Háskóli Íslands

On Wednesday July 12th, Jónas Guðnason will defend his Ph.D. thesis in Geology. The thesis title is Magma fragmentation and tephra dispersal in explosive eruptions: The 1991 and 1845 Hekla eruptions.

Opponents are Dr. Dave McGarvie Senior Lecturer, Staff Tutor In Science at The Open University in the United Kingdom and Dr. Eliza Calder Senior Lecturer in Volcanology at the School of GeoSciences at the University of Edinburgh in the United Kingdom.

Adviser is dr. Þorvaldur Þórðarson, professor of Volcanology and Petrology, Department of Earth Sciences, University of Iceland.The assessment committee included Bruce F. Houghton, Professor of Volcanology, Department of Geology and Geophysics, University of Hawai'i at Mānoa and Guðrún Larsen, Emeritus Research Professor, Institute of Earth sciences, University of Iceland


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. 

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 reached towards the end of the opening phase before transitioning into effusive activity.

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 1010 kg. The plume dispersed to the ESE of Hekla at speeds of 16–19 m s-1 and the tephra fall reached ships and islands as far away as 700–1100 km from Hekla. Over the course of the opening phase, a gradual decrease in intensity inferred both from normal grading of the tephra deposit at proximal locations and from decreasing vesicle number densities. Sedimentation of tephra from the 1845 opening phase produced bimodal grain-size distributions, primarily due to enhanced deposition of ash-sized particles by aggregation. 

Decreasing vesicle number densities with time (1991 and 1845) and across conduit (1845) show that effective separation of the gas phase increases with decreasing ascent rate. This process finally leads to the transition from explosive to effusive activity in the 1991 and 1845 eruptions. 

The results presented in this thesis contribute to enhanced understanding of vesiculation and fragmentation mechanisms during eruptions of Hekla. The patterns of sedimentation will aid in hazard mitigation and provide a basis for hazard modelling through both new and revised eruption source parameters, such as plume height, eruptive volume, mass eruption rate and total grain-size distribution.

About the Ph.D. student

Jónas Guðnason was born in Denmark in 1982. Jónas is married to Elísa Ólafsdóttir and they have two boys Emil Óli and Viggó Guðni. Jónas finished a BS degree from the University of Iceland with a focus on sulfur degassing in phreatomagmatic eruptions and their environmental effects. During his MSc studies at the University of Copenhagen Jónas focused on Argon/Argon dating of Argentinean lavas. From Copenhagen, Jónas came back to Iceland to do a PhD study at the University of Iceland with a focus on explosive volcanism.

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