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Eruption Dynamics of Magmatic/Phreatomagmatic Eruptions of Low-Viscosity Phonolitic Magmas: Case of the Laacher See Eruption (Eifel, Germany)

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Eruption Dynamics of Magmatic/Phreatomagmatic Eruptions of Low-Viscosity Phonolitic Magmas: Case of the Laacher See Eruption (Eifel, Germany) FACULTEIT WETENSCHAPPEN Opleiding Master in de Geologie Eruption dynamics of magmatic/phreatomagmatic eruptions of low-viscosity phonolitic magmas: Case of the Laacher See eruption (Eifel, Germany) Gert-Jan Peeters Academiejaar 2011–2012 Scriptie voorgelegd tot het behalen van de graad Van Master in de Geologie Promotor: Prof. Dr. V. Cnudde Co-promotor: Dr. K. Fontijn, Dr. G. Ernst Leescommissie: Prof. Dr. P. Vandenhaute, Prof. Dr. M. Kervyn Foreword There are a few people who I would like to thank for their efforts in helping me conduct the research necessary to write this thesis. Firstly, I would specially like to thank my co-promotor Dr. Karen Fontijn for all the time she put into this thesis which is a lot. Even though she was abroad most of the year, she was always there to help me and give me her opinion/advice whether by e-mail or by Skype and whether it was in the early morning or at midnight. Over the past three years, I learned a lot from her about volcanology and conducting research in general going from how volcanic deposits look on the field and how to sample them to how to measure accurately and systematically in the laboratory to ultimately how to write scientifically. I would also like to thank my promoter Prof. Dr. Veerle Cnudde for giving me the opportunity to do my thesis about volcanology even though it is not in her area of expertise and the opportunity to use the µCT multiple times. I thank the entire UGCT especially Wesley De Boever, Tim De Kock, Marijn Boone and Jan Dewanckele for helping me with preparing thin sections, drilling subsamples, explaining how Morpho+ works, solving problems when Morpho+ decided to crash etc. I would like to thank my other co-promotor Dr. Gerald Ernst for the many interesting talks we had and all the ideas he provided me with to improve this thesis. Last but not least, I would like to thank my family for supporting me these past five years. Even though they had never heard of geology five years ago, they were always there to help me when I needed them the most and I could not have written this thesis/finished my studies without them. i Table of Contents Foreword ................................................................................................................................................. i Table of Contents .................................................................................................................................... ii 1 Introduction .................................................................................................................................... 1 2 The Laacher See eruption: a phonolitic magmatic/phreatomagmatic eruption ................................. 4 2.1 Volcanic field of the Eifel .......................................................................................................... 4 2.2 Prior to the Laacher See eruption ............................................................................................. 9 2.2.1 Geography around the Laacher See volcano before the 12,900 a eruption ....................... 9 2.2.2 Pre-eruptive magma chamber .......................................................................................... 9 2.3 The Laacher See Eruption ....................................................................................................... 13 2.3.1 Lower Laacher See Tephra (LLST) .................................................................................... 14 2.3.2 Middle Laacher See Tephra (MLST) ................................................................................. 17 2.3.3 Upper Laacher See Tephra (ULST) ................................................................................... 18 2.4 Post-Eruption ......................................................................................................................... 19 3 Tephra Studies ............................................................................................................................... 23 3.1 Vesiculation and Fragmentation ............................................................................................. 23 3.1.1 Bubble Nucleation .......................................................................................................... 24 3.1.2 Bubble Growth ............................................................................................................... 26 3.1.3 Magmatic fragmentation ................................................................................................ 28 3.2 Pumice Texture ...................................................................................................................... 33 3.2.1 Natural pumice ............................................................................................................... 33 3.2.2 Analogue Modelling ....................................................................................................... 37 3.2.3 Analytical/Numerical Modelling ...................................................................................... 40 3.3 Tephra Dispersal Modelling .................................................................................................... 44 4 Methodology ................................................................................................................................. 46 ii 4.1 Field Work ............................................................................................................................. 46 4.1.1 Bulk Samples .................................................................................................................. 46 4.1.2 Individual pyroclasts for textural studies ......................................................................... 47 4.2 Laboratory Measurements ..................................................................................................... 49 4.2.1 Sieving ............................................................................................................................ 49 4.2.2 Density Measurements ................................................................................................... 49 4.3 Textural analysis .................................................................................................................... 50 4.3.1 High resolution X-ray tomography (µCT) ......................................................................... 51 4.3.2 Morpho+ ........................................................................................................................ 51 4.4 Tephra Dispersal Modelling .................................................................................................... 55 5 Results ........................................................................................................................................... 57 5.1 Laboratory Measurements ..................................................................................................... 57 5.1.1 Grain Size Distribution (GSD) .......................................................................................... 57 5.1.2 Density Measurements ................................................................................................... 59 5.2 Tephra dispersal modelling using precise density measurements ........................................... 66 5.3 Textural analysis .................................................................................................................... 69 5.3.1 Qualitative ...................................................................................................................... 69 5.3.2 Quantitative analysis ...................................................................................................... 75 6 Discussion ...................................................................................................................................... 82 6.1 Density Measurements .......................................................................................................... 82 6.2 Modelling ............................................................................................................................... 84 6.3 Texture .................................................................................................................................. 85 7 Conclusion ..................................................................................................................................... 87 References ............................................................................................................................................ 89 Nederlandse samenvatting .................................................................................................................... 94 Appendix ............................................................................................................................................... 99 iii iv 1 Introduction The science of physical volcanology has made significant advances on many different fronts in the last few decades, including detailed observations of volcanic (eruptions and) deposits in the field and laboratory, remote sensing and ground-based monitoring, analogue and numerical modelling, and detailed petrologic studies of eruptive products to investigate subsurface processes and relate them to eruptive processes. To gain a full proper understanding of volcanoes and eruptions however, it is crucial to continue combining new developments in modelling and analysis techniques with high-quality
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