From a geological point of view, the Tyrrhenian Sea can be considered a small ocean, because just like the oceans it is expanding: over time, magma has escaped from its seabed, forming new oceanic crust. Submarine volcanoes and magnetic anomalies on the seabed bear witness to this. Added to the distension of the area is the sinking of the Ionic plate under the Calabrian arc, which causes frequent earthquakes.
The Tyrrhenian Sea it is a portion of the Mediterranean Sea, located west of the Apennine peninsula between Sardinia, Corsica and Sicily. From a purely geological point of view, however, the Tyrrhenian Sea could be defined as a ocean and not a sea. This is because it is expandinga typical characteristic of the oceans: over time, in fact, magma has escaped from its seabed, giving rise to new oceanic crust. Indeed, about 2 million years ago, its expansion speed reached 20 cm/yearexceeding that of all other oceans. The numerous underwater volcanoes that are found on its seabed, including the Marsili (the largest in Europe with 3000 m of height), and the so-called “magnetic anomalies“of the ocean floor testify to this process. The area is geologically very complex because in addition to the relaxation forces there is also collapse of the ionic platepart of the African one, under the Calabrian archpart of the Eurasian plate. This process causes frequent earthquakes.
The opening of the Tyrrhenian Sea 10 million years ago
Until 10 million years agothe Tyrrhenian Sea did not exist. It was the collision between lithospheric plates, particularly between that one European and that Africanto determine its opening. The movements of these plates, in fact, brought about a block of earth’s crust which included present-day Corsica and Sardinia (Sardinian-Corsican block) to detach by rotating counterclockwise towards the south-east. This movement determined a movement towards the east of the Apennine chain that was forming, faster than that of the Sardinian-Corsican block. Due to this difference in speed, the gap opened between the Sardinian-Corsican block and the mountain range Tyrrhenian Seawhich was initially very narrow and which over time became increasingly wider. An important consequence of the opening of the Tyrrhenian Sea was the closure of the Strait of Gibraltarwith the isolation of the Mediterranean from the Atlantic Ocean, which about 6 million years ago led it to dry up through evaporation.
Oceanic features: a similar geological history
The relaxing forces caused the opening of the seabed of the Tyrrhenian Sea deep fracturesfrom which large volumes of magma emerged from the underlying mantle. This is what happens at the ocean ridges that cross the oceans, where relaxation efforts cause two adjacent lithospheric plates to move apart. The magma that comes out of the ridge solidifies, forming new basaltic crust, which pushes the old one laterally: this mechanism determines theexpansion of the ocean floor. In some places the rising magma gave rise to numerous underwater volcanoes which rise on the seabed of the Tyrrhenian Sea.
Submarine volcanoes in the Tyrrhenian Sea
Among the underwater volcanoes of the Tyrrhenian Sea, the best known are Marsili, Palinuro, Vavilov and Sisifo.
- Marsili: it is located in the southern Tyrrhenian Sea, approximately 140 km north of Sicily and 150 km west of Calabria. 70 km long and 30 km wide, it is the largest volcano in Europe. This stratovolcano rises 3000 m above the seabed and its summit is 500 m below sea level. Its last eruption occurred between 2100 and 3000 years ago. It is considered an active volcano, but if it erupted the water above it would be able to absorb the energy released by the eruption, avoiding serious consequences. Marsili, like Palinuro, is rich in hydrothermal deposits, where metals such as copper, barium, silver and lead are found.
- Palinuro: it is a quiescent volcanic complex approximately 75-90 km long located in the southern Tyrrhenian Sea off the coasts of Campania and Calabria. It extends from 3200 m deep to 80 m below sea level and is characterized by secondary volcanic phenomena, such as gas emissions. Taken together, these volcanoes represent a rift in the earth’s crust from which magmas originate from the Aeolian Islands, the central-southern Tyrrhenian Sea, and the area between Puglia and Calabria.
- Vavilov: approximately 30 km long and 15 km wide, it rises 2700 km from the seabed approximately 160 km south-east of the Campania coast and the Gulf of Naples area. It caused eruptions mainly between 300,000 and 800,000 years ago and is considered extinct.
- Sisyphus: it is located north-west of the island of Alicudi and extends from a depth of 2010 m to 1080 m below sea level. At 1.3 million years old, it is considered the oldest volcanic edifice in the entire Aeolian arc.
Other underwater volcanoes include Glauco, Glabro, Eolo, Enarete and a chain of small volcanoes between the coast of Salerno and Calabria.
The extension of the Tyrrhenian Sea, a sea in the center of the Mediterranean
The Tyrrhenian Sea not only expanded like an ocean, but in the past it did so at a surprising speed, faster than all other oceans. In 2006 the INGV (National Institute of Geophysics and Volcanology) discovered that between 1.5 and 2 million years ago the Tyrrhenian Sea has reached a expansion speed equal to 20 cm/year (that of the Pacific Ocean is 15 cm/year). This record was achieved approximately where Marsili is located. Over time, there have been periods in which expansion slowed down and others in which it accelerated. But how do we know these details?
The speed of expansion was determined thanks to the so-called “magnetic anomalies“of the ocean floor. The magma that comes out from an oceanic ridge, in fact, solidifies on both its sides, forming new basaltic crust. Its ferromagnetic minerals take the orientation of the terrestrial magnetic field present at the moment of solidification. The terrestrial magnetic field, in fact, periodically reverses its orientation (the magnetic North pole takes the place of the magnetic South pole and vice versa). The result is that on the sides of the ridge, parallel and symmetrical bands of rocks are formed whose minerals have an orientation equal to that of the current magnetic field (a normal polarity), alternating with bands in which the orientation is opposite (a reverse polarity). These bands testify to the expansion of the ocean floor and have made it possible to calculate its age by correlating the anomalies with the time scale of the reversals of the Earth’s magnetic field (reconstructed by studying sequences of lava on land). Starting from the age of the rocks and their distance from the ridge it was possible to calculate the speed of expansion of the oceans.
Geology and seismicity of the Tyrrhenian Sea
The Tyrrhenian Sea area is very complex from a geological point of view. In addition to the relaxing forces that over time have led to the opening of the pelvis, the process of subductionthat is, of sinking, of ionic plate under theCalabrian arch. The Ionic plate, made up of oceanic lithosphere, is part of the African plate and is sinking to over 500 km depth in the mantle under the Calabrian arc, made up of continental lithosphere and part of the Eurasian plate. This process causes frequent earthquakes along the coasts of Calabria and Sicily, both due to the friction at the interface between the two plates and the deformations to which both plates are subject. Some of these earthquakes have a very deep hypocenter. A particularly strong earthquake that occurred in the area in the last decade, due to the subduction of the Ionian plate, was that of 28 October 2016, with a local magnitude (ML) 5.8 and a depth of 481 km. To better understand the seismicity of the area, researchers at INGV (National Institute of Geophysics and Volcanology) study the dynamics with which the collapse of the Ionian plate occurs, based on the analysis of seismic waves.
