Eyjafjallajökull (Icelandic: [ˈeiːjaˌfjatl̥aˌjœːkʏtl̥] ; "glacier of (the mountain) Eyjafjöll"), sometimes referred to by the numeronym E15, is one of the smaller ice caps of Iceland, north of Skógar and west of Mýrdalsjökull. The ice cap covers the caldera of a volcano with a summit elevation of 1,651 metres (5,417 ft). The volcano has erupted relatively frequently since the Last Glacial Period, most recently in 2010, when, although relatively small for a volcanic eruption, it caused enormous disruption to air travel across northern and western Europe for a week.
Eyjafjallajökull consists of a volcano completely covered by an ice cap. The retreating ice cap covered an area of 66 km2 (25 sq mi) in 2019, but was previously more than 80 square kilometres (30 square miles), with many outlet glaciers. The main outlet glaciers are to the north: Gígjökull, flowing into Lónið, and Steinsholtsjökull, flowing into Steinsholtslón. In 1967, there was a massive landslide on the Steinsholtsjökull glacial tongue. On 16 January 1967 at 13:47:55 there was an explosion on the glacier. It can be timed because the seismometers at Kirkjubæjarklaustur monitored the movement. When about 15,000,000 cubic metres (530,000,000 cubic feet) of material hit the glacier a massive amount of air, ice, and water began to move out from under the glacier into the lagoon at the foot of the glacier.
The mountain itself, a stratovolcano, stands 1,651 metres (5,417 ft) at its highest point, and has a crater three to four kilometres (2 to 2+1⁄2 miles) in diameter, open to the north. The crater rim has three main peaks (clockwise from the north-east): Guðnasteinn ([ˈkvʏðnaˌsteitn̥]), 1,500 m (4,900 ft); Hámundur ([ˈhauːˌmʏntʏr̥]), 1,651 m (5,417 ft); and Goðasteinn ([ˈkɔːðaˌsteitn̥]), 1,497 m (4,911 ft). The south face of the mountain was once part of Iceland's coastline, from which, over thousands of years, the sea has retreated some 5 km (3 mi). The former coastline now consists of sheer cliffs with many waterfalls, of which the best known is Skógafoss. In strong winds, the water of the smaller falls can even be blown up the mountain. The area between the mountain and the present coast is a relatively flat strand, 2–5 km (1–3 mi) wide.
The name means "glacier of Eyjafjöll" (or more properly here "ice cap"). Eyjafjöll is the name of the southern side of the volcanic massif together with the small mountains which form the foot of the volcano. The word jökull [ˈjœːkʏtl̥], meaning glacier or ice cap, is a cognate with the Middle English word ikil surviving in the -icle of English icicle.
The name Eyjafjöll is made up of the words eyja [ˈeiːja] (genitive plural of ey, meaning eyot or island), and the plural word fjöll [ˈfjœtl̥], meaning fells or mountains, and together literally means: "the mountains of the islands". The name probably refers to the close by archipelago of Vestmannaeyjar.
The word fjalla [ˈfjatla] is the genitive plural of fjöll, and so Eyjafjalla [ˈeiːjaˌfjatla] is the genitive form of Eyjafjöll and means: "of the Eyjafjöll".
A literal part-by-part translation of Eyjafjallajökull would thus be "Islands' Mountains' glacier".
Hence the southern slopes of the mountain Eyjafjöll result in the sea side strip of land beyond being called Undir Eyjafjöllum.
The stratovolcano, whose vents follow an east–west trend, is composed of basalt to andesite lavas. Most of its historical eruptions have been explosive. However, fissure vents occur on both (mainly the west) sides of the volcano. The volcano is 800,000 years old.
The volcano is fed by a magma chamber under the mountain, which in turn derives from the tectonic divergence of the Mid-Atlantic Ridge. It is part of a chain of volcanoes stretching across Iceland. Its nearest active neighbours are Katla, to the northeast, and Eldfell, on Heimaey, to the southwest.
The volcano is thought to be related to Katla geologically, in that eruptions of Eyjafjallajökull have generally been followed by eruptions of Katla.
Eyjafjallajökull erupted in the years 920, 1612, 1821, and 2010. The Skerin Ridge eruption in 920 was a VEI 3 radial fissure eruption while the subsequent 1612 and 1821 eruptions were VEI 2 small summit eruptions. In the case of the 1821 eruption, a short explosive phase in December 1821 was followed by a year of intermittent explosive to effusive activity.
Some damage was caused by a minor eruption in 1821. Notably, the ash released from the eruption contained a large fraction of fluoride, which in high doses may damage the bone structure of cattle, horses, sheep and humans. The eruption also caused some small and medium glacier runs (jökulhlaups) and flooding in nearby rivers Markarfljót and Hólsá [ˈhɔl̥(t)sˌauː]. The eruptive phase started on 19 and 20 December 1821 by a series of explosive eruptions and continued over the next several days. The sources describe heavy ash fall in the area around the volcano, especially to the south and west.
After that event the sequence of eruptions continued on a more subdued level until June 1822.
From the end of June until the beginning of August 1822, another sequence of explosive eruptions followed. The eruption columns were shot to considerable heights, with ashfall in both the far north of the country, in Eyjafjörður, and in the southwest, on the peninsula of Seltjarnarnes near Reykjavík.
The period from August to December 1822 seemed quieter, but farmers attributed the death of cattle and sheep in the Eyjafjörður area to poisoning from this eruption, which modern analysis identifies as fluoride poisoning. Some small glacier runs occurred in the river Hólsá. A bigger one flooded the plains near the river Markarfljót. (The sources do not indicate the exact date.).
In 1823, some men went hiking up on Eyjafjallajökull to inspect the craters. They discovered a fissure vent near the summit caldera a bit to the west of Guðnasteinn.
In early 1823, the nearby volcano Katla under the Mýrdalsjökull ice cap erupted and at the same time steam columns were seen on the summit of Eyjafjallajökull.