Ground penetrating radar is shown to be an excellent tool for investigating volcanic tephra deposits due to its ability to rapidly collect nearly continuous profiles that elucidate thickening trends and other structural variations in the upper 12 m of deposit. By comparing GPR radargrams and pit data with historical records and computational models, this study provides insight into the formative depositional processes at Cerro Negro volcano, validates the application of the advection-diffusion equation to the modeling of tephra fallout, and promotes the use of research-caliber computational models to foster model literacy among students.
Interpretation of GPR radargrams collected < 1600 m from the vent indicates the tephra deposit is well characterized by three depositional regimes: 1) near vent (0 to 350 m downwind of vent), 2) proximal (350 to 1200-1400 m downwind of vent), and 3) medial (beyond 1200-1400 m downwind of the vent). The near-vent region is built by a combination of fallout from plume margins and ballistically emplaced bombs and blocks. Over-steepening of deposits results in down-slope movement of material and the formation of truncated beds and slump features within the cone edifice. Deposition in the proximal region is dominated by fallout from plume margins and results in the formation of a tephra deposit that both thins exponentially with distance from the vent and displays a Gaussian distribution of accumulated tephra fallout in the crosswind dimension. The
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medial tephra deposit is emplaced via fallout from the buoyant eruption cloud and is identified in the GPR record by an over-thickening relative to proximal deposits.
Radargrams collected in the near-vent region display evidence for the explosivity of eruptions in the form of continuous (> 200 m) reflective horizons that mantle cone slopes on the down-wind portion of the edifice, indicating that the eruption that produced this deposit had a sustained eruption column. On the tephra blanket, changes in the thickening trend of the deposit specify the location where deposition changes from that dominated by fallout plume margins to that dominated by fallout from the buoyant eruption cloud. This marker can be used to estimate the height of the eruption column, enabling determination of the volcanic explosivity index (VEI) and thus eruption magnitude. In the case of older scoria deposits that lack historical records, GPR imaging of the scoria cone edifice and/or proximal to medial tephra deposit can be used to determine whether the formative eruption was accompanied by a sustained eruption column or not. In the case of preserved proximal to medial deposits, the approximate height of that column can be estimated based on the location of the column corner.
The current generation of tephra sedimentation models does not accurately predict deposit thickness in the proximal regime, however, thickening trends evident in radargrams collected on the tephra blanket of Cerro Negro are consistent with those estimated by the advection-diffusion equation implemented with Fickian diffusion. These results indicate that, were models to incorporate an additional sedimentation regime characterized by a unique particle fall time, forecasts of deposit thickness in proximal locations could be greatly improved. Moreover, the fact that the deposit shape has such a high degree of statistical correlation (R2 = 0.98) with the morphology predicted by the
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advection-diffusion equation validates the use of advection-diffusion models for tephra sedimentation generally.
Computational models of tephra fallout have value not just for researchers attempting to gain insight into individual eruptions or to forecast the mass loading of tephra for various locations, but also in the classroom, as tools for the dissemination of skills related to both model and quantitative literacy. The Tephra2 numerical model is here reformatted for student use and made available via the VHub.org online platform. Tools to address code verification and validation, the role of simplifying assumptions, and the concepts of uncertainty and forecasting are built in to the user interface to encourage students to approach models critically. Experience using and critically assessing computational models prepares students for modern careers not only in science, technology, and mathematics disciplines, but for any field that relies on numerical modeling for decision making.
Taken together, these studies demonstrate how new methods, both in the field and in the classroom, can further our understanding of natural phenomena and the tools used to study them. Ground penetrating radar methods represent a powerful and as-yet under- utilized tool for the study of volcanic phenomena. Research-caliber computational models provide additional insight into eruptive processes and the introduction of such models into the classroom environment can promote quantitative and model literacy among students in the geosciences.
Future Work
Further analysis of the GPR dataset presented in this study will lead to improved tephra fallout models and specifically more accurate inversions of the 1992 tephra fallout
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deposit. This dataset will also enable an analysis of the distribution of bombs and blocks in the proximal region, make possible multiple independent calculations of the volume of tephra fallout on the scoria cone edifice, and provide a realistically complex initial condition for scoria cone degradation models.
