| Summary: | This thesis develops a chronology of jökulhlaup (glacier burst flood) activity from Mýrdalsjökull in southern Iceland. New geornorphological, sedimentological and tephrochronological data has identified 15 flow events during the Holocene. The majority of these were hyperconcentrated flow events originating from, or close to, the northwest area of the ice cap and are associated with subglacial volcanism. One flood originated in the Vei«!»ivötn area and on 3 occasions flooding from Katla may have been accompanied by floods from Eyjafjallajökull. A further two events relate to remobilisation of thick airfall tephra deposits. Silicic pumice found on the sandur and close to the ice margin indicates that the Markarfljót acted as a terrestrial transport route for pumice found along North Atlantic coasts, and was possibly a route for silicic Katla jökulhlaups. Additionally, flood routes and glacial landforms show that Entujökull reached a maximum Holocene extent in the mid-Holocene, extending farther downvalley than during the Little Ice Age. In prehistory, floods were directed to both the south and west of Mýrdalsjökull. Similarly timed jökulhlaups took these paths when floods also flowed from Eyjafjöll into the Markarfljót. This suggests that concurrent routing of floods to the south and west of Mýrdalsjökull is related to synchronous volcanic activity in Katla and Eyjafjallajökull. Since the 10<sup>th</sup> Century most Katla floods have been routed to the south east, possibly reflecting changes in intra-caldera eruption sites or subglacial topographic change associated with the Eldgjá eruption in c.935 AD, as suggested by Larsen (2000). The environmental impacts of these floods were significant. Late prehistoric and early historic floods had a major role in shaping the landscape faced by the earliest Norse colonisers of the region. Future flooding could pose a distinct hazard to farmland on the sandur plain downvalley and to popular tourist areas in North Þórsmörk.
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