Active hydrothermal fluids circulation triggering small-scale collapse events: the case of the 2001–2002 fissure in the Lakki Plain (Nisyros Island, Aegean Sea, Greece)

In 2001–2002, two ground collapses occurred in the island of Nisyros (Aegean Sea, Greece), which formed a 600 m long and up to 5 m wide fissure in the vegetated central part of the Lakki Plain caldera. The fissure was alternatively ascribed to tensional stress release and hydrothermal alteration. In this study, we present original data of diffuse CO2 soil fluxes, soil temperatures, mineralogical and chemical composition of the calderafilling deposits exposed on the fissure walls, and chemical and isotopic composition of interstitial soil gases collected from: the bottom of the fissure, the adjacent vegetated areas, the hydrothermal craters, and selected sites outside the caldera. The occurrence of intense hydrothermal alteration was shown by both mineralogical and chemical analyses of the fissure walls material. Typical mineral assemblage and enrichments in incompatible elements related to advanced argillic alteration, characterizing steam-heated hydrothermal environments, were recognized. Although the low-permeable sediment cover in the Lakki Plain concealed the underneath hydrothermal gas flow, preventing anomalous soil temperatures and CO2 fluxes, the chemical and isotopic composition of the interstitial soil gases revealed an active hydrothermal fluids circulation below the collapsed area, likely controlled by buried structural lineaments. Hydrothermal alteration can then be invoked as the most likely trigger mechanism for the 2001–2002 collapse event

Modelling Tephra Thickness and Particle Size Distribution of the 1913 Eruption of Volcán de Colima, Mexico

A crucial problem at most volcanoes is reconstructing past eruptions from the geological record. The rapid erosion of many volcanic terrains results in geologically recent eruptions leaving a relatively sparse record of the event. Here we consider the tephra-stratigraphic record of the 1913 eruption of Volcán de Colima, a recent but greatly eroded tephra fallout deposit. A total of 38 stratigraphic sections of the 1913 deposit have been analysed for thickness, granulometry and geochemistry. The 1913 scoria are hornblende and two-pyroxene andesites with approximately 58 wt% SiO2, providing a geochemical and petrographic signature that is distinct from earlier (1818) and later tephra fallout deposits. Tephra2, a tephra dispersion computer code based on the advection-diffusion equation, is used to model thickness variation and particle size distribution of the pyroclasts for the 1913 deposit. Based on computer simulations, the observed tephra stratigraphy is best fit with a total eruption mass of ~5.5 × 1010 kg. Computer simulations including reports of tephra accumulation from the historical record produces an alternative deposit model with a finer median particle size (~1.77 ϕ), a higher eruption column (~25 km above mean sea level, amsl), and a greater total eruption mass (~1.4 × 1011 kg). This larger eruption magnitude is supported by modelling the granulometry of the 38 stratigraphic sections. The models suggest a median deposit particle size of at least 2ϕ, a deposit mass of 1–5 × 1011 kg (VEI 4), and that significant segregation by particle size as a function of height occurred in the 1913 eruption column. This analysis highlights potential bias in eruption magnitude estimates that use only thickness of proximal deposits, and the advantage of modelling the granulometry of the deposit in such circumstances