6 research outputs found
Uncertainty quantification and sensitivity analysis of volcanic columns models: results from the integral model PLUME-MoM
The behavior of plumes associated with explosive volcanic eruptions is complex and dependent on eruptive source parameters (e.g. exit velocity, gas fraction, temperature and grain-size distribution). It is also well known that the atmospheric environment interacts with volcanic plumes produced by explosive eruptions in a number of ways. The wind field can bend the plume but also affect atmospheric air entrainment into the column, enhancing its buoyancy and in some cases, preventing column collapse. In recent years, several numerical simulation tools and observational systems have investigated the action of eruption parameters and wind field on volcanic column height and column trajectory, revealing an important influence of these variables on plume behavior. In this study, we assess these dependencies using the integral model PLUME-MoM, whereby the continuous polydispersity of pyroclastic particles is described using a quadrature-based moment method, an innovative approach in volcanology well-suited for the description of the multiphase nature of magmatic mixtures. Application of formalized uncertainty quantification and sensitivity analysis techniques enables statistical exploration of the model, providing information on the extent to which uncertainty in the input or model parameters propagates to model output uncertainty. In particular, in the framework of the IAVCEI Commission on tephra hazard modeling inter-comparison study, PLUME-MoM is used to investigate the parameters exerting a major control on plume height, applying it to a weak plume scenario based on 26 January 2011 Shinmoe-dake eruptive conditions and a strong plume scenario based on the climatic phase of the 15 June 1991 Pinatubo eruption
Insights into the formation and dynamics of coignimbrite plumes from one-dimensional models
Coignimbrite plumes provide a common and effective mechanism by which large volumes of fine-grained ash are injected into the atmosphere. Nevertheless, controls on formation of these plumes as a function of eruptive conditions are still poorly constrained. Herein, two 1-D axysymmetric steady state models were coupled, the first describing the parent pyroclastic density current and the second describing plume rise. Global sensitivity analysis is applied to investigate controls on coignimbrite plume formation and describe coignimbrite source and the maximum plume height attained. For a range of initial mass flow rates between 108 and 1010 kg/s, modeled liftoff distance (the distance at which neutral buoyancy is attained), assuming radial supercritical flow, is controlled by the initial flow radius, gas mass fraction, flow thickness, and temperature. The predicted decrease in median grain size between flow initiation and plume liftoff is negligible. Calculated initial plume vertical velocities, assuming uniform liftoff velocity over the pyroclastic density current invasion area, are much greater (several tens of m/s) than those previously used in modeling coignimbrite plumes (1 m/s). Such velocities are inconsistent with the fine grain size of particles lofted into coignimbrite plumes, highlighting an unavailability of large clasts, possibly due to particle segregation within the flow, prior to plume formation. Source radius and initial vertical velocity have the largest effect on maximum plume height, closely followed by initial temperature. Modeled plume heights are between 25 and 47 km, comparable with Plinian eruption columns, highlighting the potential of such events for distributing fine-grained ash over significant areas
The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations
Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions
Uncertainty quantification and sensitivity analysis of volcanic columns models: Results from the integral model PLUME-MoM
Assessing volcanic hazard and exposure in a data poor context: Case study for Ethiopia, Kenya, and Cabo Verde
Volcanoes produce a wide variety of hazards across varying spatial and temporal scales. When data are scarce on past eruptions and hazards, it can falsely imply low hazard recurrence and create challenges for robust hazard and risk assessment. Data quality and quantity vary considerably across different regions, volcanoes, and eruptions. Yet, there is a need for regional to global scale information on volcanic hazard and risk, where consistent and reproducible methods are applied. Such information is used by international stakeholders to inform funding priorities, risk reduction policies, and to highlight data and knowledge gaps, contributing towards the Sendai Framework's Sustainable Development Goals. Challenges in gathering this information can be most problematic where large populations are exposed to potential volcanic hazards but there are few comprehensive eruptive histories, as in sub-Saharan Africa. Here, we present a unique study to evaluate hazard and exposure for nine volcanoes in Ethiopia, Kenya and Cabo Verde, as part of an international project to develop multi-hazard Disaster Risk Country Profiles. We applied a two-stage expert elicitation process to volcanoes for the first time, and coupled the results with vent mapping, numerical hazard modelling, and GIS analysis of eight exposure categories to identify where high volcanic hazard and exposure coincide. Testing the sensitivity of our findings to input assumptions, to better understand where uncertainties lay, showed that improving our knowledge of past eruption volumes, frequencies, and dates was key to reducing uncertainty. Expert elicitations proposed that Fogo, Cabo Verde, is the most likely to erupt (eruption on average every 25 years), while Fentale (Ethiopia), Longonot and Suswa (Kenya) were elicited to have the greatest probability for a large explosive (VEI ≥ 4) eruption (on average every 400 years). Menengai and Longonot produce the larger exposure values across most VEI scenarios and categories of exposure, but population and GDP exposure was also large for more distal tephra fall and flows at Corbetti and Suswa, with order of magnitude increases expected between 2010 estimates and 2050 projections. Potentially high impact scenarios include tephra being dispersed across large cities (e.g. Nairobi, 55 km from Suswa) and key infrastructure (e.g. geothermal power station ∼2.5 km from Aluto), as well as important tourist destinations, seats of government and emergency management operations (e.g. islands east of Fogo). This study provided the first hazard and exposure assessment of its kind for these volcanoes and drew attention to volcanic risk at the levels required to inform policy and future in-country funding opportunities