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...

The 6-month long eruption at Holuhraun (August 2014-February 2015) in the Bárðarbunga-Veiðivötn volcanic system was the largest effusive eruption in Iceland since the 1783-1784 CE Laki eruption. The lava flow field covered~84 km 2 and has an estimated bulk (i.e., including vesicles) volume of~1.44 km 3. The eruption had an average discharge rate of~90 m 3 /s making it the longest effusive eruption in modern times to sustain such high average flux. The first phase of the eruption (August 31, 2014 to mid-October 2014) had a discharge rate of~350 to 100 m 3 /s and was typified by lava transport via open channels and the formation of four lava flows, no. 1-4, which were emplaced side by side. The eruption began on a 1.8 km long fissure, feeding partly incandescent sheets of slabby pāhoehoe up to 500 m wide. By the following day the lava transport got confined to open channels and the dominant lava morphology changed to rubbly pāhoehoe and 'a'ā. The latter became the dominating morphology of lava flows no. 1-8. The second phase of the eruption (Mid-October to end November) had a discharge of~100-50 m 3 /s. During this time the lava transport system changed, via the formation of a b 1 km 2 lava pond~1 km east of the vent. The pond most likely formed in a topographical low created by a the pre-existing Holuhraun and the new Holuhraun lava flow fields. This pond became the main point of lava distribution, controlling the emplacement of subsequent flows (i.e. no. 5-8). Towards the end of this phase inflation plateaus developed in lava flow no. 1. These inflation plateaus were the surface manifestation of a growing lava tube system, which formed as lava ponded in the open lava channels creating sufficient lavastatic pressure in the fluid lava to lift the roof of the lava channels. This allowed new lava into the previously active lava channel lifting the channel roof via inflation. The final (third) phase, lasting from December to end-February 2015 had a mean discharge rate of~50 m 3 /s. In this phase the lava transport was mainly confined to lava tubes within lava flows no. 1-2, which fed breakouts that resurfaced N 19 km 2 of the flow field. The primary lava morphology from this phase was spiny pāhoehoe, which superimposed on the 'a'ā lava flows no. 1-3 and extended the entire length of the flow field (i.e. 17 km). This made the 2014-2015 Holuhraun a paired flow field, where both lava morphologies had similar length. We suggest that the similar length is a consequence of the pāhoehoe is fed from the tube system utilizing the existing 'a'ā lava channels, and thereby are controlled by the initial length of the 'a'ā flows.

The effusive six months long 2014-2015 Bárðarbunga eruption (31 August-27 February) was the largest in Iceland for more than 200 years, producing 1.6 ± 0.3 km 3 of lava. The total SO 2 emission was 11 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO 2 exceeded the 350 µg m −3 hourly average health limit over much of Iceland for days to weeks. Anomalously high SO 2 concentrations were also measured at several locations in Europe in September. The lowest pH of fresh snowmelt at the eruption site was 3.3, and 3.2 in precipitation 105 km away from the source. Elevated dissolved H 2 SO 4 , HCl, HF, and metal concentrations were measured in snow and precipitation. Environmental pressures from the eruption and impacts on populated areas were reduced by its remoteness, timing, and the weather. The anticipated primary environmental pressure is on the surface waters, soils, and vegetation of Iceland.

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