Eyjafjallajökull last erupted between 1821–1823. There were also documented intrusion events in 1994 and 1999 but magma didn't reach the surface.
In the weeks prior to the eruption intense seismicity and high rates of deformation associated with the rise of magma beneath the volcano were noted by Icelandic scientists.
The eruption began at 2352 GMT on 20 March 2010 on Fimmvörðuháls, an ice-free area located between the Eyjafjallajökull and Mýrdalsjökull ice caps. The eruption was impressive but impacts were only local.
The second phase of volcanism from 14 April 2010 was located in the ice-filled summit caldera, it was explosive and generated a large ash cloud.
It is the change in style and environment of the eruption, combined with wind directions that caused the impact to the UK.
The first phase of the eruption was characterised by fire-fountains and fissure-fed lava flows, principally active from 20 March–12 April. The eruption occurred from a ~500 m long NE-SW oriented fissure with fire fountains from 10–12 vents along this fissure reaching up to ~100 m height.
Only minor amounts of ash were produced by this eruptive phase (due to the absence of ice/water) and fell within a few km of the eruption site. At the close of this phase of activity, lava flows covered an area of ~1.3 square kilometres, with an estimated average thickness of 10–20 m.
Fissures are elongate fractures or cracks in the crust along which vents may open up. Magma can erupt through the vents as a spray known as a fire-fountain. This spray consists of fragments of lava that may remain largely fluid when they hit the ground, known as spatter.
The spatter builds up along the sides of the fissure as banks. When the eruption rate is high and the spatter is hot, the molten spatter may coagulate to form a lava flow. This was observed on Fimmvörðuháls this year and is a common type of eruption in Hawaii and Iceland.
Fire-fountains are also suggested to have formed many of the extensive basalt provinces on Earth such as the Deccan Traps in India where extremely long lava flows were emplaced.
BGS are currently conducting research in the Afar Depression (Ethiopia), which is a constructive plate boundary like Iceland and is also currently experiencing episodic fire-fountain eruptions.
A swarm of earthquakes on the night of the 13 April preceded this eruptive phase. The eruption itself was accompanied by seismic tremor that is still ongoing.
The second phase of volcanism on Eyjafjallajökull started at around midnight on 14 April. It was more explosive and located in the ice-filled summit caldera. The explosions generated abundant fine particles of volcanic ash that rose to a height of about 8 km (not 11 km as previously reported). The total volume of ash (tephra) in the first three days was about 140 million m3. This can be calculated as an average magma discharge rate of about 300 m3 per second and represents 10-20 times the rate of the first eruptive episode at Fimmvörðuháls (reported by Thorvaldur Thordarson, Guðrún Larsen and Ármann Höskuldsson, The Institute of Earth Sciences, Iceland).
The second eruptive phase is described by volcanologists as ‘phreatomagmatic’, caused by the interaction of water and magma. In this case, the water comes from melting of the ice above the vents. At Eyjafjallajokull, as well as driving explosions, the meltwater caused large floods to the north and south of the volcano. So far, this is a small eruption that produces a steam-rich plume of fine ash particles that rise into the atmosphere along with volcanic gases.
Researchers at the University of Iceland suggest that the relationship between these two phases of volcanism can be explained by magma movement in the earth’s crust — see preliminary interpretations of chemical analysis of tephra. Primitive basalt from deep in the crust flowed up to the surface causing the 20 March 2010 eruptive phase at Fimmvörðuháls. The rising basalt magma (~1200°C) also found a route towards the central summit part of the volcano where it mobilised an existing cooler and more viscous body of magma. Eruption of this more evolved magma into the ice-filled summit caldera resulted in the explosive second phase of the eruption.
By 19 April, only one vent was active and much of the ice around it had melted. The height of the ash plume had reduced to around 4 km, and researchers on observation flights observed spatter around the margins of the crater, suggesting that the absence of water had led to a further change in the eruptive style of the volcano.
Contact Dr Sue Loughlin for further information.