Abstract:

Caldera collapses are relatively rare with nine events documented globally over the last 100 years. Two of the most recent events, those of Kilauea in 2018 and Bárðarbunga in 2014-2015 stand out in terms of the detailed observations and monitoring. In both cases, the collapses were caused by lateral withdrawal of magma from underneath the caldera. The collapse in Bárðarbunga was of the downsag type, with 2 km3 drained from a magma chamber at several kilometers depth, causing the Holuhraun eruption, 40 km to the northeast of Bárðarbunga. It was the largest eruption in Iceland in the last 250 years. Subsidence exceeded 1 m over an area of 110 km2, extending 1-2 km outside the caldera rims, with a maximum of 65 meters in the northeast part of the caldera. The caldera is filled with ice, up to 800 m thick. In order to track any possible changes following the collapse, repeated gravity surveying has been carried out at Bárðarbunga since 2015. The results show that gravity has increased significantly within the caldera, and the shape of the anomaly that has been developing indicates maximum depth of source at 600-800 m, placing it at the glacier-bedrock interface. The emergence and gradual growth of this anomaly indicates that the caldera floor has been rising, and that a significant part of the 2014-15 collapse has now been reversed by this rise. These observations have implications for the understanding of how highly active basaltic calderas in rifting environments may operate.

All are welcome.