The crater of Astroni belongs to the volcanic district of Flegrei Fields. It was produced by 7 following eruptions that took place between 4.100 and 3.800 years ago.
The crateral building is perfectly preserved, it presents an oval edge: 247 hectares wide, 6 km long with a diameter of 2km and a drop of 9 metres and 261 metres s.l.m. (sperone of Torre Nocera) inclusive.
The crater is called "ring of ash" because it is composed by ashes and volcanic slags created by big freato-magmatic eruptions, that is characterised by the contact between the incandescent magma and the groundwater table that are in the underground. Because of these violent eruptions, every crater of the flegrea area doesn't present the classic structure of "cone" and lavic rocks aren't traceable.
Astroni are one of the few areas where it is possible to observe lavas called "trachiti", which form a rocky wall called " La Caprara" or "cava trachitica", since it was used for the building exploitation of the present rocks.
At the end of the crater there are three ranges called "colle dell'imperatrice" (76 m s.l.m.), "colle della Rotondella" (59 m s.l.m.) and "Pagliaroni" (17 m s.l.m.),they are composed by volcanic slags. In the lower part of the crater there are three little mirrors of water called "lago Grande", "Cofaniello piccolo" e "Cofaniello grande", they lack in sources ,that's why they are fed by the groundwater table below.
The thermal sources has completely disapperead.
Although Astroni don't present a volcanic activity, the Vesuvian Observatory has carried out researches to understand better the evolutions of the flegrea area