Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Abstract

40Large volcanic eruptions on Earth commonly occur with collapse of the roof of a crustal magma 41 reservoir, forming a caldera. Only a few such collapses occur per century and lack of detailed 42 observations has obscured insight on mechanical interplay between collapse and eruption. We use Calderas are 1 -100 km diameter depressions found in volcanic regions of Earth and other planets. basaltic andesite) intrusive activity and eruptions (2,(9)(10)(11)(12). 59The consensus from field and modelling studies is that caldera collapse progresses from initial 60 surface downsag to fault-controlled subsidence (1, 8, 13, 14). The pre-collapse topography is obtained by subtracting the subsidence observed at the surface. As we recorded the caldera subsidence mainly on the ice (Fig. 1, Fig. S1), we made corrections and (Fig. 3A). We therefore conclude that suggestions of a large increase in ice flow out of the caldera 147 during these events (25) cannot be fitted with our data. 148Bedrock subsidence exceeding 1 m occurred within an area of 110 km 2 that extended beyond the 149 pre-existing caldera (Fig. 1, Fig. S1). After termination of collapse the total subsidence at the pre-150 existing caldera rims amounted to 3 to 11 meters ( Fig. 1D and 1E). Using subglacial radio-echo GPS station in the center of the caldera (Fig. 1A), including the rate of vertical rate of ice surface Cumulative number of M>4 caldera earthquakes, with magnitude evolution colored in red, blue and 176 grey representing clusters on the southern rim, the northern rim and smaller clusters, respectively 177 (see Fig. S5). E) Cumulative seismic moment for M>4 caldera earthquakes. from analysis of subaerial gas measurements (Fig. 4). This depth concurs with our regional on FTIR and Multi-GAS measurements (24). 194Seismicity and subsurface structure 195 We used seismic data and Distinct Element Method (DEM) numerical modelling (24), to 196 characterize the deeper collapse structure as the reactivation of a steeply-inclined ring fault (Fig. 5). 197We mostly observed seismicity at depths of 0-9 km beneath the northern and southern caldera rims 198( Fig. 5B), with earthquakes being more numerous on the northern rim. This spatial pattern of 199 seismicity is consistent with fracturing above a deflating magma reservoir that was elliptical in (Fig. 5C, D). Our best fitting models had preexisting faults dipping out at 80-85¡ from the caldera 207 center on the north side and at 85-90¡ toward the caldera center on the south side. The modeled pre- 208existing faults lay at 1-2 km below the surface on the north side and 3-4 km on the south side. 209Modeling of a more complex fault geometry or the inclusion of greater material heterogeneity may 210 further improve the data fit, but presently lacks robust geophysical constraints. components of the observed earthquakes at B ‡rdarbunga. We, however, narrowed down on 222 plausible solutions by using the micro-earthquakes (Fig. 5A). The moment tensor solutions are well 223 constrained, but the inferred d...

Synthesis of global satellite observations of magmatic and volcanic deformation: implications for volcano monitoring & the lateral extent of magmatic domains

“…For example, the maximum displacement rate of a transient, but rapid, acceleration is likely to be underestimated if its duration was a small fraction of satellite repeat time. The highest deformation rates in our datasets (metres per day) were detected during caldera collapse at Bar∂a-bunga (Gudmundsson et al, 2016), and dyke opening during eruptions or rifting events (e.g., Pagli et al, 2012) where duration could be defined from independent data sources. Despite measurement limitations, some general trends can be observed in the relationship between displacement rate and other deformation signal parameters.…”

Modelling satellite-derived magma discharge to explain caldera collapse

“…The collapse was coeval with a series of magnitude M 5 earthquakes whose distribution correlates with the margins of a subsiding piston ~7 km in diameter (Riel et al, 2015). Caldera floor subsidence decelerated exponentially with time and, by the end of the eruption on 27 February 2015, had formed a bedrock depression ~65 ± 3 m deep, ~1.8 ± 0.2 km 3 in volume (Gudmundsson et al, 2016).…”

“…The Holuhraun eruption followed 15 d of sustained seismicity that accompanied the propagation of a 45-km-long segmented dike, initiated at 10-12 km beneath the caldera of Bárðarbunga volcano (Sigmundsson et al, 2015;Ágústsdóttir et al, 2016;Gudmundsson et al, 2016). The effusive activity persisted for ~180 d and was accompanied by the slow collapse of the ice-covered summit caldera of Bárðarbunga which began a few days after the beginning of the seismicity (Gudmundsson et al, 2016). The collapse was coeval with a series of magnitude M 5 earthquakes whose distribution correlates with the margins of a subsiding piston ~7 km in diameter (Riel et al, 2015).…”

Tremor-rich shallow dyke formation followed by silent magma flow at Bárðarbunga in Iceland

“…The boiling temperature of water at 2 km depth is about 350 • C and 441 • C under hydrostatic and lithostatic pressure conditions, respectively 21 . These temperatures are achievable given a temperature of more than 1100 • C for the e using lava 22 . However, for hydrothermal boiling we would expect that an increase in available water and decrease in pressure (upwards movement) would lead to an increase in observed tremor amplitude.…”