Here is some info on the most significant volcanoes in Iceland:
1. The 1783 eruption of Laki (also known as Skaftáreldar) is considered to be the most significant eruption in recorded history. The Eruption lasted 8 months, ejected over 42 billion tonns of Lava, the estimated death toll was over 2 million (with Iceland losing over 20% of its population, roughly 10,000 people, due to plague and famine).
Experts have even said that it most likely was partly responsible for the french revolution in that 120 million tons of sulfur dioxide were emitted into the atmosphere which blocked the suns heat to a great extent and caused intolerable food shortages.