Geomorphic Reconstruction and 3D Visualisation of a Tenth Century Volcanic Jökulhlaup at Sólheimajökull Glacier, South Iceland

摘要

Jökulhlaup (glacial outburst floods) present a recurring natural hazard to the inhabitants and infrastructure along Iceland’s south coast. The high-energy, debris-rich floods are capable of eroding, transporting and depositing vast quantities of sediment; and in some cases, can completely re-shape sandur topography within a single flood event. They can be triggered by drainage of ice-dammed lakes or by meltwater released during sub-glacial volcanic eruptions; or a combination of both. This research reconstructs a Tenth Century jökulhlaup at Sólheimajökull; a southern outlet glacier of Mýrdalsjökull ice cap overlying Katla volcano. The flood permanently altered sandur topography and marked an important shift in changing flood routes and resultant flood deposits at the site since. Collection and synthesis of field evidence provides detailed information about the jökulhlaup, the route it took, the contemporaneous glacier margin, and environmental conditions at the time. As Sólheimasandur’s last major jökulhlaup, this event is important, not just in terms of scale and magnitude (comparable with the largest floods known to have occurred at the site); but also because of its geomorphic and sedimentary signature. This research records and compiles the remnant features of the Tenth Century flood, some of which are still evident at Sólheimasandur today. The outcome is a template for linking jökulhlaup sources, pathways and deposits, focusing in particular on ice- proximal flood routeways as a determining control over the nature of flood deposit. Geomorphic, sedimentological and tephrostratigraphic techniques have been used to recreate the origin, mechanics, and impacts of this flood through analysis of its deposits. In the same manner the regional Holocene jökulhlaup chronology is refined for Sólheimasandur, supported by a dating framework of volcanic eruptions and glacial fluctuations. An innovative 3D visual simulation, VolcVis, has been created, which models the field site as it might have looked in the Tenth Century, based on field data collected in reconstructing the Tenth Century flood. The ability of the prototype simulation to assist in palaeo-geomorphic reconstruction, is tested by visually simulating the Tenth Century volcanic jökulhlaup for the first time. The model distills complex and often spatially disparate field data into an interactive, user-friendly tool, with wider applicability in communicating results to the general public, aiding hazard mitigation authorities and informing the designation of evacuation routes. Identification of the pathways, magnitudes and frequencies of past jökulhlaup at Sólheimajökull allows a greater accuracy in predicting contemporary flood routes, when other major subglacial eruptions occur in this region of Iceland. The value of this research is timely given recent volcanically-generated jökulhlaup at Eyjafjallajökull (2010) and Grímsvötn (2011); illustrating how heightening awareness of past (and present) glacio-volcanic hazards in this region of South Iceland ensures better preparedness for future events.