High-Resolution Grain-Size Distributions: Insight into Tephra Dispersal and Sedimentation during Plinian Eruptions
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Date
2024-01-09
Authors
Wiejaczka, Joshua
Journal Title
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Publisher
University of Oregon
Abstract
Detailed field studies of past eruptions contribute to constraining the input parameters used to forecast tephra dispersion and mitigate potentially fatal volcanic hazards. It is thus of the utmost importance to understand the relationships between the characteristics of tephra deposits and these input Eruption Source Parameters (ESPs). In this dissertation, I determine the ESPs for the ~7.7 ka Cleetwood eruption of Mount Mazama (Crater Lake/giiwas, Oregon, USA). This eruption is an important historic eruption because it immediately preceded the climactic caldera-forming eruption, at the same location, and is similar to the only observed silicic volcanic eruptions that have transitioned from explosive to effusive activity (2008 Chaitén and 2011-2012 Cordón Caulle, [Chile]). The Cleetwood eruptive sequence consisted of two consecutive VEI 4 eruptions: the main lower Cleetwood unit and smaller upper Cleetwood units, in order from oldest to youngest. The lower Cleetwood phase alone, produced a ~14.4 km plume and emplaced ~0.85 km3 of tephra. Altogether, the explosive phase of the Cleetwood eruption deposited ~1.1 km3 (non-DRE) of material and transitioned to an effusive stage that emplaced a ~0.6 km3 rhyodacitic lava flow. Furthermore, I develop a novel approach which combines laser diffraction and dynamic image analysis to produce a continuous set of high-resolution grain-size distributions (HR-GSDs) for samples spanning a range of sizes of ejected tephra from less than a micron to a few centimeters. Through this approach, I show the ability for these HR-GSDs to provide insights into magma fragmentation and tephra transport. Next, through detailed wind analysis and the use of these ESPs as the inputs for Tephra2, a volcanic ash transport and dispersal model, I estimate the geometry and dimensions of the volcanic plume that emplaced the lower Cleetwood unit. Here, I show the standard version of Tephra2, which uses a vertical line source, does well to reproduce mass loads and grain-size distributions separately but fails to fit both simultaneously with a single set of empirical inputs. To overcome this, I adapt Tephra2 outputs to simulate deposition via an umbrella cloud. Applying this adaptation and a grid search approach over reasonable plume heights and umbrella cloud geometries gives the best results for a plume with a 4x40 km2 elliptical geometry. This approach improves overall GSDs without degrading mass loads. Lastly, I combine detailed componentry and HR-GSDs on samples I collected from the products of hybrid phase of the 2011-2012 eruption at Cordón Caulle. This analysis suggests that ash sintering after fragmentation produced a dense plug that obstructed the shallow conduit. This caused the system to re-pressurize and subsequently shatter pieces of the plug during the next explosive event. This pattern continued until permeable outgassing dominated over re-pressurization, facilitating the transition to a solely effusive stage.
Description
Keywords
eruption source parameters, explosive volcanism, fractal dimension, grain-size distribution, magma fragmentation, tephra dispersal