The Astroni volcano: the only example of closely spaced eruptions in the same vent area during the recent history of the Campi Flegrei caldera (Italy)

Abstract

The Astroni volcano formed during the third and most recent epoch of activity (4.8^3.8 ka) of the Campi Flegrei caldera (CFc). The activity of the volcano was dominated by explosive, mostly phreatomagmatic eruptions, with only subordinate lava effusions. We have grouped the sequence of deposits into seven distinct units, separated by erosional unconformities or very thin paleosols. The units include mostly surge beds, with subordinate strombolian deposits and lavas, and one plinian fallout layer. The total volume of erupted magma is 0.45 km3 (DRE), while the total mass is 1.12U1012 kg. The magma feeding the first five eruptions was alkali-trachytic and slightly zoned, while the last two eruptions tapped a magma batch resulting from mixing of the previously extruded alkali-trachytic and a less evolved trachytic magma. The volcano grew at the northwestern edge of the polygonal volcano-tectonic collapse, northwest southeast elongated, which accompanied the Agnano^Monte Spina eruption (4.1 ka), the largest of the third epoch. Available radiometric dates and stratigraphical data constrain the age of the volcano in the final part of the 4.1^3.8 ka time span. This implies that the seven eruptions followed each other at very short time intervals. This conclusion is also supported by constancy in archaeological facies of findings within the paleosols between variable Astroni units, in the plain north of the caldera. The sequence of close eruptions in the same area, although with a slight migration of the vent from northwest to southeast, makes the Astroni volcano peculiar in the recent history of the CFc. Therefore, the definition of its history is very important in order to understand one of the past phenomenologies of the caldera, relevant elements to forecast its behavior.