Submarine volcanoes
The seafloor of the Tyrrhenian Sea is distinctively shaped by numerous underwater ridges and volcanic features. Many island and coastal volcanoes extend far beneath the surface, with significant portions lying underwater. For example, approximately 95% of the total structure of the Stromboli volcano lies below sea level. In addition to these partially submerged systems, there are entirely underwater volcanoes that can be comparable in size or even larger than those found on land.
Underwater volcanoes are particularly challenging to study due to the lack of direct access. Nevertheless, marine geological studies conducted over the past few decades have led to a greater understanding of their nature and how they function. Observations and sample collection are typically performed using research vessels.
In the case of the Italian seas, submarine volcanic activity is primarily concentrated in parts of the Tyrrhenian Sea and the Strait of Sicily, where the Earth's crust is thinner and more fractured. Some submarine volcanoes are still active and occasionally reveal their presence through gas emissions and very slow deformation, while others, now extinct, form true seamounts. Their activity differs from that of terrestrial volcanoes, as they are surrounded by seawater, which rapidly cools the emitted products and can fragment the magma, generating small explosive events. The resulting materials are partly deposited on the seafloor and partly dispersed by sea currents.
In addition to Marsili, the best-known underwater volcano in the Tyrrhenian Sea, it is important to mention other notable volcanoes such as Palinuro, Glabro, Eolo, and Enarete, as well as the numerous volcanic systems in the Strait of Sicily. In this region, the undersea eruptions recorded off the coast of Sciacca in 1831 and off the coast of Pantelleria in 1891 constitute the only documented historical evidence of this type of activity.
It is the largest volcano in Europe, with a length of about 50km and a width of 20km. It has a height of 3km above the surrounding sea floor, and its "ridge" reaches depths of less than 1000m. It is formed by a series of volcanic buildings of different sizes. The western flank consists of conical buildings, while the northwestern flank is characterized by several "flat-topped volcanoes" and a staircase of stacked lava terraces.
Although an ongoing eruption has never been observed, Marsili's activity is evidenced by the circulation of high-temperature fluids that deposit sulfides of lead, copper, zinc, and oxides and hydroxides of iron and manganese on the seafloor.
Palinuro is a volcanic complex about 75km long, consisting of at least 8 major edifices aligned roughly east-west, while Glabro lies not far from the Palinuro volcanic complex along the same alignment. The summit of these two volcanoes is around 100m below sea level.
The Alcione volcano and the twin apparatuses of the Lametini are located intermediate between the Palinuro-Glabro alignment and the Aeolian Islands arc. They are conical volcanoes, about a thousand meters high relative to the surrounding sea floor.
In the Aeolian apparatus all the major buildings have emerged and given rise to islands, although minor apparatuses exist, particularly around the Vulcano-Lipari-Salina alignment. West of the archipelago are the Eolo and Enarete volcanoes, which are about a thousand meters high. Eolo is notable for its flat summit.
Enotrio has a rugged landscape, with sharp ridges and steep cliffs. Ovidio, by contrast, is the easternmost volcano and the one closest to the coast; it consists of at least five main cones that are rounded and heavily eroded by ocean currents.
The Strait of Sicily is an area of widespread volcanism due to its high tectonic activity. This volcanism, partly still active, has given rise to the islands of Pantelleria and Linosa, as well as numerous submarine volcanic edifices, such as Ferdinandea (or Graham), Terribile, Unnamed, Nerita, and Bannock, which are mainly aligned along northwest–southeast and north–south trends.
Ferdinandea Island formed in 1831, off the coast of the town of Sciacca, through the accumulation of volcanic materials. This small volcanic cone was subsequently eroded and destroyed by wave action within a few months. During its brief emergence, the Kingdom of the Two Sicilies, England, and France each claimed sovereignty over the island, assigning it different names: Ferdinandea, Graham, and Giulia.
Today, the volcanic structure lies at a minimum depth of about 20 meters below sea level and exhibits ongoing gas emissions, forming columns tens of meters in diameter.
A recent hypothesis interprets the Graham Bank, together with the Terribile and Nerita Banks, as parts of a large submarine volcanic complex named Empedocles, comparable in size to Mount Etna. However, conclusive evidence to support this hypothesis is still lacking.