It is a common misconception that eruptions take place in Iceland every 4 to 5 years. That is the average number of years between eruptions. The actual picture of the eruption rate in Iceland is far from this simple. Eruption activity in Iceland happens in periods of 50 to 80 years. With long quiet periods between them that last up to 50 to 80 years. The period of high activity and low activity are about equally long according to scientific data. The longest period of quiet took place in the year 1720 to 1880, that is a quiet period of 160 years. The shortest period of quiet took place 1620 to 1720, this is a period of 100 years. The rate of eruptions during the active areas is 4 to 11 eruptions over a period of 40 years, but during the quiet period that number is 0 – 4 eruptions over a period of 40 years. In every case but one the eruptions where in Grímsfjall volcano.
Last peak in eruptions in Iceland took place in the years 1880 to 1900. Last eruption low took place in the year 1960 in Iceland. Before that this low in activity took place in the year 1820. The reason for this behaviour is that activity in the rift zone happens in jumps. It is not even activity with long quiet periods between them. [Speculation] If I use basic maths to figure out when next peak in activity is going to start, it puts it down in the year 2020. If that is going to happen is something that remains to be seen.
Reference material in Icelandic
Sigurður Steinþórsson. „Er eldvirkni á Íslandi sveiflukennd?“. Vísindavefurinn 19.7.2000. http://visindavefur.is/?id=672. (Skoðað 29.5.2014).
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