| Summary: | The research presented here has been carried out at the Faculty of Earth Sciences, University of Iceland. The primary aim of this project has been to investigate and characterise the explosive phases of the 10th century Eldgjá eruption. Using data gathered from this eruption more general conclusions have been reached regarding processes active during fissure eruptions and subglacial eruptions. The main results are presented as papers I-III. The papers are either submitted, or in preparation to be submitted, to peer-reviewed international journals. 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 ...
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