The volcanic nature of the Campi Flegrei, or Phlegraean Fields, was known already in ancient times. The various hot springs and fumaroles were a popular holiday destination for wealthy Romans and in 79 AD the nearby Mount Vesuvius erupted, destroying the famous city of Pompeii. In 1539 a smaller volcanic eruption formed the 403-foot-high cone of the Monte Nuovo, the new mountain.
Victorian geologist Charles Lyell argued in 1830 that a magma chamber deep underground would not only explain the active volcanoes, but also a strange phenomenon he had observed. On the columns of a Roman ruin he noted boreholes in the rocks, made by marine mollusks and now found 22 feet above recent sea-level. The only possible explanation to Lyell was a large magma chamber, periodically refilling with molten rock, would first pull the columns below sea-level, then after some time, push the ground up and lift the columns again above the sea.
The Phlegraean Fields, the Monte Nuovo is shown as larger red area, northwest of the harbor of... [+]
Earthquakes were recorded in the 1950s, 1970s and 1980s, followed by a measurable ground uplift. Today we know that the magma chamber of the Campi Flegrei is located at a depth of 1.8 miles below ground level. How exactly the magma causes the observed ground movements is still not fully understood. The classic model argues that as the magma chamber fills, the larger volume pushes simply the overlying ground up. An alternative model suggests that hot fluids from the magma heat the rock and a large body of groundwater, found in the underground of the Campi Flegrei. As hot water possesses a larger volume, it expands and the ground is pushed up.
In any case, simulations done by a research team of the University College London and the Vesuvius Observatory in Naples and published under the title Progressive approach to eruption at Campi Flegrei caldera in southern Italy, have shown that this periodical uplift causes an accumulation of stress in the rocks above the magma chamber, making it more likely that magma will find a way to the surface, increasing the possibility of an eruption. Other volcanic activity, like gas released from the underground, already showed that the Phlegraean Fields are still quite active.
Since 2005 the ground around the city of Pozzuoli, located almost at the center of the Campi Flegrei caldera (a large, partially collapsed volcanic crater), has risen by 1.25 feet. Both effects are not unusual in the area. The columns described by Lyell (and also located in Pozzuoli) show that uplift occurred repeatedly in the last 2,000 years and the ground even periodically deflated again. However, the new model on how the rocks, covering the magma chamber, react to the phases of uplift showed an unexpected behavior.
During uplift the rocks deform. Under tension, rocks become more brittle, making it more easy for the magma to break the rocks and form a volcanic conduit to the surface. Former geological models assumed that as soon as the pressure in the magma chamber decreased, the tension in the overlying rocks would decrease as well. However, the new model suggests that tensions tend to constantly accumulate over time. When the ground is uplifted, the rocks break and magma is injected from the deep-seated magma chamber into shallower areas of the Phlegraean Fields. The magma, even if it not erupts to the surface, cools and solidifies in the formed crevasses. This newly formed rock-volume tends to maintain the surrounding rock under pressure and tension. With every phase of uplift, the rocks are exposed to higher tension forces, becoming more brittle and prone to break each time.
This new model of rock behavior under increased stress by magma movements can't predict a specific eruption. However, it suggests that eruptions will more likely occur in the future, as the rock strength tends to decrease, slowly but constantly over time. Instead of one large uplift, a number of minor uplifts will suffice to break the already weakened rocks, causing the eruption of a new volcano.
