The final stage before a big bang?

November 20, 2018

The Phlegraean Fields near the bustling metropolis of Naples is one of the world's most active and volatile volcanic regions. Its calderas were formed by enormous eruptions that took place 39,000 and 15,000 years ago, in addition to countless minor eruptions. Smaller volcanoes also erupted repeatedly during the period in between. The Phlegraean Fields have become more active again in recent years.

In an article in the journal Science Advances, a team of vulcanologists, led by ETH Zurich's Francesca Forni and Olivier Bachmann, argues that the Phlegraean Fields are subject to caldera cycles. It concludes that the volcanic area is at the early stage of a new cycle, which could culminate in another gigantic eruption.

A cycle begins with the accumulation of magma in a large reservoir in the Earth's crust, a process that takes millennia. This stage is characterised by long periods of dormancy and small eruptions of differentiated magma. A further injection of magma into the magma chamber triggers an enormous eruption. The reservoir is emptied abruptly, the roof collapses, and a new caldera is formed - the cycle begins anew.

Minerals as data loggers

Rock samples from 23 earlier eruptions at the Phlegraean Fields provided the researchers with evidence of the start of a new cycle. In particular, rock material from Monte Nuovo, which last erupted in 1538, resembles in its composition rock ejected just before the last two major eruptions.

The researchers took advantage of the fact that the chemical composition of minerals from magmatic rock stores information on the conditions under which it originates. A comparison of the chemical signatures of rock from different eras enables vulcanologists to reconstruct the conditions in the crust at the time of its formation. This allows them to determine the current stage of the magma system. The vulcanologists also created a model of the cycle.

"The importance of this study is that we can reconstruct from past eruptions the rhythm that super volcanoes have, and hopefully predict where they stand in their cycle. Here of course, in the Phlegraean Fields, but doable in principle in any super volcanoes on this planet," explains ETH Professor Bachmann.

Despite these advances, the researchers cannot predict when the next major eruption at the Phlegraean Fields may occur. But former ETH doctoral student Forni is certain: "We do not have to worry about a catastrophic eruption in the next 20,000 years. The magma reservoir underneath the Phlegraean Fields fills up only very slowly. We, the next generations and perhaps the entire human race will not be here to witness a massive eruption.

Nonetheless, it is important to continue to monitor the evolution of the Phlegraean Fields on a sustained basis. Forni warns that even a small eruption, which can occur during the early stages of a cycle, would wreak havoc on the region. Early warning signs of an impending eruption of a magma chamber include land elevation and changes in the composition of the gases emitted by the Phlegraean Fields.

A massive volcanic eruption would be devastating not only for the Naples region but the entire world. Supervolcanoes have caused short-term climate catastrophes, crop failure and famines in the past. The eruption of the Indonesian supervolcano Tambora in 1815 is a well-documented example: the following summer was dubbed the "year without a summer", and even Switzerland suffered crop failure.

ETH Zurich

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