Háskóli Íslands

Friday the 14th of June, William Michael Moreland will defend his Ph.D. thesis in Geology. The thesis is titled Explosive activity in flood lava eruptions: a case study of the 10th century Eldgjá eruption, Iceland.

Opponents are Dr. Valentin R. Troll, Professor of Petrology, Geocentrum, University of Uppsala, Sweden and Dr. Corrado Cimarelli, Faculty staff, Department of Earth and Environmental Sciences, Ludwig Maximilians Universität, München, Germany.

Adviser is Dr. Þorvaldur Þórðarson, Professor of Volcanology and Petrology, University of Iceland. The assessment committe included Guðrún Larsen, Emeritus Research Professor, University of Iceland and Dr. Bruce F. Houghton, Professor and Gordon A. Macdonald Chair in Volcanology, Department of Geology and Geophysics, University of Hawai'i at Mānoa Abstract.

Chairing the defense is Dr. Magnús Tumi Guðmundsson, Head of Faculty and Professor at the Faculty of Earth Sciences at the University of Iceland.

Abstract

The 10th century Eldgjá flood lava eruption, southern Iceland, was the most voluminous eruption on Earth in the last 1100 years, erupting up to 21.0 km3 of transitional alkali basaltic magma of rather uniform composition. While 19.7 km3 was erupted as lava in the form of two extensive lava fields covering 780 km2 in total, 1.3 km3 (dense rock equivalent) was erupted as tephra in at least 16 explosive phases. The Eldgjá vents form a ~70-km discontinuous mixed cone-row which stretches from beneath Mýrdalsjökull to the edge of Vatnajökull.

Explosive activity took place as discrete events restricted to distinct lengths of the fissure and alternated between subglacial and subaerial until phase 10 after which all activity was subaerial. Each phase contributed a tephra unit to what became a thick composite tephra deposit over 2 m thick 10 km away from source. Eruption column heights are estimated to have reached between 11 and 18 km, well above the 10-km tropopause above Iceland. The combination of subaerial and subglacial vents lead to both magmatic and phreatomagmatic tephra being produced. Individual explosive phases have been classified as Plinian and Phreatoplinian.

Vesicle-size analysis reveals that the magma beneath all Eldgjá fissure segments had identical vesiculation histories. However, total grain-size distributions of magmatic and phreatomagmatic tephra exhibit stark differences with the magmatic having one medium lapilli mode and the phreatomagmatic having one broad peak of medium lapilli to medium ash and a narrower peak of very fine ash. Thermal granulation is suspected as having reduced lapilli-sized magmatic foam into finer particles but without adding intensity to the eruption.

About the Ph.D. student

William grew up with three siblings in rural Northumberland, north-east England where his parents ran a village pharmacy. Having been inspired by British Geological Survey geologists working in the area, William went to the University of Edinburgh to study Geology. He graduated with a Master of Earth Science with Honours Geology, having completed a Bachelor’s mapping project on the Isle of Eigg, Inner Hebrides, Scotland; and a Master’s research project on the geochemistry of Arthur’s Seat Volcano, Edinburgh. After graduating, William immediately moved to Iceland to begin his PhD.

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