A massive volcanic eruption helped convert Iceland's Vikings to Christianity more than a thousand years ago, according to new research.
Memories of the largest lava flood in the history of Iceland, recorded in a medieval poem, were used to drive the island's conversion to Christianity, suggests the study.
A team of scientists and historians, led by Cambridge University researchers, has used information contained within ice cores and tree rings to accurately date the massive eruption, which took place soon after the island was first settled.
Having dated the eruption, the researchers found that Iceland's most celebrated medieval poem, which describes the end of the pagan gods and the coming of a new, singular god, describes the eruption and uses memories of it to stimulate the Christianisation of Iceland.
The eruption of the Eldgjá in the 10th Century is known as a lava flood: a rare type of prolonged volcanic eruption in which huge flows of lava engulf the landscape, accompanied by a haze of sulphurous gases.
Iceland specialises in this type of eruption - the last example occurred in 2015, and it affected air quality hundreds of miles away in Ireland.
The Eldgjá lava flood affected southern Iceland within a century of the island's settlement by Vikings and Celts around 874.
But until now the date of the eruption has been uncertain, hindering investigation of its likely impacts.
Scientists say it was a "colossal" event with around 20 cubic kilometres of lava erupted - enough to cover all of England up to the ankles.
The Cambridge-led team pinpointed the date of the eruption using ice core records from Greenland that preserve the volcanic fallout from Eldgjá.
Using the clues contained within the ice cores, they found that the eruption began around the spring of 939 and continued at least through the autumn of 940.
At a glance | Deadliest volcanic eruptions
Study first author Dr Clive Oppenheimer, of Cambridge's Department of Geography, said: "This places the eruption squarely within the experience of the first two or three generations of Iceland's settlers.
"Some of the first wave of migrants to Iceland, brought over as children, may well have witnessed the eruption."
Once they had a date for the eruption, the team then investigated its consequences.
First, a haze of sulphurous dust spread across Europe, recorded as sightings of an exceptionally blood-red and weakened Sun in Irish, German and Italian chronicles from the same period.