Hekla is one of Iceland's largest and most active volcanoes. It has a recorded history of eruptions dating back to 1104, 130 years after the date most commonly given for the initial colonization of Iceland (historians and archaeologists argue about this, but that's another story), and geological evidence of eruptions dating back over seven thousand years. The volcano is thought to have produces about five cubic kilometers of tephra (volcanic ejecta) in the last thousand years; about 10% of all the tephra produced in Iceland during this time. Hekla is unique among Icelandic volcanoes in that it produces calc-alkaline lavas, more typical of subduction zones than the mid-oceanic rift on which Iceland sits. This makes ash beds caused by Hekla's activity distinctive, enabling geologists to say with some confidence that ash layers as far away as Scotland and Scandinavia originate from Hekla.
Mount Hekla.
The volcano takes the form of a stratovolcano (classic volcanic cone) at the end of a volcanic ridge. In major eruptions a fissure up to 5 km long can open along the ridge.
In addition to being calc-alkaline, ash from Mount Helka is unusually rich in fluorine, which has led to a number of mass poisoning incidents associated with ash falls from Hekla. The volcano has also been known to throw lava bombs for over a kilometer, with one in 1947 being recorded as falling 32 km away.
Ash fall deposits associated with Hekla in Europe have been dated to 5050 BC, 2310 BC and 950 BC. In 1846 an eruption caused ash falls in the Faroes, Shetlands and Orkneys that were associated with widespread livestock deaths. In 1947 an eruption caused ashfalls as far away as Helsinki.
The prospect of an eruption from Helka is clearly a worrying prospect, particularly after the recent eruptions from Eyjafyallajokull and Grimsvötn volcanos in Iceland caused major disruption to air traffic in Europe. Hekla has a track record of sudden eruptions, not proceeded by earthquakes.
On the 7th of July 2011 the media contained a number of reports of an imminent eruption on Hekla. Icelandic geophysicists Pall Einarsson of the Faculty of Earth Sciences at the University of Iceland, and Ari Trausti Guđmundsson, a geoscientist and science writer, were quoted as saying that there had been some activity around Mount Hekla and that the volcano was due an eruption.
Unfortunately 'due an eruption' means slightly different things to a journalist and a geologist. To a journalist this implies within the next week, to a geologist, the next decade or so. Einarsson and Guđmundsson found themselves trying to retract the story, and becoming frustrated with the press. Einarsson went as far as telling English language website IceNews that British journalists had disgraced their profession by the way the story had been reported (although he may not have fully realized what else the UK press has been up to lately).
Hekla is situated on, and fed by the Mid-Atlantic ridge. Eruptions on Hekla follow a pattern; when the volcano has been quiet for a long time, pressure will have built up for a long time, leading to a massive eruption. The devastating 1947-8 occurred after a century of inactivity. The last eruption was in 2000, which was not particularly notable - before then Hekla had erupted in 1991, 1981, 1980 and 1971. So it is likely that if Hekla does erupt in the near future then it will not be a major eruption. But that doesn't make a very good story.
See also The Grímsvötn Volcano and Volcanoes on Sciency Thoughts YouTube.
