From a geological point of view, the hill of Posillipo represents the edge of the ancient caldera that originated from the eruption of the Neapolitan Yellow Tufo about 15,000 years ago. The area of Posillipo is part of the Phlegraean volcanic system. For this reason it is still subjected to volcano-tectonic activity, which currently manifests itself in the form of bradyseism and in the presence of underwater gaseous emissions (fumaroles) visible even from the surface. The narrow and sharp offshoot of Punta Cavallo, which represents the western limit of the Marine Protected Area in the Trentaremi Bay, is the evidence of the intense volcanic activity characterizing this stretch of coast. In fact, Punta Cavallo represents the wreck of the ancient volcano of Coroglio which was subsequently covered by the pyroclastic deposits emitted during the eruption of the Neapolitan Yellow Tufo and partly eroded by the sea.

The small islands of Gaiola, separated from the mainland by a narrow stretch of sea on average 50 metres wide, ideally divide the coastal strip of the Park into two sectors: the eastern one, where the hill slopes gently towards the sea, and the western one, characterized by the high cliffs surrounding the Bay of Trentaremi. From a geomorphological point of view, the current coastal conformation of the MPA is the result of the interaction between natural erosion, bradyseism and the intense anthropic reworking of the tuff cliffs that took place in different historical periods. As evidence of this, the majestic Caves of Trentaremi, which open onto the eastern side of the bay of Trentaremi, are the result of centuries of mining, which began most likely in the Greek-Roman era.