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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I have been studying material on magma conduits being interconnected, the material makes sense.
One thing that came to the fore was that over periods of time (as you say) there may be no significant activity,
but the seisomgraphic readings show considerable activity. The interconnected conduit theory is that the conduits fill up the various magma chambers. In some chambers as little as a third may be liquid magma. However as the various chambers reach capacity, then things quiten down, but pressure continues to build. There is a belief that not all data on Yellowstone is reaching the public domain, and that this is because there are fears that a major event will take place. The same fears exist about Iceland, so much so that the UK government has commissioned study http://www.bbc.co.uk/news/science-environment-27239321 which is being carried out by the Met Office.
Thanks for the link to the BBC News. I had missed it. As for the working of a volcano. It is still not properly known what happens inside a volcano when it erupts or before it erupts. It is going to take a good deal longer and a lot more studies until we do know what happens inside a volcano before an eruption is triggered.