Gudrún Larsen

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.

Geochemistry of historical-age silicic tephras in Iceland

The major element chemistry of nine silicic tephras of historical age from Iceland is assessed as a key step in the development of the recent tephrochronology of the North Atlantic region. The tephras include the largest such layers produced by each of the five central volcanoes Hekla, Ö ræfajökull, Eyjafjallajökull, Torfajökull and Askja since the ninth century ad (H 1104, Ö1362, E 1821, Landnám tephra c. 870, A 1875) and four other tephras (H 1158, H 1510, H 1947, Ö1727). The determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra, and allows links to be made between Icelandic source areas (with precise dating evidence) and distal deposits elsewhere in the North Atlantic region. Although major element data can be used to discriminate between tephra layers produced by the different central volcanoes, on its own it cannot be used to identify all the Holocene layers produced by each central volcano. However, integration with other stratigraphic and chronological data can resolve ambiguous cases, permitting the confident identification of precise isochrones.

Recent volcanic history of the Veidivötn fissure swarm, southern Iceland — an approach to volcanic risk assessment

Abstract The recent volcanic history of the southwestern part of the Veidivotn fissure swarm, southern Iceland, provides a basis for assessment of volcanic risk in an area of large hydropower potential. Local tephrastratigraphy and regional tephrochronology provide relative and absolute dating of individual eruptions as well as information on the volume and distribution of the products formed in each eruption. Three large eruptions took place in this area in ∼ 1480 A.D., ∼ 900 A.D. and ∼ 150 A.D., respectively. Each eruption produced approx. 1 km3 (DRE) of basaltic, and minor amounts of silicic lava and tephra on fissures up to 42 km long. No evidence is found of smaller eruptions during this period. The estimated eruption frequency, one eruption every 600–800 years, implies that this part of the Veidivotn fissure swarm is inactive for long periods between relatively large volcanic events. A change in the mode of eruption from effusive to explosive took place during this period. The hazards posed by this area include far-reaching lava flows, widespread heavy tephra fall with thicknesses in excess of 2 m at distances of 10 km, and damming of a large glacial river with the consequent formation of unstable lakes. A volcano-tectonic model, which explains the observed eruption frequency and provides a basis for a long-term monitoring program, is proposed. Eruptions on the Veidivotn fissure swarm are interpreted as corollaries of rifting episodes initiated in the Bardarbunga central volcano. Volcano-tectonic episodes affect the fissure swarm at an average interval of 100 years. Minor episodes are limited to the central volcano and adjacents parts of the fissure swarm. During the less frequent major episodes, rifting and volcanic activity extends to the extreme southwestern part of the fissure swarm. Seismic monitoring of the Bardarbunga central volcano could provide an early warning of renewed activity on the Veidivotn fissure swarm. A major rifting episode resulting in eruption on its southwestern part can be expected during the next 100 to 300 years.

Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment

On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

The 1991 eruption of Hekla, Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s−1. After this peak, the effusion rate declined rapidly to 10–20 m3 s−1, then more slowly to 1 m3 s−1, and remained at 1–12 m3 s−1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.

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.

Correlation of late Holocene terrestrial and marine tephra markers, north Iceland: implications for reservoir age changes

The tephrochronology of the last 3000 years has been investigated in soil sections in north Iceland and in a marine sediment core from the north Icelandic shelf, 50 km offshore. Tephra markers, identified with major element geochemical analysis of volcanic glass shards, serve to correlate the marine and terrestrial records. Hekla 3, the largest Holocene tephra marker from the volcano Hekla, in south Iceland, dated to 2980 years BP, is used as the basal unit in the tephra stratigraphy. AMS 14C dating of molluscs in the sediment core shows variable deviation from the tephrochronological age model, indicating that the reservoir age of the seawater mass at the coring site has varied with time. A standard marine reservoir correction of 400 14C years appears to be reasonable at the present day in the coastal and shelf waters around Iceland, which are dominated by the Irminger Current. However, values over 500 years are observed during the last 3000 years. We suggest that the intervals with increased and variable marine reservoir correction reflect incursions of Arctic water masses derived from the East Greenland Current to the area north of Iceland.

