T. Thordarson

UK monitoring and deposition of tephra from the May 2011 eruption of Grímsvötn, Iceland

Mapping the transport and deposition of tephra is important for the assessment of an eruption’s impact on health, transport, vegetation and infrastructure, but it is challenging at large distances from a volcano (> 1000 km), where it may not be visible to the naked eye. Here we describe a range of methods used to quantify tephra deposition and impact on air quality during the 21–28 May 2011 explosive basaltic eruption of Grímsvötn volcano, Iceland. Tephra was detected in the UK with tape-on-paper samples, rainwater samples, rainwater chemistry analysis, pollen slides and air quality measurements. Combined results show that deposition was mainly in Scotland, on 23–25 May. Deposition was patchy, with adjacent locations recording different results. Tape-on-paper samples, collected by volunteer citizen scientists, and giving excellent coverage across the UK, showed deposition at latitudes >55°N, mainly on 24 May. Rainwater samples contained ash grains mostly 20–30 μm long (maximum recorded grainsize 80 μm) with loadings of up to 116 grainscm-2. Analysis of rainwater chemistry showed high concentrations of dissolved Fe and Al in samples from N Scotland on 24–27 May. Pollen slides recorded small glass shards (3–4 μm long) deposited during rainfall on 24–25 May and again on 27 May. Air quality monitoring detected increased particulate matter concentrations in many parts of the country. An hourly concentration of particles < 10 μm in diameter (PM10) of ∼413 μgm-3, was measured in Aberdeen at 02:00hrs on 24 May 2011. Significant peaks of non-anthropogenic PM, which is most likely to have a volcanic origin, could be tracked as far south as the English Midlands (> 53°N) on 24 May but no negative effects on health were reported. Although the eruption column reached altitudes of 20 km above sea level, air mass trajectories suggest that only tephra from the lowest 4 km above sea level of the eruption plume was transported to the UK. This demonstrates that even low plumes could deliver tephra to the UK and suggests that the relative lack of basaltic tephra in the tephrochronological record is not due to transport processes.

Contrasting styles of welding observed in the proximal Askja 1875 eruption deposits II: Local welding

Welded fall deposits on the northern caldera rim at Askja volcano are associated with the Plinian phase of the 1875 eruption. Two welding units occur within the proximal Plinian fall centered on stratigraphic sub-units which, where non-welded, are poorly sorted and ash-rich with high abundances of fluidal and needle-like ash particles. Welding has formed due to two discrete processes; a) the sintering of hot ash and lapilli which forms the two distinct units that are laterally continuous on distance scales of tens of meters (termed ‘regional welding’), and b) creation of welding halos enclosing large, dense, discrete, nonto poorly vesicular spatter bombs that are up to 9 m in diameter (termed ‘local welding’). This paper is concerned with the nature of regional welding and the companion paper (this issue) focuses on the phenomenon of local welding. Three case studies documenting the range of welding patterns observed in regional welding are presented here. Vertical and lateral profiles of welding intensity, together with the deposit characteristics reveal that welding could only occur when the accumulation rates were sufficient and that grain size and thickness are second order factors facilitating welding. Rapid and reversible shifts in both thickness and welding grade are observed on a ~10 m scale laterally along the caldera rim suggesting considerable unsteadiness of the transport regime, which promoted localized fluctuations of the accumulation rate. The welded deposits prompt re-examination of both the dynamics of the Plinian phase of the 1875 eruption and the distribution of source vents. The dispersal of the welding units is not compatible with deposition from the full height of the Plinian plume. Similarly to the ultra-proximal deposits of Novarupta or Tarawera, these clasts probably fell from heights of hundreds of meters to b4 km, retaining sufficient heat to weld after deposition. The E–W elongated distribution of the welded units is also not compatible with a single source vent, and favors several vents that were in a fountaining phase, and located adjacent to the northern rim.

