Near-Vent Sedimentation

Most tephra fallout models rely on the advectiondiffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection-diffusion equation to tephra sedimentation using data collected on the proximal (350 to~1,200 m from the vent) to medial (greater than~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and highresolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection-diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2-3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700-1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of~500 m 2 s −1 estimated for the deposits, confirming that the advection-diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than~2 km; and (3) a medial zone, in which particles fall from~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud-a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.

A VEI 5 dacite eruption emplaced the Orange Tuff about between 34.3 cal kBP and 17.2 cal kBP. Gunung Salak is the unit's source and the Orange Tuff represents the most recent such eruption from any of the volcanoes southwest of Bogor, Indonesia. The Orange Tuff is the region's first such documented tephra-fall deposit whose characteristics and phenocryst geochemistry make it readily identifiable over at least 1250 km 2. Magnetite compositions and temperature and fO 2 estimates inferred from Fe-Ti oxide compositions are particularly useful for identifying the unit. Deposit characteristics suggest that the eruption lasted 1-11 h with mass eruption rates of 1.0-8.3 × 10 8 kg/s and a column height of 31-40 km. The eruption's column height and the deposit's 2.5-11 km 3 volume suggest that the unit was dispersed over a much wider area than mapped. The unit is a marker bed throughout its mapped distribution and has potential to be applied over a much broader area as a regional marker bed. The large population and infrastructure proximal to Salak suggest that the unit should be considered in hazards assessments despite its age and the lack of subsequent similar eruptions.

Most tephra fallout models rely on the advectiondiffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection-diffusion equation to tephra sedimentation using data collected on the proximal (350 to~1,200 m from the vent) to medial (greater than~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and highresolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection-diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2-3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700-1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of~500 m 2 s −1 estimated for the deposits, confirming that the advection-diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than~2 km; and (3) a medial zone, in which particles fall from~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud-a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.

Ground penetrating radar (GPR) has been shown to be a useful tool for mapping geometry and thicknesses for volcanic fall, surge, granular flow, and lahar deposits. However, the success of GPR surveys is highly dependent on soil properties and the nature of stratigraphic layering. The efficacy of the method at a given site can be difficult to predict. Small-scale (10s to 100s of meters) test surveys with ground penetrating radar show that geologic features of interest can be resolved to depths up to 20 m on both Poás and Irazú volcanoes in Costa Rica. The antenna frequencies used in this pilot study, 50 MHz, 100 MHz, and 200 MHz, produce wavelengths too long to resolve most individual layers (mm's to cm's thick) in the near-surface tephra fall and surge deposits. However, these GPR profiles clearly show the attitude of beds and resolve some distinct contacts at depth, particularly the base of the 1963-1965 intracrater deposits on Irazú. On Irazú GPR profiles also confirm field observations that 1963 surge deposits thin consistently with distance from the crater rim, while packages from 1723 and older show uniform thicknesses or increasing thickness with distance from crater rim, suggesting reworking or reverse flow of surges returning from the Playa Hermosa caldera wall. On Poás, bright reflectors are present at depths below 2 m on near-vent profiles, but the lack of nearby stratigraphic observations precludes geological interpretation. The test profiles on both volcanoes also clearly show diffractions produced by blocks on the order to 5 cm or more in diameter embedded in the surficial deposits, as well as evidence of sag beneath blocks. Resolution of blocks decreases with depth, presumably due to both the inherent loss in lateral resolution with wave travel distance and to clearly observed dispersion (loss of high frequencies). Future studies, particularly with higher frequency antennas on Poás, could be useful for tracking depositional units between exposures, and for resolving the distribution of blocks or bombs in deposits in the uppermost few meters.

Most tephra fallout models rely on the advectiondiffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection-diffusion equation to tephra sedimentation using data collected on the proximal (350 to~1,200 m from the vent) to medial (greater than~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and highresolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection-diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2-3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700-1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of~500 m 2 s −1 estimated for the deposits, confirming that the advection-diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than~2 km; and (3) a medial zone, in which particles fall from~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud-a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.

