The year 1783 is something that is going to live in historical memory of the Icelandic people for long time to come. The historical memory of Icelanders still remember the eruption of Laki that took place 229 years ago. This eruption is among Iceland biggest eruption in historical times and since Iceland got populated around the year 600 (or around that time, according to new data). It is unclear when the exact eruption started in Grímsfjall volcano and in Þórðarhyrna volcano (a smaller volcano south-west of Grímsfjall volcano, it is inside Grímsfjall volcano fissure swarm). But what is known is that this eruption started sometimes in May.
The eruption started in Grímsfjall volcano it self, it is unclear where exactly it started. Creating a glacier flood that did go down Þjórsá that same month in some part. But was documented that Þjórsá was darker and had worse smell then normally and had more water in it. It was also documented that Skaftá did have more water in it and was darker and had bad smell early spring in Iceland (that means late April or early May). But the glacier flood that must have taken place after an eruption started are not well documented in history best to my knowledge. From what I can read it seems that the eruption of Grímsfjall volcano started in the main volcano (unknown where exactly). The years 1755 and to the year 1785 seems to have been extremely active in terms of volcano activity in Iceland. But that year there was also an eruption on the Reykjanes Ridge that created a short lived Island. It did however only last few months at the best. It is possible that the eruption on the Reykjanes Ridge was in the area of Eldey or close to it. But it is impossible to confirm it for sure.
It is not known when the actual eruption started in Grímsfjall volcano or in Þórðarhyrna volcano. But it has been assumed from what I can gather that Þrórðarhyrna volcano was also erupting during this eruption. But the reason for that might well be due to dike intrusions from Grímsfjall volcano might have started an eruption in Þórðarhyrna volcano. But best to my knowledge, there have not been many eruptions in Þórðarhyrna volcano since the eruption in the year 1783 (or around that time).
According to documented history, the fissure eruption started around 8. June 1783. This fissure eruption had growing earthquake swarms before it took place. This was due (but they did not know this at the time for obvious reasons). When the eruption started, it started with a large ash cloud that did go down Skaftárhrepp and nearby area. Following this ash cloud was a lot of gas pushing up with the erupted material. But blue haze was reported in historical documents about this eruption. The problem with the carbon-monoxide did not get any better . Given the year this did happen, there are holes in the event as they did happen. But by 12th of June 1783 the lava had already filled several canons at least and had already destroyed a lot of farmland and farms. But it is documented that lava bombs where known to have been found up to 112 km distance from the eruption. The total fissure length is around 25 km long. It is among Iceland longest eruption fissure in historical times. But Askja volcano had an fissure eruption that was around 20 km long in its eruption episode around the year 1875 to the year 1961, when it took a break that is continuing to this day.
Based on historical documents. It seems that this eruption had two phases. The the first one started with an eruption in the south part of what is now the eruption fissure of Laki. The second one was when eruption craters did open up north part of the fissure after several months of eruption on the south part of the eruption fissure. But by that time the eruption had already diminished in strength at the same time, or before that. It is hard to know from historical documentation what actually did happen at this time. But the eruption is believed to have stopped on 26 May 1785.
Many of the eruption craters did make ash, not lava. The ash layers from those craters where up to 2 meters thick at the distance of 500 meters in some areas. It is also believed that the eruption strokes from many of the craters where up to 800 to 1400 meters high when they where at the tallest. During this eruption there was a sour rain, along with other deadly effects that did follow this eruption. It did also happen during this eruption that many animals did die due to ash poisoning, and gas poisoning from the eruption. Just as humans did at this time. But at this time Iceland did not have any functional government that was able to handle this type of natural disaster. The amount of volcanic gases also had effects around northern hemisphere. But this effects did create huge problems in Europe, Canada, U.S and in Russia from best of my knowledge (it might however not be correct).
Many people might believe that after such a big eruption Grímsfjall volcano would stop for a long time. This was not the case. Only 22 years after the major eruption of the year 1783 to 1785 Grímsfjall volcano had an new eruption. This was smaller, only having the VEI scale of 2. But Laki eruption has the VEI scale of 4+. Before the Laki eruption, there had been an other eruption in the year 1774, but it was an minor VEI=2 eruption. Last eruption in Þórðarhyrna and Grímsfjall volcanoes was in the year 1902. That was an VEI=4 eruption that lasted from December 1902 and until January 1904.
For this reasons I classify Grímsfjall volcano (including also Þórðarhyrna volcano) one of the dangerous volcanoes in Iceland. I do think it might be even be more dangerous the Katla volcano all considered. But this is just my personal opinion and nothing else.
Based on following references
Skaftáreldar 1783 (Icelandic)
The eruption that changed Iceland forever (BBC News)
Blog post updated at 01:49 UTC on 22.03.2012.