A tale of two volcanic cycles
The island of Ponza is the exposed remnant of a much larger volcanic complex, its geology recording a history of eruptions and dormancy driven by continental-scale tectonics. The island's rocks were emplaced during two primary cycles of activity, separated by a long period of dormancy. This volcanic activity is directly linked to the opening of the Tyrrhenian Sea, a back-arc basin that stretched and thinned the continental crust, allowing magma to ascend.
The first and most voluminous cycle occurred during the Pliocene epoch. Geochronological studies date this activity to between 4.2 and 3.7 million years ago. During this time, the volcanism was primarily submarine. Highly viscous, silica-rich rhyolitic magma erupted onto the seafloor. The interaction with seawater caused the lava to shatter, forming extensive deposits of hyaloclastite—a type of volcanic breccia that constitutes about 90% of the island's older formations. These Pliocene eruptions produced high-K calc-alkaline rhyolites, a chemical composition typical of magmas formed in subduction zones. The famous white and pale-colored tuff cliffs seen today, such as at Chiaia di Luna, are products of this initial phase.