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http://www.newscientist.com/article/dn18786-get-ready-for-decades-of-icelandic-fireworks.html

In 1998 Gudr�n Larsen from the University of Iceland in Reykjavik and colleagues used 800 years' worth of data from lava layers, ice cores and historical records to show that Iceland's volcanism goes through cycles of high and low activity. The peaks of these cycles seem to be strongly linked to bursts of earthquakes, which release the build-up of strain on tectonic faults near Iceland caused by the rifting of the Atlantic Ocean.

In addition, the periodicity may be linked to pulses of magma coming from the mantle and pressure fluctuations at the surface caused by glaciers melting and geothermal activity.

Larsen and colleagues showed that the Vatnaj�kull ice cap region � which includes the highly active Gr�msv�tn and B�rdarbunga volcanoes � experienced between 6 and 11 eruptions every 40 years during phases of high activity, compared with no more than three eruptions per 40 years during low-activity phases. Other regions of Iceland appear to follow a similar pattern to Vatnaj�kull.

As well as becoming more frequent, eruptions seem to get more intense during the high-activity phases. A number of Iceland's most devastating eruptions � including that of the volcano Laki in 1783 that killed over half of Iceland's livestock and led to a famine that wiped out about a quarter of the human population � have occurred when the Atlantic rift system has been active. "If we are entering a more active phase, these bigger eruptions will become more likely," says Thordarson.


Judging by recent volcanic and earthquake activity, Thordarson and his colleagues believe that Iceland is entering its next active phase and estimate it will last for 60 years or so, peaking between 2030 and 2040.


http://www2.norvol.hi.is/Apps/WebObjects/HI.woa/swdocument/1015806/Eyjafjallajokull_Katla_west_hazard+assessment_summary_2006.pdf
-- Eyjafjallaj�kull - ice covered stratovolcano - Eruptions once every few hundred years - Results - Hazard assessment