Magnús T. Gudmundsson

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland

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.

Eight centuries of periodic volcanism at the center of the Iceland hotspot revealed by glacier tephrostratigraphy

A record of volcanic activity within the Vatnajokull ice cap has been obtained by combining data from three sources: tephrostratigraphic studies of two outlet glaciers, a 415-m-long ice core from northwestern Vatnajokull, and written records. The record extends back to a.d. 1200 and shows that the volcanic activity has a 130–140 yr period. Intervals of frequent eruptions with recurrence times of three to seven years alternate with intervals of similar duration having much lower eruption frequency. In comparison with other parts of the plate boundary in Iceland, eruption frequency is greater, episodes of unrest are longer, and intervals of low activity are shorter. The high eruption frequency may be the result of a more sustained supply of magma, owing to the areas location above the center of the Iceland mantle plume. When combined with historical data on eruptions and earthquakes, our data indicate that rifting-related activity in Iceland as a whole is periodic and broadly in phase with the volcanic activity within Vatnajokull.

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

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

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.

Aggregation-dominated ash settling from the Eyjafjallajökull volcanic cloud illuminated by field and laboratory high-speed imaging

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

Center of the Iceland hotspot experiences volcanic unrest

The recent Eyjafjallajokull (Iceland) eruption strikingly underlined the vulnerability of a globalized society to the atmospheric dispersal of volcanic clouds from even moderate-size eruptions. Ash aggregation controls volcanic clouds dispersal by prematurely removing fine particles from the cloud and depositing them more proximally. Physical parameters of ash aggregates have been modeled and derived from ash fallout deposits of past eruptions, yet aggregate sedimentation has eluded direct measurement, limiting our ability to predict the dispersal of volcanic clouds. Here we use field-based, high-speed video analysis together with laboratory experiments to provide the first in situ investigation and parameterization of the physical features and settling dynamics of ash aggregates from a volcanic cloud. In May 2010, high-speed video footage was obtained of both ash particles and aggregates settling from the Eyjafjallajokull volcano eruption cloud at a distance of 7 km from the vent; fallout samples were collected simultaneously. Experimental laboratory determinations of the density, morphology, and settling velocity of individual ash particles enable their distinction from aggregates. The combination of field and experimental analyses allows a full characterization of the size, settling velocity, drag coefficient, and density distributions of ash aggregates as well as the size distribution of their component particles. We conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud, aggravating the ash hazard locally and modifying cloud dispersal regionally. This study provides a valuable tool for monitoring explosive eruptions, capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecast.

Melting of ice by magma-ice-water interactions during subglacial eruptions as an indicator of heat transfer in subaqueous eruptions

A volcanic eruption beneath the Vatnajokull ice cap in central Iceland (Figure 1) began on September 30,1996, along a 7-km-long fissure between the volcanoes Bardarbunga and Grimsvotn. The eruption continued for 13 days and produced ˜0.5 km3 of basaltic andesite. Meltwater from the eruption site flowed into the caldera lake of the Grimsvotn volcano, where it accumulated beneath a floating ice shelf. The lakes ice dam was lifted off the glacier bed on November 4, and in the next two days more than 3 km3 of water drained out beneath the glacier and flushed down to the south coasts alluvial plain, causing extensive flooding and damage to transportation and communication systems.

Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934-91, deduced from in-situ measurements of subglacial lake volume

Eruptions within glaciers are characterized by fast cooling of volcanic deposits, rapid melting of ice and heating of meltwater. Heat transfer rates in subglacial eruptions may be monitored through melting rates of ice and simple calorimetric calculations used to infer heat fluxes and estimate the efficiency of heat transfer from magma. Cooling models of effusive basaltic eruptions forming pillow lava indicate that thermal efficiency of such eruptions is 10-45%, and highest when the eruption rates are low and pillows are exposed to surrounding meltwater for a comparatively long time. When magma fragmentation occurs by non-explosive granulation or explosive activity the glass particles formed have diffusion times mainly in the range 10 - 3 s to 10 2 s depending on grain size, the mean being of the order of 1 s. Limited observational data on ice-melting rates and models of cooling times suggest that the efficiency of heat transfer from fragments may commonly be 70-80%. Correspondingly, total heat transfer rates associated with fragmentation are several times higher than for pillow lava at the same eruption rate. The contrasts in efficiency imply that variation in heat transfer rates during fragmentation may closely correlate with variations in magma eruption rate, whereas for pillow lava eruptions changes in heat transfer lag well behind changes in eruption rate. Though pillows may still have molten cores when buried in a growing volcanic pile, the temperature of volcanic glass created during subaqueous fragmentation should be no greater than 250-300°C at the time of deposition.

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

A record of volumes of jokulhlaups from the subglacial Grimsvotn lake, Vatnajokull, Iceland, has been derived for the period 1934-91. The change in lake volume during jokulhlaups is estimated from the lake area, ice-cover thickness and the drop in lake level. The jokulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power. The two jokulhlaups in the 1930s each discharged about 4.5 km (peak discharge 25-30×10 3 m 3 s −1 ). In the 1980s, jokulhlaup volumes were 0.6.-1.2 km 3 (peak discharge 2×10 3 m 3 s −1 ). The lake level required to trigger a jokulhlaup has risen as an ice dam east of the lake has thickened. Water flow in a jokulhlaup ceases when the base of a floating ice shelf covering Grimsvotn settles to about 1160 m a.s.l. Apparently, the jokulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side. The decline in melting rates has resulted in a positive mass balance of the 160-170 km Grimsvotn ice-drainage basin. Comparison of maps shows that the average positive mass-balance rate was 0.12 km 3 a −1 (25% of the total accumulation) in the period 1946-87. A gradually increasing positive mass balance has prevailed since 1954, reaching 0.23 km 3 a −1 in 1976-86 (48% of total accumulation )

Gravity and magnetic studies of the subglacial Grímsvötn volcano, Iceland: Implications for crustal and thermal structure

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.