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Title: Spatial and temporal variations in the causative processes of caldera formation : an example from Nisyros, Greece
Author: Kinvig, Helen Susan
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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Collapse calderas are associated the most violent volcanic eruptions on Earth. Their destructive potential makes understanding their processes of formation crucial in assessing their vo1canic hazard. This thesis investigates the formation of a central caldera on Nisyros volcano,Greece. It reviews the volcanic stratigraphy of Nisyros; particularly addressing the deposits of its most recent explosive eruptions: the Lower Pumice and Upper Pumice. The eruption stratigraphy of the Lower Pumice and Upper Pumice is reconstructed and discussed, and their contribution to caldera formation is assessed. Lithic component analyses of xenoliths contained within the Lower and Upper Pumice pyroclastic deposits reveal important insights into their eruption chronologies, and vent locations . Circum-caldera lithic variations indicate the use of multiple vents during the caldera forming eruptions. Tephra distribution maps are used to estimate physical eruption parameters for the Pumice eruptions, and aid identification of vent-sites. It is concluded that the Nisyros caldera formed by incremental collapse, involving the cumulative magma withdrawal associated with the Lower pumice and Upper Pumice, and additionally the intercalated, high volume Nikia Rhyolite. A new model of Nisyros caldera formation is presented, in which regional tectonics are considered to have been an important influence on collapse and vent location. A new geological map implementing a revised volcanic stratigraphy is additionally presented. Finally, numerical models that implement an FEA method to investigate the effects of crustal layering on local stress field distribution during an eruption are presented and compared with existing criteria for ring fault initiation. Their results indicate that mechanical layering may be influential in facilitating or inhibiting caldera collapse. The results of the numerical study are discussed with reference to Nisyros volcano.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available