A great deal of publicity surrounded the ash clouds from Eyjafjallajökull in April and May 2010, and Grímsvötn in May 2011, which caused disruption to European air traffic. In this article we review Iceland in a geological context, including the types of volcanoes and volcanic activity, and what historical data can reveal about the probability of future eruptions assuming that the future will be similar.
Iceland's volcanism can be attributed to its location on the Mid Atlantic Ridge in the North Atlantic Ocean, where the Eurasian and North American plates are moving apart a few centimeters per year. In Iceland, this produces volcanic rift zones, regions where the Earth's crust is being pulled apart and fractured, and here molten rock, or magma, rises up, and some reaches the surface and erupts as lava and/or ash. Eruptions can occur from central summit vents or flank vents of a volcano, or from linear 'fissures', metres to kilometers in length, which develop parallel to the rift zone. Iceland is also widely considered to be underlain by a 'mantle plume', a hot zone in which there is increased melting of rock in the earth's mantle. Such a 'hotspot' causes enhanced volcanic activity in addition to that already occurring due to the spreading movement of the plates. The island's geological setting, as well as climatological circumstances, results in unusually diverse styles of volcanism in the region (Thordarson and Larsen, 2006), for example, some volcanoes are beneath the sea (submarine) and some are beneath ice caps (subglacial).
Detailed investigations of the eruptive history of Icelandic volcanoes can reveal important clues as to the probability of future eruptive episodes and the likelihood of these producing significant ash. Data from field studies of ice cores and terrestrial soils, and written records where available, have been used by scientists to try to determine the frequency, size and type of volcanic eruptions over time. A study of the last 11 centuries reveals over 200 eruptions, with around three quarters of these explosive, and with an average frequency of 20-25 events per 100 years (Thordarson and Larsen, 2006). The apparent increase in eruption frequency over the last few centuries can be accounted for by improved documentation of eruptive events. Studies of longer timescales e.g. the last 10,000 years since the last ice age, suggest similar eruption rates to historic times.
Icelandic volcanic eruptions in historic time (Thordarson and Larsen, 2006). Enlarge
There are around 30 volcanic systems in Iceland, but much of the volcanic activity originates from several frequently erupting volcanoes, which include Grímsvötn, Hekla and Katla. Some eruptions are large enough to result in widespread ashfall, and ash from Icelandic volcanoes has been found in peat bogs and sediments in Scotland and other parts of northern Europe (Swindles et. al., 2011).
Rarely, on a timescale of perhaps hundreds of years, Iceland is the source of some very large and long lasting volcanic eruptions. The well documented Laki flood lava eruption of 1783-84 lasted 8 months and had catastrophic consequences locally in Iceland. Also, sulphur dioxide was emitted in such large quantities as to produce far reaching effects, with a sulphuric haze pervading for months over large parts of Europe. Health, environmental and climatological impacts, as well as the risk of major disruption to air traffic, could be expected in many regions of the northern hemisphere in the event of a future eruption of similar or larger magnitude.
Eruptions can be effusive (producing mainly lava), explosive (producing mainly tephra), or mixed. Tephra is a term used to describe the solid fragments of magma ejected explosively from a volcano, and is categorised by size, with fragments less than 2 mm in diameter termed ash. Explosive eruptions may occur if the magma is viscous (has high total silica content) or where any composition of magma may interact with water (e.g. an ice-capped volcano). In the latter, magma coming into contact with meltwater from glacial ice leads to a type of explosive activity known as hydromagmatic, and certain phases of the Eyjafjallajökull 2010 eruption were attributed to this type of interaction. Due to the high level of fragmentation of material in hydromagmatic eruptions, the ash clouds they produce may contain a significant proportion of fine ash, increasing the likelihood of long-distance transportation in the atmosphere.
The eruptions of Eyjafjallajökull and Grímsvötn in 2010 and 2011 respectively, are unlikely to be considered out of the ordinary in light of the historical record. It could be argued that up until recently, we have been fortunate: for example during the Grímsvötn eruption of November 2004, the ash cloud was carried eastwards toward Scandinavia, causing only limited air travel disruption. The interval between individual volcanic eruptive episodes is far from predictable, but with a volcano erupting in Iceland on average every 5 years, and with around three-quarters of eruptions producing ash, we need to remain alert to the threat.
Deborah Lee, February 2012
Thanks to Sue Loughlin of British Geological Survey, and John A. Stevenson of the University of Edinburgh for their assistance with resources and reviews. Thanks also to the Icelandic Met Office for supplying the map of volcanoes.
Swindles, G. T., Lawson, I. T., Savov, I. P., Connor, C. B., and G. Plunkett (2011), A 7000 yr perspective on volcanic ash clouds affecting northern Europe, Geology, v. 39, 9, 887-890.
Thordarson, T., and G. Larsen (2006), Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history, Journal of Geodynamics, 43, 118-152.