Eyjafjallajokull (pronounced AY-uh-fiat-luh-YEU-koot-luh, with the 'EU' as in liqueur) is a small glacier near the south coast of Iceland which lies on top of the glacial stratovolcano Eyjafjoll. Eyjafjoll is part of Iceland's East Volcanic Zone, which also includes Katla.
The Eyjafjallajokull volcano has erupted 4 previous times in CE 500, 920, 1612, and 1821. Once the Eyjafjallajokull starts erupting, it continues to erupt for up to 2 years. In historic times, ever since Iceland was first inhabited in CE 874, records show that Katla always erupted shortly afterward. However, Katla is not the only current threat. Scientists estimate the probability of a second Icelandic volcano erupting this year at greater than 80%. Eight different Icelandic volcanoes have a greater than 40% chance of erupting during 2010.
Fortunately, in historical times, Katla eruptions which followed Eyjafjallajokull eruptions have not been Katla's strongest eruptions. However, they have still been hot enough to melt the glacial cover and cause jokulhlaups, or glacial flood surges. For Katla and the Eyjafjallajokull volcano, these jokulhlaups have sometimes been over 150 feet deep.
The most recent time the Eyjafjallajokull volcano erupted was in March 1821, although most contemporary accounts begin with the explosive eruption of December 19, 1821. The volcano continued to vent and spew ash until January, 1823. This is the best documented of all 4 historic Eyjafjallajokull volcanic eruptions.
The original March eruption released a cloud of ash and gas. Like most Hekla-type eruptions, the March 1821 eruption was high in fluorides, which are poisonous to livestock. Large numbers of cattle and sheep died of fluoride poisoning due to eating fluorine-contaminated grass during 1821. The current Eyjafjallajokull eruption is also a Hekla-type eruption but is not as high in fluorides, with only 1/3 the concentration of the 1821 eruption.
The March 1821 eruption also melted the glacier, which triggered several jokulhlaups. As the eruption continued, more and more ice melted, causing waves of glacial flooding.
On December 19, 1821, the Eyjafjallajokull volcano exploded through a new crater 5 miles south of Holt, sending the remaining ice around it into the air. Lava bombs and large pieces of ice, some up to 60 feet across, were thrown up to 5 miles away. A strong smell of sulfur also accompanied the eruption. The red glow of the ejected flame was so bright that the people of Holt could "read as perfectly within their houses at night as if it had been day."
On December 20, the ashfall began. Powdered pumice fell like snow on everything for miles around, covering much of Iceland. Between 28% and 40% of the ash layer left behind by the 1821 eruption consisted of silicon dioxide, one of the most common substances on earth. In the current eruption, up to 58% of the ash is silica.
On the ground, silicon dioxide is usually plain sand. When heated to the melting point and then suddenly cooled in the upper atmosphere, it becomes millions of tiny glass shards. This is why ash from explosive eruptions is usually sharp. Some of the ash fell on Iceland, enough to cause local eye problems due to constant irritation. Most of the ash was ejected high into the atmosphere, where it dispersed naturally over time. In 1821, there were no aircraft engines to be damaged by high concentrations of glass shards.
Fortunately for those near the volcano, the conversion of silica to glass kept it from crystallizing. The air was thick with ash, but none of it was the crystalline silica dust which causes silicosis.
Heavy storms struck the area on the 25th, 26th, and 27th of December. These storms may have been caused in part by the volcanic ash and steam. Lightning is often a part of stratovolcanic eruptions, as it has been during the current Eyjafjallajokull volcanic eruption. Although the eruption eased and snow covered the Eyjafjallajokull glacier again, buried vents below the glacier emitted steam throughout the winter. Some of the steam came from evaporating glacial ice. Most of the water vapor was ejected by the volcano. By February 1822, the Eyjafjallajokull volcano was visibly smaller than it had been.
In March 1822, the volcano erupted again, and then yet again from June until August. Each of these eruptions ejected fluorides, steam, lava bombs, and extensive ashfalls which covered most of the country. Livestock continued to fall sick and die. At the time, the causes were not clearly understood. Today, volcanoes such as Eyjafjallajokull are known to release enough fluorides to contaminate outdoor feed and cause skeletal fluorosis in foraging livestock. Icelandic farmers have been warned not to allow their livestock to graze on land downwind from the volcano, even if it is not covered with ash.
During each eruption, new jokulhlaups swelled glacial rivers such as the Holtsa and flooded the countryside. The largest of these jokulhlaups flooded the Markarfljot river flats toward the end of 1822. However, Markarfljot flooding is usually caused by Katla eruptions. Katla finally melted its icecap completely and sent up its own ash cloud in the spring of 1823.
Somewhere around the same time, Eyjafjallajokull went dormant. Its last recorded activity of that eruptive period was in January 1823.