IMAGINE living over a giant volcano, watching the ground move by metres and not knowing if the Earth is about to explode. That鈥檚 what life is like for two million people living in Campi Flegrei, Italy, who built their homes in an ancient volcanic crater. But now a simple monitoring programme may show which rumblings are innocent, and when people should run for it.
Craters or 鈥渃alderas鈥� such as Campi Flegrei were formed by huge supervolcanoes many thousands of years ago. Some, like those at Yellowstone National Park and Long Valley ski resort in the US, and Campi Flegrei itself, still have molten rock bubbling beneath them. If one erupts again, the explosion could spew a layer of ash metres deep, causing mass destruction and changing the Earth鈥檚 climate. Such massive eruptions are so rare that calderas are often ignored in favour of smaller but more active volcanoes like Mount Etna. But they are more likely than a catastrophic asteroid impact (快猫短视频, 20 July, p 23).
And the potential for smaller eruptions alone makes Campi Flegrei one of the most dangerous volcanoes in Italy. It last erupted in 1538, but bouts of movement in the 1970s and 1980s caused serious concern. 鈥淭he ground went up 1.5 metres in one-and-a-half years,鈥� says Dave Hill, chief scientist for the Long Valley caldera, who was in Italy during an evacuation in 1983. 鈥淚t was fairly unnerving.鈥�
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Now Hazel Rymer, Joachim Gottsmann and their colleagues at the Open University in Milton Keynes propose that by monitoring tiny changes in gravity and ground height within the caldera, they can understand what鈥檚 going on below. If the ground is rising and the changes in gravity indicate that its density is decreasing, for example, the magma below is probably filling with gassy bubbles that could build up pressure and trigger an explosion. And when the ground is falling and density is on the rise, a subterranean chamber is probably collapsing as magma flows out of it, fracturing the surrounding rock. Magma can then escape through the fractures in an eruption.
By mapping out the changes that would occur in these situations, the team developed a graph that has 鈥渄anger zones鈥�, alongside areas of relative stability (see Graphic).
For most mountainous volcanoes, plotting data collected from around a cone results in a messy scatter of points on the graph, giving no clue to whether an eruption is imminent. With calderas, however, Rymer and Gottsmann found that the data generally forms a straight line, so it鈥檚 easy to tell which zone it falls into (Bulletin of Volcanology, vol 64, p 338 and Journal of Volcanology and Geothermal Research, vol 113, p 379).
The team says that watching the line should give years of warning. 鈥淚t tells you way before any other precursors whether dangerous activity is on the way,鈥� says Gottsmann. That can help city officials plan well ahead. If all the data lands in a bad spot, Rymer says, 鈥淚 would be very concerned that an eruption was imminent 鈥� well within a decade.鈥�
Giovanna Berrino from the National Institute of Geophysics and Volcanology in Naples, who has been working with the team to analyse 20 years of Campi Flegrei data, says she鈥檚 impressed by the technique. The researchers found that during the 1983 evacuation, the crater was actually in a stable zone. 鈥淚t鈥檚 a very interesting and useful approach. We鈥檙e continuing with this research,鈥� says Berrino.