Matthew J. Roberts

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.

Eruptions of Eyjafjallajökull Volcano, Iceland

The April 2010 eruption of Eyjafjallajokull volcano (Figure 1), located on Icelands southern coast, created unprecedented disruptions to European air traffic during 15–20 April, costing the aviation industry an estimated

Ice fracturing during jökulhlaups: implications for englacial floodwater routing and outlet development.

250 million per day (see the related news item in this issue). This cost brings into focus how volcanoes can affect communities thousands of miles away. Eyjafjallajokull rises to 1666 meters above sea level and hosts agricultural land on its southern slopes, with farms located as close as 7 kilometers from the summit caldera. In the past 1500 years, Eyjafjallajokull has produced four comparatively small eruptions. The eruption previous to 2010 began in December 1821 and lasted for over a year, with intermittent explosive activity spreading a thin layer of tephra (ash and larger ejected clasts) over the surrounding region. In contrast, the explosive 2010 eruption, sourced within the ice-capped summit of the volcano, so far is larger and characterized by magma of a slightly different composition. This may suggest that deep within the volcano, the 1821 magma source is mixing with new melt, or that residual melt from past intrusive events is being pushed out by new magma.

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

Theoretical studies of glacial outburst floods (jokulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jokulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðararjokull and Solheimajokull. Video footage and aerial photographs, taken before, during and after the Skeiðararjokull jokulhlaup and immediately after the Solheimajokull jokulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment.

Glaciohydraulic supercooling in Iceland

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

Forecasting and monitoring a subglacial eruption in Iceland

We present evidence of glaciohydraulic supercooling under jokulhlaup and ablation- dominated conditions from two temperate Icelandic glaciers. Observations show that freezing of sediment-laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes. Intraglacial frazil ice propagation under normal ablation-dominated conditions can trap copious volumes of sediment, which forms anomalously thick sections of debris-rich ice. Glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development. Extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling, which may explain the concentration of englacial sediments deposited in Heinrich layers in the North Atlantic during the last glaciation.

The role of multiple glacier outburst floods in proglacial landscape evolution: The 2010 Eyjafjallajökull eruption, Iceland

The recognition of geophysical precursors to volcanic activity is a primary challenge in volcano monitoring. That challenge was successfully met by scientists at the Icelandic Meteorological Office (IMO) before the 1 November 2004 eruption of Grimsvotn, a subglacial volcano beneath the Vatnajokull ice cap,Iceland (Figure 1). Seismic and geodetic precursors were properly recognized, leading to a timely eruption forecast and warning announcements. During the eruption, IMOs monitoring capability was greatly expanded by employing geophysical and meteorological observation tools, which enabled real-time hazard assessment.

Localized uplift of Vatnajökull, Iceland: Subglacial water accumulation deduced from InSAR and GPS observations

The 2010 Eyjafjallajokull eruption in Iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jokulhlaups (glacial outburst floods) (>140 events). Time-lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jokulhlaup of 14 April 2010 composed 61% of the 57 × 106 m3 total discharge of the combined events, and had the highest peak discharge for the two main flood events, but only deposited 18% of the total volume of sediment in front of Gigjokull glacier. The majority of sediments (67% of a total volume of 17.12 × 106 m3) were deposited by the 15 April 2010 jokulhlaup, and this was followed by extensive reworking by a series of smaller jokulhlaups over the following 29 days that deposited 15% of the total sediment. The geomorphological and sedimentary signatures of the two largest jokulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits. Consequently, the ice-proximal, posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jokulhlaups. The findings support a complex-response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water:sediment ratios during the eruption.

Ice–water interactions during floods from Grænalón glacier-dammed lake, Iceland

We report on satellite and ground-based observations that link glacier motion with subglacial hydrology beneath Skeiðararjokull, an outlet glacier of Vatnajokull, Iceland. We have developed a technique that uses interferometric synthetic aperture radar (InSAR) data, from the European Remote-sensing Satellite (ERS-1/-2) tandem mission (1995–2000), to detect localized anomalies in vertical ice motion. Applying this technique we identify an area of the glacier where these anomalies are frequent: above the subglacial course of the river Skeiðara, where we observed uplift of 0.15–0.20md during a rainstorm and a jokulhlaup, and subsidence at a slower rate subsequent to rainstorms. A similar pattern of motion is apparent from continuous GPS measurements obtained at this location in 2006/07. We argue that transient uplift of the ice surface is caused by water accumulating at the glacier base upstream of an adverse bed slope where the overburden pressure decreases significantly over a short distance. Most of the frictional energy of the flowing water is therefore needed to maintain water temperature at the pressure-melting point. Hence, little energy is available to enlarge water channels sufficiently by melting to accommodate sudden influxes of water to the base. This causes water pressure to exceed the overburden pressure, enabling uplift to occur.

The Digital Signal Processing Platform for the Low Frequency Aperture Array: Preliminary Results on the Data Acquisition Unit

Abstract This paper explores changing ice–water interactions during jökulhlaups from Grænalón, a 5 × 108m3 subaerial lake dammed by Skeiðarárjökull, Iceland. Unstable drainage of Grænalón since the early 20th century has resulted in 45 jökulhlaups whose hydrologic character has varied enormously. Geomorphic observations and geophysical measurements from the inlet and outlet zones of the subglacial floodwater tract constrained the hydromechanical factors governing ice–water interactions at Grænalón. To date, three distinct drainage regimes have occurred in response to the changing surface elevation of Grænalón. Shifts from one drainage regime to another involved pronounced changes in jökulhlaup magnitude, timing and cyclicity. Present hydraulic conditions for lake drainage differ from the classical view of a pressure-coupled lake draining directly beneath an ice dam. Instead, low-amplitude drawdown occurs at regular, frequent intervals when hydrostatic pressure in a shallow, rock–ice trench enables water to flow beneath a sagging ice barrier. Floodwater exits Skeiðarárjökull in a supercooled state due to rapid hydraulic displacement from an overdeepened subglacial basin.