Investigating the inner controls on explosive eruptions using textural analyses and petrology

• Main Author: Bernard, Olivier
• Other Authors: Caroline Bouvet De La Maisonneuve
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Geology::Volcanoes and earthquakes; Science::Geology::Petrology
• Online Access: https://hdl.handle.net/10356/155178
• Institution: Nanyang Technological University

During an eruption, the level of threat a volcano poses to its surroundings is related to the style of activity. An effusive eruption generating slow-moving lava flows will be a lot less destructive than a violent explosive eruption producing large pyroclastic density currents (PDC). Recently, it has been shown that most volcanoes around the globe and particularly in a subduction-setting, are able to produce both effusive and violently explosive eruptions with sometimes rapid transitions between eruption styles in the span of a few hours to minutes. However, predicting how a volcano will erupt proves challenging. In this thesis, I investigate the role of pre-eruptive storage conditions, volatile contents and magma ascent rates in controlling eruption styles using petrological, geochemical, textural and petrophysical methods on fresh volcanic deposits. First, I focus on the Rabaul caldera system (Papua-New-Guinea) where effusive, strombolian, vulcanian and sub-plinian eruptions occured in the past 120 years without any noticeable chemical changes in the magma compositions. I find that slight changes in the pre-eruptive state of the magma such as phenocryst-content and exsolved volatile contents influenced magma ascent rates and subsequent eruption styles. Second I use apatite to finely study the volatile budgets and degassing rates of the magma batches that fed the sub-plinian, effusive and vulcanian phases of the 2006 Rabaul eruption. I find that characterizing the plumbing system configuration is important for interpreting the monitoring records of Rabaul caldera, and may explain the sudden changes in eruption styles at other volcanic systems. Third, I focus on three PDC deposits similar in aspect in the field but of varying compositions to probe the processes that could explain the emplacement of mafic PDCs by using silicic and intermediate PDCs as a benchmark. I find that sudden decompression-induced crystallization of the matrix due to important undercooling of the melt plays a major role in promoting mafic magma fragmentation and generating mafic PDCs. The methodological approach taken during this Ph.D brings new insights into understanding controls on eruption styles at mafic and intermediate systems. It highlights the need for multidisciplinary approaches combining petrology, geochemistry and monitoring to build solid knowledge on volatile budgets, crystal-bubble interactions, and viscosity evolution during magma ascent, degassing processes, and decompression rates that are all fundamental factors influencing eruption dynamics.