Magma systems beneath Icelandic volcanoes: Exploring Eyjafjallajökull and Torfajökull volcanoes through InSAR time series

Abstract

Mapping magmatic plumbing systems beneath active and moderately-active volcanoes contributes to our understanding of how magma spreads up to the crust surface. Additionally, the time that magma takes to accumulate in the earth’s interior before an eruption could be better estimated if we knew in detail the shape, size and depth of the magma chamber.
Radar interferometric observations can used to measure displacements associated with crustal deformation and to subsequently infer magma chamber features through geophysical modelling. This is normally applied to highly active volcanoes, or volcanoes with an eruption imminent. However, less is known about long-lasting magma chambers beneath volcanoes that have not erupted for centuries.
In this study, we explore the magma systems of two different Icelandic volcanoes using InSAR time series. Eyjafjallajökull, which erupted in April 2010 and caused significant disruption to air traffic, had not erupted prior to that for nearly two centuries, and only three times in total in the previous 1200 years.In 1992 an increase of seismicity signalled a change from its previous low activity. InSAR- and GPS-derived models of Eyjafjallajökull from these last 20 years of unrest, show a complex network of sills and dikes rather than a single established magma chamber. In 1994 and 1999 two deformation episodes were modelled as sill intrusions between 4.5 and 6.5 km depth under the southeastern flank. Prior to the 2010 eruption, further sills intruded at similar depths. Shortly before the onset of eruption, one or more dikes propagated upwards and eventually reached the surface. Curiously, the source of deflation during the explosive eruption has a different location and geometry to the sills modelled in the pre-eruptive phases.
Torfajökull, also erupts infrequently, with only two eruptions in the last 1200 years, the latest of which was over 5 centuries ago. However, ongoing seismicity, deformation and geothermal activity indicate the continued presence of a long-lasting magma chamber. InSAR time series show subsidence of the southwestern part of the caldera with rates of up to ~13 mm yr-1, which has been interpreted as a cooling magma chamber. Although historical eruptions have been relatively small, the large caldera (12 km diameter) is evidence of a massive “supervolcano” eruption in the past, and the potential for a further eruption of similar size is unknown.
Here we compare and contrast the two volcanoes and explore what we might expect in terms of warning signals in the case of a future eruption of Torfajökull.