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 39-day long eruption at the summit of Eyjafjallajökull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011 kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km3) transported by 3–10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010 kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010 kg (2%) are considered to have been transported longer than 600–700 km with <108 kg (<0.02%) reaching mainland Europe.

Magma flow during volcanic eruptions causes surface deformation that can be used to constrain the location, geometry and internal pressure evolution of the underlying magmatic source 1 . The height of the volcanic plumes during explosive eruptions also varies with magma flow rate, in a nonlinear way 2,3 . In May 2011, an explosive eruption at Grímsvötn Volcano, Iceland, erupted about 0.27 km 3 denserock equivalent of basaltic magma in an eruption plume that was about 20 km high. Here we use Global Positioning System (GPS) and tilt data, measured before and during the eruption at Grímsvötn Volcano, to show that the rate of pressure change in an underlying magma chamber correlates with the height of the volcanic plume over the course of the eruption. We interpret ground deformation of the volcano, measured by geodesy, to result from a pressure drop within a magma chamber at about 1.7 km depth. We estimate the rate of magma discharge and the associated evolution of the plume height by differentiating the co-eruptive pressure drop with time. The time from the initiation of the pressure drop to the onset of the eruption was about 60 min, with about 25% of the total pressure change preceding the eruption. Near-real-time geodetic observations can thus be useful for both timely eruption warnings and for constraining the evolution of volcanic plumes.

[1] During the eruption of the ice-covered Eyjafjallajökull volcano, a series of images from an airborne Synthetic Aperture Radar (SAR) were obtained by the Icelandic Coast Guard. Cloud obscured the summit from view during the first three days of the eruption, making the weather-independent SAR a valuable monitoring resource. Radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera, as well as the formation of cauldrons to the immediate south of the caldera. Additionally, radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site. The eruption breached the ice surface about four hours after its onset at about 01:30 UTC on 14 April 2010. The first SAR images, obtained between 08:55 and 10:42 UTC, show signs of limited supraglacial drainage from the eruption site. Floodwater began to drain from the ice cap almost 5.5 h after the beginning of the eruption, implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera. Heat transfer rates from magma to ice during early stages of cauldron formation were about 1 MW m À2 in the radial direction and about 4 MW m À2 vertically. Meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods. After the third day of the eruption, meltwater generation at the eruption site diminished due to an insulating lag of tephra.

basaltic effusive eruption at Fimmvörðuháls, southern Iceland, was an important opportunity to directly observe interactions between lava and snow/ice. The eruption site has local perennial snowfields and snow covered ice, and at the time of eruption it was covered with an additional $1-3 m of seasonal snow. Syn-eruption observations of interactions between lava and snow/ice are grouped into four categories: (1) lava advancing directly on top of snow, (2) lava advancing on top of tephra-covered snow, (3) snow/ice melting at lava flow margins, and (4) lava flowing beneath snow. Based on syn-and post-eruption observations in 2010/11, we conclude that the features seen in the lava flow field show only limited and localized evidence for the influence of snow/ice presence during the eruption. Estimated melting rates from radiant and conductive heating at the flow fronts are too slow (on the order of 5 m/hr) to allow for complete melting of snow/ice ahead of the advancing lava flows, at least during periods of observed rapid lava advance rates (15-55 m/hr). Thus we conclude that during those periods, which largely established the aerial extent of the lava flow field, lava advanced on top of snow; that this likely was the predominant mode of lava emplacement for much of the eruption is supported by many syn-eruption field observations. Examination of the lava flows subsequent to the eruption has so far only found subtle evidence for interactions between lava and snow/ice; for example, locally lava flows have fractured and are collapsing, or have developed marginal rubble aprons from melting of snow banks that were partly covered by lava flow margins.