Evidence of the Eldgjá (Iceland) eruption in the GISP2 Greenland ice core: relationship to eruption processes and climatic conditions in the tenth century:

Glaciochemical studies and the evaluation of tephra in the GISP2 ice core provide information on the characteristics and potential environmental and climatic effects of the mid- to late AD 930s voluminous fissure eruption of Eldgjá, Iceland. The similarity in the chemical composition of basaltic glass shards found in a section of core dated at AD 938 ± 4 compared to proximal glass from the Eldgjá eruption verifies the presence of Eldgjá debris. A dacitic glass present in the same layer probably originated from Eldgjá as well, in which case Eldgjá was the primary contributor of sulphur-rich aerosols to the atmosphere in the late AD 930s. We cannot completely exclude the possibility that another explosive eruption in the AD 930s produced this dacitic glass. Estimated maximum stratospheric loading is 100 X 1012 g H 2SO4 over a 3-6 year period following the eruption, but loading could be as low as half of that value. A search of historical and proxy records for the late AD 930s to early 940s fail to show a consistent period of climatic cooling, especially considering the lack of an absolute date for the Eldgjá eruption. This inconsistent response is similar to that observed after the equally voluminous AD 1783 fissure eruption of Laki, Iceland, using the same proxy data sets. However, a marked drop in surface temperatures in the Northern Hemisphere follows the Laki eruption.

Provenance of basaltic tephra from Vatnajökull subglacial volcanoes, Iceland, as determined by major- and trace-element analyses:

The Holocene eruption history of subglacial volcanoes in Iceland is largely recorded by their tephra deposits. The numerous basaltic tephra offer the possibility to make the tephrochronology in the North Atlantic area more detailed and, therefore, more useful as a tool not only in volcanology but also in environmental and archaeological studies. The source of a tephra is established by mapping its distribution or inferred via compositional fingerprinting, mainly based on major-element analyses. In order to improve the provenance determinations for basaltic tephra produced at Grímsvötn, Bárdarbunga and Kverkfjöll volcanic systems in Iceland, 921 samples from soil profiles around the Vatnajökull ice-cap were analysed for major-element concentrations by electron probe microanalysis. These samples are shown to represent 747 primary tephra units. The tephra erupted within each of these volcanic system has similar chemical characteristics. The major-element results fall into three distinctive compositional groups, all of which show regular decrease of MgO with increasing K2O concentrations. The new analyses presented here considerably improve the compositional distinction between products of the three volcanic systems. Nevertheless, slight overlap of the compositional groups for each system still remains. In situ trace-element analyses by laser-ablation-inductively-coupled-plasma-mass-spectrometry were applied for better provenance identification for those tephra having similar major-element composition. Three trace-element ratios, Rb/Y, La/Yb and Sr/Th, proved particularly useful. Significantly higher La/Yb distinguishes the Grímsvötn basalts from those of Bárdarbunga and Rb/Y values differentiate the basalts of Grímsvötn and Kverkfjöll. Additionally, the products of Bárdarbunga, Grímsvötn and Kverkfjöll form distinct compositional fields on a Sr/Th versus Th plot. Taken together, the combined use of major- and trace-element analyses in delineating the provenance of basaltic tephra having similar major-element composition significantly improves the Holocene tephra record as well as the potential for correlations with tephra from outside Iceland.

The Recent Sedimentation History of Thingvallavatn, Iceland

The sediments in the Thingvellir lake basin have been successfully dated by tephra layers back to ca. AD 900, the time of Nordic Settlement in Iceland. Nine of the twelve tephra layers known to have been deposited within or near the Thingvallavatn area since that time have been identified in sediment cores, providing an excellent source of information on the rate of sedimentation in the lake basin. The rate of net sedimentation increases proportionally with water depth below a critical limit of 30 m water depth, indicating a hydrologically controlled sedimentation