Since the settlement, Eyjafjallajökull has erupted four times: first around 920, then in 1612, 1821 and now in 2010. The Manuscript Collection of the National and University Library of Iceland holds a description of the eruption of 1821. It is in the manuscript ÍB 11 fol, a draft history of Iceland’s volcanic eruptions compiled by the poet and naturalist Jónas Hallgrímsson in 1840–1845. For this compilation Jónas drew on various annals.
Of the eruption of 1821 it says:
“This eruption began as follows: at dusk and in the evening of Wednesday 19 December 1821, flashes were often seen in clear weather, and the next day at about mid-afternoon a small whitish cloud was seen at the top of Eyjafjallajökull; in an instant it rose high into the air and became a thick column of smoke that grew darker as time went on. The weather was clear and calm. At sunset the smoke disappeared, but soon afterwards an even greater column rose from the glacier, with flashes and fiery sparks high in it, so that people were sure this was a real eruption.
From the 21st to the 27th the column rose fiercely in a north-easterly wind and spread south and west of the mountain; the glacier there darkened with the ashfall, but the column grew ever larger and spread through the sky, and there was heavy ashfall under Ytri-Eyjafjöll and in Austur-Landeyjar. Now and then fire was seen on the ground below the column, and in the first days the lakes and rivers rose somewhat, and west along Eyjafjallajökull fearful booms were heard in it, as if it might burst forth at any moment. The weather was warm all those days. The ashfall eased soon after the New Year of 1822 and never reached people except in the districts named above.
After that smoke rose from the fissure all the time and the cracking in the glacier went on now and then, but fire or real eruptions were seldom seen until towards Midsummer, when the ashfall began again; it was very light but long-lasting, and did most damage in the middle districts under Austur-Eyjafjöll. People did not abandon their farms for good, though some went away with their livestock for a while, and the animals that stayed behind gave very little and did poorly on the hay the following winter. Those districts were so hard pressed that they came close to destitution, and other districts had to help them with gifts. The ash sifted down from the glacier now and then until the end of the year, and a whitish steam was still rising from it more than two years later.
District officer Magnús Sigurðsson went up onto the glacier with two companions in the spring of 1823 and examined the fissure. It ran from north to south where it was said to be before, and seemed to them about 1,000 fathoms long and 30 fathoms wide at its narrowest, a bottomless rocky gorge to look at. In three other places, at some distance from each other south-west of the fissure, smoke was then rising from the glacier through holes.”
(The Icelandic original is quoted in modern spelling)
You can view Jónas’s manuscript here on handrit.is.
It is worth mentioning that Chief Justice Magnús Stephensen wrote about the eruption in his journal Klausturpósturinn in 1822, which can be read on the Library’s website www.timarit.is.