Holocene jökulhlaups, glacier fluctuations and palaeoenvironment, Mýrdalsjökull, South Iceland

Abstract

This thesis develops a chronology of jokulhlaup (glacier burst flood) activity from Myrdalsjokull in southern Iceland. Throughout the Holocene, the interaction of the volcano Katla and the overlying ice cap of Myrdalsjokull have triggered many jokulhlaups. Crucially however, our knowledge of the possible flood routes is incomplete. Flood activity to the south and east of the ice cap has been well constrained from historical and geomorphological studies, but routes to the west of the ice cap have not been fully investigated. This is important for understanding the interaction of Katla and its overlying ice cap as well as from the perspective of hazard assessment.New geomorphological, sedimentological and tephrochronological data have 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 Veidivotn area and on 3 occasions flooding from Katla may have been accompanied by floods from Eyjafjallajokull. A further two events relate to re-mobilisation of thick airfall tephra deposits. Silicic pumice found on the sandur and close to the ice margin indicates that the Markarfljot acted as a terrestrial transport route for pumice found along North Atlantic coasts, and was possibly a route for silicic Katla jokulhlaups. Additionally, flood routes and glacial landforms show that Entujokull reached a maximum Holocene extent in the mid-Holocene, extending further downvalley than during the Little Ice Age.In prehistory, floods were directed to both the south and west of Myrdalsjokull. Similarly timed jokulhlaups took these paths when floods also flowed from Eyjafjoll into the Markarfljot. This suggests that concurrent routing of floods to the south and west of Myrdalsjokull is related to synchronous volcanic activity in Katla and Eyjafjallajokull. Since the 10th 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 Eldgja 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 downvalley and to popular tourist areas in North Forsmork.