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The rapid resurgence of the ice-covered Bárdarbunga after the 2014-2015 caldera collapse, evidence from repeated gravity surveys and other data

Magnús T. Gudmundsson , Thórdís Högnadóttir, Eyjólfur Magnússon, Finnur Pálsson, Hannah Reynolds, Tom Winder, Elías Rafn Heimisson, Bryndís Brandsdóttir, Freysteinn Sigmundsson

Abstract

Caldera collapses are relatively rare, with nine events documented globally over the last 100 years.  Two of the most recent events, those of Kīlauea 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 magma withdrawal from underneath the caldera.  At Bárðarbunga, where the caldera is filled with up to 800 m thick ice, the collapse classifies as downsag, as ~2 km3  of magma were drained laterally from underneath the caldera, causing the Holuhraun eruption, 40 km to the northeast.  Maximum subsidence of 65 meters occurred in the northeast part of the caldera.  Earthquake fault plane solutions in recent years and other observational data are consistent with resurgence at Bárðarbunga since 2015.  To track possible changes following the collapse and quantify any resurgence, repeated gravity surveying has been carried out since 2015.  The results show a large gravity increase within the caldera with peak amplitude exceeding 1 mGal. The shape of the anomaly that has been developing indicates maximum depth of source shallower than 1 km, 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 been reversed in only 8 years.  These observations have implications for the understanding of how highly active basaltic calderas in rifting environments may operate, where resurgence occurs through upwards movement along the caldera faults.