SURGE · Uncapping subglacial eruption dynamics and glacier response

Uncapping subglacial eruption dynamics and glacier response

The overall aim of SURGE was to investigate the dynamics of volcanic eruptions that occur beneath glaciers. We aimed to determine the physical eruption processes that have occurred at Katla volcano, south Iceland, in times of greater glacial extent and to use the volcanic products to reconstruct the past glacial environment at the time of eruption. The Katla central volcano is a 30 km-wide mountain with an ice cap (Mýrdalsjökull) covering most of it. In the last 12,000 years, Katla has erupted frequently through the ice cap, leading to explosive eruptions that have scattered ash across Iceland and the North Atlantic. Despite this, the geology of this large mountain, and the volcanic and glacial processes that have built it, have not been well studied. In addition, while it is known that Iceland has been intermittently covered with ice and that the extent of the ice sheet has varied, there is little empirical evidence for ice thickness and extent at different times around Katla. The objectives of the SURGE project were to: 1) determine the eruption processes that contributed to mountain-building at three areas around Katla; 2) reconstruct the glacier thickness and extent in these areas; 3) determine the age of lavas in the three areas. It has been shown that an increase in volcanism in Iceland accompanied the end of the last glaciation. As the ice sheet retreated, pressure was released from the volcanoes leading to more eruptions. How an individual volcano that exhibits a variety of eruption styles, such as Katla, might behave physically under different ice thicknesses is becoming increasingly important to understand as glaciers around the world retreat. In addition, a better understanding of the range of past eruptions at different glacial thicknesses will inform our understanding of the likely hazards during future eruptions at Katla and other similar volcanoes. This project has contributed three new age-constrained estimates for the minimum thickness of the glacier and it is clear that at the times of eruption, Mýrdalsjökull was thicker and would have been more extensive than the present day. We have found that basaltic lavas erupted underwater on the western flank of Katla (Morinsheiði) at 800 m above sea level 10-20 thousand years ago. This area was covered by the Icelandic Ice Sheet at this time, which allowed a meltwater lake to form and remain stable during the eruption. This shows that the ice surface must have been at least 800 m above current sea level. It is likely Mýrdalsjökull converged with Eyjafjallajökull and an ancient glacier in the Markarfljót valley to the north and that this situation persisted after the last glacial maximum. We have also found that rhyolitic explosive and lava-producing eruptions occurred ~12 thousand years ago, depositing tephra and lava on the northwest and east side of Katla (Enta and Klakkfjöll). These deposits do not show any evidence for a meltwater lake forming in the glacier, but do indicate the eruptions occurred through the glacier with enhanced cooling by the ice and/or water. This suggests that meltwater drained freely from the eruption site. At this time, the glacier was at least 150-250 m thicker than present day.

Data: CORDIS, © European Union