Whether Eyjafjallajökull is a stratovolcano or a shield volcano is controversially discussed in the professional world. What is certain is that the subglacial volcano has a caldera at its summit, which is covered by a 150 - 200 m thick ice shield. The caldera has a diameter of 3 x 4 kilometers and contains several craters.
The formation of the volcano began about 800,000 years ago. 12 eruption cycles of the Eyjafjallajökull are known from the time before Iceland was taken and only 4 eruptions since humans have been on the island. These took place in the years 920, 1612 - 1613 and 1821 - 1823. The last eruption so far took place in spring 2010 and the three previous eruptions were followed by the neighbouring volcano Katla a short time later.
Eyjafjallajökull lies on a NE-SW striking fissure system that forms the eastern volcanic zone of Iceland. The divergence of this fault zone is due to the fact that here the European continental plate is moving away from the North American plate. This creates a spreading zone along which magma can rise from the mantle. The eastern volcanic zone stretches from the Westman Islands, across the Eyjafjallajökull-Katla system and further across the Laki Fault to below the Vatnajökull glacier. There, some other subglacial volcanoes like Barabunga and Grimsvötn were formed.
The volcanic rock of Eyjafjallajökull consists mainly of a transition basalt between tholeiite and alkaline basalt. In addition, andesite, dacite and trachyte were also mined. These rocks indicate that basaltic magma differentiates over long periods of time in the magma chamber.
As different as the rocks are the eruptive processes at the volcano; purely effusive eruptions are known, as well as explosive eruptions and volcanic eruptions that combined both eruption types. Explosive eruptions are predominantly of the volcanic eruption type; however, volcanologists have also discovered evidence of a large plinian eruption in prehistoric times. The 1612 eruption began with a magmatophreatic eruption and ended with strombolian activity.
Eyjafjallajökul eruption in 2010The last eruption in spring 2010 took place in 2 phases. First, an effusive fissure opened on the western flank of the volcano. The fissure was formed on a saddle between Eyjafjallajökull and Myrdalsjökull and interrupted the hiking trail at Fimmvörduhals Pass at 1000 m altitude. The hiking trail connected the Skogar waterfall on the south coast with the valley of Thormörk in the north of the glacier. During this eruption phase red-hot lava fountains and lava flows were produced. Shortly after the end of the first eruption phase, the second phase followed: an explosive eruption of the volcanic type which started from the summit caldera of Eyjafjallajökull. Meltwater increased the explosiveness of the eruption and a huge ash cloud rose up to 9 km high. It was directed south by strong air currents. Already on the second day of the volcanic eruption the airspace over large parts of Europe had to be closed, including Germany.
The ice cover makes it difficult to explore the volcano. Before the volcanic eruption in 2010 relatively little was known about Eyjafjallajökull. Only in the course of the eruption the interest was awakened and modern measuring methods were used. Radar images of the ice-covered caldera were taken from an airplane. Some of the data, such as seismic and LiveCam, are available to the public via the Internet.
A glacier tongue (Gígjökull) starts from the summit caldera and descends to an altitude of 1200 m. Above it, meltwater flows into the Markarfljót river. This river flows near Thorsmörk into the Sander plain on Iceland's south coast. Volcanic eruptions under the ice of the glaciers hold a special danger: a glacier run (Jokulhlaup) can be caused by meltwater that is formed by the heat of the eruption and dams up under the glacier. It usually takes days or weeks for the meltwater to break through at the base of the glacier and flood the sand plains in an enormous flood.