1978– 1980, 1986–1996, 2006–2016). We here present a novel technique to deal with the long time series of the chemical compositionPublished213-2334V. Processi pre-eruttiv