Magma fragmentation and tephra dispersal in explosive eruptions: The 1991 and 1845 Hekla eruptions

• Main Author: Guðnason, Jónas
• Other Authors: Þorvaldur Þórðarson, Jarðvísindadeild (HÍ), Faculty of Earth Sciences (UI), Verkfræði- og náttúruvísindasvið (HÍ), School of Engineering and Natural Sciences (UI), Háskóli Íslands, University of Iceland
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Iceland, School of Engineering and Natural Sciences, Faculty of Earth Sciences 2017
• Subjects: Hekla; Tephra; Total grain-size distribution; Vesicle size; Eldgos; Gjóska; Gjóskudreifing; Doktorsritgerðir; Iceland
• Online Access: https://hdl.handle.net/20.500.11815/315

Iceland is one of the most volcanically active terrestrial regions on Earth. Hazards posed from volcanic eruptions to local population and life stock is well known in Iceland and the wider effects from volcanic activity have been elevated in recent years with increasing globalization, tourism and air traffic. Even small to medium size explosive eruptions can influence areas beyond their immediate surroundings. The high eruptive frequency, in combination with short warning times and dispersion of air borne pyroclasts, make Hekla an important volcano to study and monitor. The aim of this thesis is to add on to the growing collection of studies that focus on explosive volcanism in Iceland, by studying the opening phase of the 1991 and 1845 Hekla eruptions. Paper 1 focuses on the 1991 eruption. Temporal variation in plume activity during the 50-minute-long opening phase was revealed by mapping out the proximal tephra fall. The 1991 eruption produced an eruption plume that rose to 11.5 km (a.s.l.) in 10 minutes. The mass eruption rate for the opening phase was 2.6 x 106 kg s-1, and total mass of tephra 8.6 x 109 kg. The principal axis of tephra sedimentation was to the NNE of Hekla, with systematic grain-size fractionation during the first 65 km of transport. Beyond 65 km from the source, the tephra layer has a consistent grain-size distribution but decreasing mass, indicating sedimentation from a laminar regime in the volcanic plume. The opening phase of the 1991 eruption was pulsating, as revealed from changes in grain-size and vesicle number density with time. The vesicle number density indicates that peak intensity was reached towards the end of the opening phase before transitioning into effusive activity (paper 3). In paper 2, the 1845 tephra fall is mapped and the one-hour-long opening phase constrained. The average plume height during the 1845 eruption was 19 km (a.s.l.), with a mass eruption rate of 2.1 x 107 kg s-1 and total tephra volume of 0.13 km3 (0.03 km3 dense rock equivalent) and total mass of 7.5 x ...