Sicily’s Mount Etna erupted this week, spewing lava and sending up plumes of ash.
But while dramatic, it was not on the scale of the eruption on Christmas Eve, December 2018.
Scientists who have been studying that eruption now believe signs were there for more than seven months that the lid was about to blow.
In the immediate aftermath, Italy's National Institute for Geophysics and Vulcanology (INGV) counted more than 130 seismic shocks in the zone, with the strongest reaching a magnitude of 4.0.
After studying the eruption, they found it began with a long prelude stage that started in 2017, in which magma accumulated deep in the volcano’s reservoir and began degassing, leading to a pressure build-up six to seven months before the eruption.
Antonio Paonita, of Rome’s National Institute of Geophysics and Volcanology and colleagues determined that Mount Etna was” extraordinarily over-pressurised” in the weeks leading up to that eventful Christmas Eve.
Writing in the journal Science Advances, the researchers said they investigated the lead-up to the eruption by comparing estimated magma recharge deep beneath the surface with measurements of volcanic degassing, using a procedure that may help scientists predict future volcanic eruptions.
Previous research has suggested that the eruption of volcanoes may be driven by the build-up of gases in magma chambers long before an eruption occurs.
With its frequent eruptions and established monitoring networks, Mount Etna offers an opportunity to explore this phenomenon.
To understand the pressure build-up that led to the eruption in December of 2018, Mr Paonita’s team analysed data collected by continuous monitoring networks installed on the volcano and sampled from specific sites between January 2017 and February 2019. The researchers looked at changes in helium isotope ratios, carbon dioxide, sulphur dioxide, and hydrochloric acid.
They found activity in gas vent sites began a long, upward trend as early as the first half of 2017.
Etna erupted once more this week.
The activity caused ash and lapillus to fall on some villages on the slopes of Europe’s largest active volcano, according to the Catania Institute for Geophysics and Volcanology.
There was no impact on the operations of the nearby Catania international airport.
Etna is the largest of Italy’s three active volcanoes. The others are Stromboli, on the Sicilian island of the same name, and Mount Vesuvius near Naples, which last erupted in 1944.
Before this week, Etna’s last eruption was on August 10.