Benedikt Ofeigsson

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18 years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5 mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05 km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

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

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual near-exponential decline of both the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

2 Katla and Eyjafjallajökull Volcanoes

Abstract The Katla volcano is covered by the Mýrdalsjokull ice cap and is currently one of the most active volcanoes in Iceland. It has erupted twenty times the past 1,100 years. The neighbouring volcano Eyjafjallajokull has erupted twice, simultaneously with Katla. As glaciers cover both volcanoes, their eruptions are phreato-magmatic by nature. The volcanoes are located directly south of where surface expressions of the rift cease. Seismically, Katla is one of the most active volcanoes in Iceland, showing an annual cycle in activity, observed from at least 1960 and less pronounced since 2004. From 1999 to late 2004, GPS measurements revealed steady inflation of the volcano, showing uplift and outward horizontal displacement. Until 1990s, Eyjafjallajokull had been seismically quiet for several decades. Seismic activity there was high in 1994 and again in 1999, related to the emplacement of two intrusions.

Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground- and satellite-based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003 to 2007, we observe a net micro-gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that, here, magma movement and/or a steam cap in the geothermal system of Askja at depth are responsible for the observed micro-gravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular, variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation.

Pressure sources versus surface loads: Analyzing volcano deformation signal composition with an application to Hekla volcano, Iceland

The load of lava emplaced over periods of decades to centuries induces a gradual viscous response of the Earth resulting in measurable deformation. This effect should be considered in source model inversions for volcanic areas with large lava production and flow emplacement in small centralized regions. If deformation data remain uncorrected, constructive load and pressure source interference may result in an overestimate of depth and volume of a magma reservoir whereas destructive signal interference may cause these values to be underestimated. In both cases the source geometry preference could be biased. The ratio of horizontal and vertical displacements aids the identification of composite signals. We provide a method to quantify and remove the lava load deformation signals, using deformation at Hekla volcano, Iceland as an example.

Crustal movements due to Iceland's shrinking ice caps mimic magma inflow signal at Katla volcano

Many volcanic systems around the world are located beneath, or in close proximity to, ice caps. Mass change of these ice caps causes surface movements, which are typically neglected when interpreting surface deformation measurements around these volcanoes. These movements can however be significant, and may closely resemble movements due to magma accumulation. Here we show such an example, from Katla volcano, Iceland. Horizontal movements observed by GPS on the flank of Katla have led to the inference of significant inflow of magma into a chamber beneath the caldera, starting in 2000, and continuing over several years. We use satellite radar interferometry and GPS data to show that between 2001 and 2010, the horizontal movements seen on the flank can be explained by the response to the long term shrinking of ice caps, and that erratic movements seen at stations within the caldera are also not likely to signify magma inflow. It is important that interpretations of geodetic measurements at volcanoes in glaciated areas consider the effect of ice mass change, and previous studies should be carefully reevaluated.

Chapter 18 Tectonic and volcanic deformation at São Miguel Island, Azores, observed by continuous GPS analysis 2008–13

Abstract We use a Global Positioning System (GPS) to unravel the complex geodynamics of the Azores Triple Junction where tectonic and volcanic activities coexist. The temporal analysis of densely distributed continuous GPS data on São Miguel for the period 2008–13 provides an improved understanding of interactions between present-day plate boundary kinematics and volcanic deformation. We find a high-strain-rate (0.28 ppm a–1) zone between Congro and Furnas, which accommodates about 50% of the Eurasian–Nubian plate spreading as predicted by the MORVEL plate angular velocity model. The seismic unrest of Fogo–Congro (2011–12) shows a strong similarity with the Matsushiro (Japan) earthquake swarm (1965–66) and the Campi Flegrei (Italy) volcanic unrest (1969–72 and 1982–85), in that an edifice-scale inflation associated with intense high-frequency earthquakes and inflation–deflation reversals coincided with a sharp drop in seismicity. We propose the following hypothesis for the Fogo unrest: (1) the primary inflation source beneath Fogo promotes lateral diffusion of fluids that is selectively guided by existing cracks/fissures formed from regional extension; (2) an influx of fluids increases pressure in cracks/fissures and generates lower-frequency earthquakes; and (3) discharge of fluids causes pressure decrease and dilatancy recovery (i.e. seismic quiescence).

Magma Movements in Volcanic Plumbing Systems and their Associated Ground Deformation and Seismic Patterns

Abstract Improving our understanding of volcanic hazards requires better knowledge of the location, volume and properties of magma bodies in the roots of active volcanoes, as well as information on melt supply and magma transfer. This requires a good understanding of both the geometric structure of the volcanic and igneous plumbing system, as well as observations of sub-surface magma movements and their interpretation. Arrival of new magma in volcano roots often causes volcanic unrest expressed by one or more of the following: increased seismicity, ground deformation, volcanic gas release and ground temperature changes. Recent eruptions and magmatic events in Iceland have provided opportunities to apply repeated geodetic observations at volcanoes to measure ground deformation and interpret these measurements together with seismic observations in terms of subsurface magmatic processes.