GPR data collected at Cerro Negro volcano clearly demonstrates that the proximal and medial zones are well-characterized by advection-diffusion methods. By incorporating a plume characterization that releases coarse particles from the eruption column at heights defined by the location of the column corner (~ 1–2 km at Cerro Negro) and fine particles from the height of neutral buoyancy (~ 6–7 km at Cerro Negro), fallout models will be able to better characterize both the magnitude and standard deviation of tephra thickness in the proximal region. This can be tested by inverting the combined GPR-pit dataset presented here with such a model. If the proposed change in fallout regime is a better approximation to the processes governing proximal to medial deposition, then we can expect an improved fit with tephra thickness values in these regions, agreement with observed eruption parameters including column height and wind speed, and a more accurate estimation of the total mass of tephra ejected by the eruption. The diffusion coefficient could either be set at the value estimated for the 1999 – present deposit (~600 m2 s-1) or found via the inversion in which case we would expect it to be within the range calculated for proximal-medial deposits (262 – 762 m2 s-1). Inverting the Cerro Negro combined GPR-pit dataset with this updated tephra fallout model will help to validate the plume model described here, thereby improving our understanding of volcanic processes at Cerro Negro and similar volcanoes.
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Also in the proximal zone, GPR radargrams contain abundant evidence of buried bombs and blocks greater than 10 cm in diameter. The spatial distribution of these pyroclasts can be analyzed based on the existing data and compared to the distribution visible at the surface of the deposit to determine whether the surface distribution is representative of the total distribution of bombs and blocks ejected by an eruption. Additionally, both down-wind and off-wind profiles can be analyzed to determine if significant variations in the spatial distribution of pyroclasts exist. Such analysis could indicate either some directionality to pyroclast ejection or an increased or decreased likelihood for pyroclasts to roll down slope upon impact. Determining whether the slope angle and grain size distribution (finer particles having greater volumes on down-wind slopes) influence the likelihood of projectiles rolling down slope upon impact will enhance models of scoria cone construction.
The minimum estimate of the volume of material contributed to the edifice from the eruption column was here calculated based on the prominence of evidence for tephra fallout (e.g., laterally continuous reflections, reflections mantling topography) vs ballistic or remobilized deposits (e.g., pinch outs, discordant reflections) in the GPR data. Alternative methods include advection-diffusion modeling of tephra fallout onto cone slopes based on our parameterization of proximal-medial deposits or calculations based on the difference in slope angle between downwind and off-wind cone profiles. Values obtained via these independent methods can be compared and checked for consistency.
Finally the morphologic features evident in GPR radargrams, including dip angles and the presence or absence of bed truncation, can be used as the initial conditions for advection-diffusion models of scoria cone degradation. Cerro Negro can be used as a
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starting point to explore how true cone morphology evolves with age and what quantifiable changes in the locations, thicknesses, and dips of stratigraphic layers evident in GPR radargrams are predicted when modeling millions of year of erosion. Model predictions can be compared to radargrams collected at Rattlesnake Maar scoria cone, AZ, in 2012. If erosional features can be approximated via forward modeling of actual cone morphology and be identified via GPR methods, then scoria cones around the globe could benefit from this type of study as a new relative dating technique.
In this study (Chapters 2 and 3), the detailed GPR profiles collected at Cerro Negro volcano are combined with pit data and interpreted via comparison with existing qualitative models of cone formation and quantitative models of tephra fallout. These radargrams contain additional information that can be incorporated into fallout models or used to determine the relative contribution of high energy violent Strombolian and low energy Strombolian processes to scoria cone construction. Radargrams can be used to enhance models of scoria cone degradation and provide insight into the distribution of pyroclastic bombs and blocks about the volcano. These studies will further our understanding of scoria cones generally and will enable determination of the explosivity and relative ages of older scoria cones that lack historical observations of eruption(s).
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