Some think that it was water vapor, either escaping from within the magma as it depressurized or boiling off from an underground water source encountering rising magma. Others, including this particular research team, think that it was magma pooling at around a depth of 3 kilometers (2 miles), and then spreading out laterally.
As for this new inflation period, the team are confident – based on decades of geochemical analysis of near-boiling water effusing from the ground – that it’s down to the pressurized release of water from superheated fluids circling near the magma chamber. So this time, it’s good news: magma isn’t moving upwards. The team, however, were keen to point out that their interpretation is far from certain.
“Campi Flegrei is still a very volatile place,” lead researchers Roberto Moretti, a professor at The Second University of Naples, told the conference. “We're not in a position to say that everything is well.”
The Phlegraean Fields last erupted in 1538, but this was a relatively calm explosion compared to some of its ancient blasts. The caldera’s formation eruption 40,000 years ago registered as 6 to 7 on the Volcanic Explosivity Index (VEI) – the most powerful types of eruptions possible.
This blast produced between 100 and 500 cubic kilometers (20 to 120 cubic miles) of volcanic debris, some of which made it as far as Greenland. Although not many people were affected by it back then, over a million people are now in the kill zone, making the Phlegraean Fields one of the most dangerous – and consequently, heavily monitored – volcanoes in the world.
The Bay of Naples, as seen from space. A million people are sitting right on top of the dangerous caldera. NASA/ESO