Pāhoehoe, ‘a‘ā, and block lava: an illustrated history of the nomenclature

Lava flows occur worldwide, and throughout history, various cultures (and geologists) have described flows based on their surface textures. As a result, surface morphology-based nomenclature schemes have been proposed in most languages to aid in the classification and distinction of lava surface types. One of the first to be published was likely the nine-class, Italian-language description-based classification proposed by Mario Gemmellaro in 1858. By far, the most commonly used terms to describe lava surfaces today are not descriptive but, instead, are merely words, specifically the Hawaiian words ‘a‘ā (rough brecciated basalt lava) and pāhoehoe (smooth glassy basalt lava), plus block lava (thick brecciated lavas that are typically more silicic than basalt). ‘A‘ā and pāhoehoe were introduced into the Western geological vocabulary by American geologists working in Hawai‘i during the 1800s. They and other nineteenth century geologists proposed formal lava-type classification schemes for scientific use, and most of them used the Hawaiian words. In 1933, Ruy Finch added the third lava type, block lava, to the classification scheme, with the tripartite system being formalized in 1953 by Gordon Macdonald. More recently, particularly since the 1980s and based largely on studies of lava flow interiors, a number of sub-types and transitional forms of all three major lava types have been defined. This paper reviews the early history of the development of the pāhoehoe, ‘a‘ā, and block lava-naming system and presents a new descriptive classification so as to break out the three parental lava types into their many morphological sub-types.

Survival of the Mýrdalsjökull ice cap through the Holocene thermal maximum: evidence from sulphur contents in Katla tephra layers (Iceland) from the last ∼8400 years

Abstract The climate in Iceland was drier and warmer during the Holocene thermal maximum than it is today and it has been suggested that ice caps disappeared entirely. Katla, a volcano covered by the Mýrdalsjökull ice cap in southern Iceland, has erupted rather steadily throughout the Holocene. Pre-and post-eruption sulphur concentrations in its products have been determined in previous studies, through melt inclusions trapped in phenocrysts (pre-eruption mean values of 2155±165 ppm) and fully degassed magmatic tephra (post-eruption mean values of 445±130 ppm). The phreatomagmatic tephra has much more variable S contents (550–1775 ppm) and spans the compositional gap between magmatic tephra and melt inclusions. These variable sulphur values are attributed to arresting of degassing as the magma is quenched upon contact with external water in the shallow levels of the volcano conduit. Sulphur in Katla tephra can thus be used to evaluate whether Mýrdalsjökull survived the warm spells of the Holocene. In this study, sulphur concentrations in tephra layers representing the last ∽8400 years of the volcano’s eruption history were measured, revealing concentrations in the phreatomagmatic range (600–1600 ppm). Hence, we conclude that over the last ∽8400 years, explosive activity at Katla has been dominated by phreatomagmatic eruptions, implying that the Mýrdalsjökull ice cap has been present throughout the Holocene.

Conclusion: recommendations and findings of the RED SEED working group

Abstract RED SEED stands for Risk Evaluation, Detection and Simulation during Effusive Eruption Disasters, and combines stakeholders from the remote sensing, modelling and response communities with experience in tracking volcanic effusive events. The group first met during a three day-long workshop held in Clermont Ferrand (France) between 28 and 30 May 2013. During each day, presentations were given reviewing the state of the art in terms of (a) volcano hot spot detection and parameterization, (b) operational satellite-based hot spot detection systems, (c) lava flow modelling and (d) response protocols during effusive crises. At the end of each presentation set, the four groups retreated to discuss and report on requirements for a truly integrated and operational response that satisfactorily combines remote sensors, modellers and responders during an effusive crisis. The results of collating the final reports, and follow-up discussions that have been on-going since the workshop, are given here. We can reduce our discussions to four main findings. (1) Hot spot detection tools are operational and capable of providing effusive eruption onset notice within 15 min. (2) Spectral radiance metrics can also be provided with high degrees of confidence. However, if we are to achieve a truly global system, more local receiving stations need to be installed with hot spot detection and data processing modules running on-site and in real time. (3) Models are operational, but need real-time input of reliable time-averaged discharge rate data and regular updates of digital elevation models if they are to be effective; the latter can be provided by the radar/photogrammetry community. (4) Information needs to be provided in an agreed and standard format following an ensemble approach and using models that have been validated and recognized as trustworthy by the responding authorities. All of this requires a sophisticated and centralized data collection, distribution and reporting hub that is based on a philosophy of joint ownership and mutual trust. While the next chapter carries out an exercise to explore the viability of the last point, the detailed recommendations behind these findings are detailed here.

