Despite the small distances that separate them, the islands of the Tuscan Archipelago have a very diversified origin and geological history: if Capraia, the youngest, was generated by two volcanic eruptions (about 7 million and 4.5 million years ago), Elba includes – in the easternmost part – a portion of the African continent about half a billion years old, here translated by the movements of the earth’s crust that have occurred over the ages; other areas of Elba, as well as the Giglio and the entire Montecristo, are made up of granite rocks formed at great depths from cooling magma pockets and then raised up by tectonic movements; lifting and remodeling due to the action of the sea also originate Gorgona (formed by metamorphic rocks), Pianosa and Giannutri (characterized by limestone of sedimentary origin). In particular, Giannutri is the only island of the archipelago composed of calcareous-dolomitic formations that show strong similarities with the rocks of the nearby Apennine mountains, so that its origin and its emergence from the bottom of the sea must certainly be related to the orogeny of the Apennines (occurred about 20 million years ago by the clash between the African plate and the Eurasian plate).
The well-known phenomena of partial drying up of the Mediterranean Sea that occurred 5 million years ago due to the closure of the Strait of Gibraltar make it clear that in the distant past Giannutri, similar to the other islands of the Tyrrhenian, appeared for long periods as an arid mountain peak emerging from a salt desert. Even after the waters came back to submerge the Mediterranean basin, the climatic changes due to the cyclical nature of the glaciations caused rises or lowering of the sea level that helped to shape the island’s appearance. The proof is, for example, the submerged arch that is located a few meters deep not far from Cala Ischiaiola, along the western coast of the island, produced a few millennia ago by the breaking of the waves against the coast at a lower sea level than the current one, during a colder climate phase. Even the deep gully that characterizes the sea bed of Cala Spalmatoio, in the stretch where the inlet opens in the Gulf of the Spalmatoi, is the product of erosion caused by the flow of meteoric waters in a time when the Mediterranean surface was at a significantly lower level than today’s.
Giannutri also returned fossils of megaloceros, or giant deer, in the gravel that fills the fissures of the rocks at Cala Maestra. The animal, lived in a period between 400,000 and 10,000 years ago, testifies to the presence of deer on the island, which presumably did not appear as such: it is known that some glacial periods of particular rigor have produced a lowering of the sea level of the Mediterranean about 100 m below the current level, allowing the emergence of the highest parts of the seabed to the point of creating tongues of land that could be crossed by the animals that colonized the Italian peninsula. The presence of the giant deer, about 2 m tall at the withers (like a modern horse) and with antlers up to 3 m wide, confirms that Giannutri, like the rest of Italy, experienced periods of cold climate similar to that today found in the forests of northern Europe, which was at that time covered by ice.