Volcanic hazard of Colli Albani.

Although controversy exists about the age of its most recent eruption (either 36 ka or ,23 ka), Colli Albani volcano is unanimously considered to be quiescent and not exinct. During the Holocene, several lahars were generated by overflows from Albano crater lake up to the fourth century BCE, when the Romans excavated a drainage tunnel to keep the lake level below the crater rim. Such recent activity, together with the frequent occurrence of seismic swarms underneath the crater zone, the ongoing uplift of the volcanic edifice and the magmatic affinity of the emitted gas, indicate the presence of an active magma chamber. The most likely site for a new eruption is the deep crater hosted in the southern part of the Lake Albano, where the last eruptive events occurred. Any eruption would have a strong explosive character enhanced by the interaction of magma with the water of the lake and would endanger a densely inhabited area up to the outskirts of Rome. The hazard of a new overflow from Lake Albano is very low because of the present low level of the lake. There is instead a potential for CO2 release from the deep lake water following the occurrence of rollovers, which would threaten the lake shore, a site where thousands of people spend their vacations in the summer. However, the content of dissolved CO2 is presently far from saturation and no Nyos-type events will occur today. Presently, the main hazard is related to strong gas emissions (CO2, H2S and Rn) from fractured zones and gas blowouts from wells reaching shallow gas-pressurized aquifers.