MeMoVolc consensual document: a review of cross-disciplinary approaches to characterizing small explosive magmatic eruptions

A workshop entitled “Tracking and understanding volcanic emissions through cross-disciplinary integration: a textural working group” was held at the Université Blaise Pascal (Clermont-Ferrand, France) on the 6–7 November 2012. This workshop was supported by the European Science Foundation (ESF). The main objective of the workshop was to establish an initial advisory group to begin to define measurements, methods, formats and standards to be applied in the integration of geophysical, physical and textural data collected during volcanic eruptions. This would homogenize procedures to be applied and integrated during both past and ongoing events. The workshop comprised a total of 35 scientists from six countries (France, Italy, Great Britain, Germany, Switzerland and Iceland). The four main aims were to discuss and define: standards, precision and measurement protocols for textural analysis; identification of textural, field deposit, chemistry and geophysical parameters that can best be measured and combined; the best delivery formats so that data can be shared between and easily used by different groups; and multi-disciplinary sampling and measurement routines currently used and measurement standards applied, by each community. The group agreed that community-wide, cross-disciplinary integration, centred on defining those measurements and formats that can be best combined, is an attainable and key global focus. Consequently, we prepared this paper to present our initial conclusions and recommendations, along with a review of the current state of the art in this field that supported our discussions.

Interaction between central volcanoes and regional tectonics along divergent plate boundaries: Askja, Iceland

Activity within magmatic divergent plate boundaries (MDPB) focuses along both regional fissure swarms and central volcanoes. An ideal place to investigate their mutual relationship is the Askja central volcano in Iceland. Askja consists of three nested calderas (namely Kollur, Askja and Öskjuvatn) located within a hyaloclastite massif along the NNE-SSW trending Icelandic MDPB. We performed an extensive field-based structural analysis supported by a remote sensing study of tectonic and volcanic features of Askja’s calderas and of the eastern flank of the hyaloclastite massif. In the massif, volcano-tectonic structures trend N 10° E to N 40° E, but they vary around the Askja caldera being both parallel to the caldera rim and cross-cutting on the Western side. Structural trends around the Öskjuvatn caldera are typically rim parallel. Volcanic vents and dikes are preferentially distributed along the caldera ring faults; however, they follow the NNE-SSW regional structures when located outside the calderas. Our results highlight that the Askja volcano displays a balanced amount of regional (fissure-swarm related) and local (shallow-magma-chamber related) tectonic structures along with a mutual interaction among these. This is different from Krafla volcano (to the north of Askja) dominated by regional structures and Grímsvötn (to the South) dominated by local structures. Therefore, Askja represents an intermediate tectono-magmatic setting for volcanoes located in a slow divergent plate boundary. This is also likely in accordance with a northward increase in the spreading rate along the Icelandic MDPB.

The Volume of Lava Erupted During the 2014 to 2015 Eruption at Holuhraun, Iceland: A Comparison Between Satellite‐ and Ground‐Based Measurements

The 31 August 2014 to 27 February 2015 eruption at Holuhraun created the largest lava flow field in Iceland since the 1783–1784 Laki eruption. Emplacement of a basaltic flow field of this magnitude onto an effectively flat surface (<0.1°) is a rare occurrence. Lava discharge rate, a fundamental variable that controls flow field emplacement, allows us to estimate the total volume of lava erupted when integrated over time. Thus, discharge rate data are important for volcano monitoring and lava flow modeling. Here we compare discharge rates estimated using data from National Aeronautics and Space Administration’s MODerate Resolution Imaging Spectroradiometer (MODIS) and the method of Harris et al. (1997a, https://doi.org/10.1029/96JB03388; 1997b, https://doi.org/10.1007/s004450050174; 2007, https://doi.org/10.1007/s00445-007-0120-y), with estimates of discharge rates derived from comparing intermittent ground-based measurements of flow field volume. The time-averaged discharge rates (TADR) reveal a pulsed increase in the first few days of the eruption. Although the trends of the satelliteand ground-based discharge rates are similar, the ground-based estimates are systematically higher than the satellite-derived estimates (about 2 to 3 times higher) in the first 30 days of the eruption, and relatively close (within 30%) for the next 20 days. Conversely, during the final 130 days, the satellite-based estimates are systematically higher than the ground-based estimates (about 2 times higher). This difference likely arises from the assumption of the lava flow surface temperature used in the space-based calculation, which may not be entirely representative of this uniquely large and intense basaltic eruption. However, the satellite-based technique yields a total erupted volume of about 1.21 km in good agreement with the 1.2 km (84 km) derived from field observations and mapping.