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Title: Dynamics and deposits of large explosive eruptions
Author: Engwell, Samantha Louise
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Characterisation of explosive volcanic eruptive processes from deposit interpretation is key to long-term volcanic hazard and risk assessment, particularly for large explosive eruptions which occur relatively infrequently and whose deposits are often transient in the geological record. Of particular importance is estimation of eruption size, or magnitude, which is typically determined by measurement and interpretation of tephra fall deposits. The accuracy of individual tephra thickness measurements is analysed using the Fogo A deposit, Sao Miguel, Azores. Statistical analysis shows measurements are highly clustered spatially and the area represented by a single measurement ranges between 0.5 and 10 km2 . Tephra thickness measurement error (9 %) is much less than error associated with natural variance (10 - 40 %). The volume uncertainty related to thickness uncertainty for the Fogo A deposit is estimated to be 1.3 %, equivalent to a volume of 0.02 km2 , but this value is highly dependent on the number of measurements. Uncertainties in tephra volumes were further investigated by application of a statistical model to thickness data using cubic B-splines under tension. Volume uncertainties are correlated with number and spatial distribution of data points and decrease from as much as 40 % for 30 measurements to 10 % for 120 measurements. Large explosive eruptions are commonly multiphase, with each phase resulting in different hazards. Generally, proximal deposits are used to interpret eruption dynamics and hazard. Analysis of tephra from the Magnitude 8 Campanian 19nimbrite (Cl) eruption from distal deep-sea sediments show that thickness trends are better preserved in the deep sea than in sub-aerial environments. Grain size trends are consistent regardless of depositional environment. Plinian ash is identified to distances of 1500 km and within 150 - 900 km constitutes 40 ± 5 % of the deposit volume. The grain size information is used to provide estimates of the total grain size distribution of the Campanian Plinian and co-ignimbrite phases. The Plinian grain size distribution is highly dependent on the samples used, while the co-ignimbrite distribution does not change. The co-ignimbrite TGSD is much finer than the Plinian TGSD or the TGSD determined using all of the deposits. Results for both eruption phases vary little with method used. Results have implications for tephra dispersal modeling and highlight a need for the eruption phases to modeled separately. Because grain size trends are similar for all co-ignimbrite deposits, for real time modelling purposes, a total grain size distribution such as that presented here may be used, with intensity, magnitude and local wind speeds likely to be the more variable factors Ash transport and deposition is studied in a fluid mechanical model of umbrella clouds based on gravity current dynamics controlled by buoyancy forces and interaction with the wind. Deposit thinning trends in medial-to-distallocations are presented and results related to conditions near the source. Resulting deposition trends are complex and have implications for isopach production methods.
Supervisor: Sparks, R. S. J. ; Aspinall, W. P. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available