Volcanic ash is the most widespread of all volcanic hazards and has the potential to affect hundreds of thousands, or even millions, of people in the densely populated islands of Indonesia. There is limited information available for this region on the hazard posed by volcanic ash, particularly from volcanoes that have not erupted in recent times. There is a need for computational models capable of accurately predicting volcanic ash dispersal at ground level when coupled with field observations of historical or ongoing eruptive activity. To maximise the effectiveness of such models, they should be readily accessible, easy to use and well tested. Geoscience Australia in collaboration with the Australia-Indonesia Facility for Disaster Reduction and the Indonesian Centre for Volcanology and Geohazard Mitigation has collaboratively adapted an existing open-source volcanic ash dispersion model for use in Indonesia. The core model is the widely used, open-source volcanic ash dispersion model FALL3D. A Python wrapper (name here python-FALL3D) has been developed, which modifies the modelling procedure of FALL3D in order to simplify its use for those with little or no background in computational modelling. The modified procedure does not alter the core functionality of FALL3D, but simplifies the modelling procedure by streamlining the installation process, automating both the pre-processing of input meteorological datasets and configuring and executing each utility program in a single-step process. An application example was presented using python-FALL3D for an active volcano in West Java, Indonesia. The example showed that communities located on the western side of Gunung Gede are always susceptible to volcanic ash ground loading regardless of the seasonal variations in wind conditions, whereas communities on the eastern side of Gunung Gede have a marked increase in susceptibility to ground loading during rainy season conditions when prevailing winds include a strong easterly component.

Studies of grain-size distributions of explosive volcanic eruptions provide important insights into fragmentation mechanisms and eruptive conditions and are crucial to the modeling of tephra dispersal. As a result of sedimentation processes and plume dynamics, grain-size features vary significantly both in the downwind and crosswind directions and are difficult to characterize. We have analyzed grain-size features in the downwind and crosswind directions of the two largest eruptions of the last 2000 years of Cotopaxi volcano activity (Ecuador). Crosswind grain-size variations are similar for both eruptions (i.e., layers 3 and 5), while at any given downwind distance from vent, the layer 3 deposit is coarser than the layer 5 one. This suggests that layers 3 and 5 were characterized by similar plume height but that layer 3 was advected by a stronger wind. In addition, both deposits are coarsest along the dispersal axis and become richer in ash in the crosswind direction showing a Gaussian decreasing rate. Deposit thickness also shows a Gaussian crosswind decay, but layer 3 is significantly thicker at all points than is layer 5 due to the former's larger erupted mass. Based on both quantitative analysis of field data and on numerical simulations, we show that tephra deposits associated with large explosive eruptions (i.e., plume height of 30 km) should be sampled out to at least 200 km from the vent (depending on wind speed and tropopause height) in order to derive complete grain-size distributions that are not depleted in fines. Eruptions occurring in a strong wind field at high latitudes (e.g., Iceland) require lesser representative-sampling distances because of the lower tropopause heights.

Most tephra fallout models rely on the advectiondiffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection-diffusion equation to tephra sedimentation using data collected on the proximal (350 to~1,200 m from the vent) to medial (greater than~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and highresolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection-diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2-3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700-1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of~500 m 2 s −1 estimated for the deposits, confirming that the advection-diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than~2 km; and (3) a medial zone, in which particles fall from~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud-a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.

Regardless of the recent advances in geophysical monitoring and real-time quantitative observations of explosive volcanic eruptions, the characterization of tephra deposits remains one of the largest sources of information on Eruption Source Parameters (ESPs) (i.e. plume height, erupted volume/mass, Mass Eruption Rate — MER, eruption duration, Total Grain-Size Distribution — TGSD). ESPs are crucial for the characterization of volcanic systems and for the compilation of comprehensive hazard scenarios but are naturally associated with various degrees of uncertainties that are traditionally not well quantified. Recent studies have highlighted the uncertainties associated with the estimation of ESPs mostly related to: i) the intrinsic variability of the natural system, ii) the observational error and iii) the strategies used to determine physical parameters. Here we review recent studies focused on the characterization of these uncertainties and we present a sensitivity analysis for the determination of ESPs and a systematic investigation to quantify the propagation of uncertainty applied to two case studies. In particular, we highlight the dependence of ESPs on specific observations used as input parameters (i.e. diameter of the largest clasts, thickness measurements, area of isopach contours, deposit density, downwind and crosswind range of isopleth maps, and empirical constants and wind speed for the determination of MER). The highest uncertainty is associated to the estimation of MER and eruption duration and is related to the determination of crosswind range of isopleth maps and the empirical constants used in the empirical parameterization relating MER and plume height. Given the exponential nature of the relation between MER and plume height, the propagation of uncertainty is not symmetrical, and both an underestimation of the empirical constant and an overestimation of plume height have the highest impact on the final outcome. A ± 20% uncertainty on thickness measurements, area of isopach contours, integration limit for the power-law fit and deposit density result in ESP uncertainties ≤± 20% for plume height and erupted volume/mass. Finally, a third case study has also been used to explore the sensitivity of the Voronoi Tessellation strategy for the determination of TGSD and the inversion on both mass/area and grain-size data for the determination of erupted mass and plume height. Results confirm the validity of the methods but also the strong dependence on the distribution and number of observations.