At least 500 people were forced to evacuate as scientists warned that the far more dangerous Katla volcano always erupts after Eyjafjallajokull begins to spew.
Vulcanologists say that Mount Katla could become “enormously powerful” if it awakens from beneath a nearby glacier.
An eruption in the mid-1780s of another powerful Icelandic volcano, Laki, had disastrous consequences downwind in Europe.
Airborne volcanic debris altered weather patterns, causing crop failures and lowering temperatures for years.
Poisonous gas from the eruption killed many people in the British Isles after it was carried across the far North Atlantic by prevailing winds.
Past eruptions of Katla have caused massive flooding in Iceland, and sent house-sized boulders tumbling into valleys and onto roadways.
Photos: Top - Orvar Atli Þorgeirsson
Bottom - Ragnar Axelsson