Mediterranean Sea

Marine geology tries to answer these questions. It is a complex branch of studies because direct observation by humans is difficult in these areas and are necessarily limited to depths of up to few tens of metres only. Therefore, sophisticated equipment is used, that enables the observation of the sea floor with different “eyes”, such as sound waves, sonar equipment and seismic waves that enable the investigation of areas below the seabed, inside the earth’s crust.
With complex and costly equipment it is also possible to sample materials embedded in the sea floor and to drill even further into the layers, taking sample “cores” of the sediments. These are an extremely precious archive of geological and climatic data regarding the evolution of the seabed and the surrounding areas. Together with the study of the specimens collected on dry land, this has enabled the reconstruction of the geological history of the Mediterranean and its present form. Knowledge about the geological processes that are taking place below the sea floor, enables us to comprehend and foresee the possible geological problems, and also in a minor way, to take the necessary measures to keep them under control.

Many basins, only one sea

The Mediterranean accounts for 0.7% of the surface of all the seas. It is a closed sea, between emersed land, with only one point of contact and exchange with the Atlantic Ocean, at the Strait of Gibraltar. It has a surface area of 2.5 million km2 and extends 4,000 km from Gibraltar to the Bosphorus. Its coasts are never more than 400 km apart. For this reason, since ancient times, the Mediterranean has been one of the most populated and navigated seas, and on its shores, some of the most important civilizations have flourished. The Mediterranean, seen from the surface, looks like a single sea divided into a number of basins with different characteristics, a different geological history, and also different morphologies of the sea floor.

A long story

The geological history of the Mediterranean area is very old; it began over 180 million years ago, when the fragmentation of Pangaea created an internal sea that progressively widened between what later became the European and the African continents. The real Mediterranean Sea began to take shape much later, 50-60 million years ago, when the collision of the African plate and the European plate separated it from the Atlantic Ocean, which in turn was separating Africa and Europe from North America.

Let us try to understand what happened

A fragment of the European continent became detached from the rest of Europe and began to move away towards the East, and rotating anti-clock wise, a new sea was opened. This fragment, that geologists call the Sardinian-Corsican block, was made up of Corsica and Sardinia and a small part of the Calabria region. The movement of this fragment of Europe led to the formation of the western Mediterranean basins: the Alboràn Sea, the Algeria-Provence basin, and a part of the Tyrrhenian Sea. Therefore these are relatively young basins, approximately 30 million years old. This movement stopped about 16-18 million years ago, in the Early Miocene Epoch.

Some more history

Beginning 10-12 million years ago, the series of straits that permitted communication with the Atlantic began to close, under the thrust of Africa against Europe. Since at the time, the climate was quite arid, evaporation was greater than the amount of water fed by the rivers, and the Mediterranean survived only because of the exchange with the Atlantic. The progressive decrease in this exchange led to a gradual lowering in the sea level. About 6 million years ago (during the Messinian period), communication with the ocean closed definitively and the lowering in the level of the Mediterranean ended in the so called “salinity crisis”. The Mediterranean almost evaporated completely, transforming into an arid landscape of deep desert canyons and small hyper-salty basins where 6% of the salt in all the oceans collected.

The various minor basins into which the Mediterranean Sea can be subdivided are the result of different geological processes, that affect the morphology of the sea bottom. The continental shelf is a submerged “rim” that outlines the continents separating them from the dominion of the sea, strictly speaking, with a slope at a depth of 150-200 m. We can imagine this area as an extension of the continents under the sea level. The continental shelf is not a typical characteristic in the Mediterranean area. In fact, it extends only in the northern part of the Adriatic Sea, near the coasts of Tunisia, between Sicily and Malta, around Corsica and Sardinia, and in the Aegean Sea. Thus there is no “connection” between the continents and the sea bottom, with a passage from one to the other that is often rapid and sudden and characterized by a brusque drop in the bathymetry.

The Alboran Sea

It extends from the Strait of Gibraltar to the Balearic Basin. Its maximum depth is 1,500 m, that drops to 1,800 m in the Alboran rift, that separates it from the Algerian basin. In the centre there is a small volcanic island, 10 m above s.l., that rises from the sea bottom that is 1,500 m below. This part of the Mediterranean receives the direct influence of the Atlantic, because it is where the sea water mixes with the ocean water. The water here is generally colder and less salty and rich with organisms coming from the Atlantic.

The Algerian- Provence basin

This is the vastest basin of the western Mediterranean area. Leaving the Alboran Sea to the west, it extends with a triangular shape from the Gulf of Valencia to the Ligurian Sea. Its maximum depth is 2,800 m, near the western coasts of Sardinia. It is characterized in its most western part, by the large deep-sea cone of the Ebro River, where the continental shelf reaches a width of 60 km. Along the northern coasts, up to Genoa, the continental shelf is practically absent, it is no wider than 3-9 km. Here the sea bottom descends rapidly to depths over 2,000 m and is characterized by a number of submarine canyons that cut across it. These canyons carry large quantities of material from the erosion of the emersed land toward the abyssal depths.

The Tyrrhenian Sea

The Tyrrhenian Sea is an almost triangular shaped depression, between Sardinia and peninsular Italy, and is the youngest of the deep Mediterranean basins. It has a depth of 3,800 m and is the deepest of the western basins. From a geological point of view, it is a little ocean (the youngest in the planet) as on its bottom there are two small abyssal plains formed by the ocean crust. Morphologically, it is a deep basin surrounded by sharp and deep slopes, cut by deep submerged valleys. It communicates with the other basins through 4 passages : a 300-400 m deep channel puts it in communication with the Ligurian Sea; a wide, 2,000 m deep channel between Sicily and Sardinia connects it to the Algerian basin; the Boniface Strait (that is max. 50 m deep) connects it to the Provence basin; and finally, the Strait of Messina is the connection (100 m deep) with the Ionian Sea.

The Adriatic Sea

The Adriatic basin has geological and morphological characteristics that are quite particular. Over one third of the area of the sea bottom is no more than 50-60 m deep, unlike the Ionian basins where there are the greatest depths of the Mediterranean. The Adriatic basin lies between the Apennine Mountain range and the area of the Balkans. It is a zone of great compression with the margin of the European plate dipping below the Adria plate. It is not a very deep sea, it filled rapidly with the sediments from the erosion of the two mountain ranges facing each other, and in a near geological future it is destined to disappear. It is subdivided into three different basins. The northern part, or Upper Adriatic Sea, is entirely covered with the alluvional deposits of the large rivers of the Northeast, specially of the Po River, and is characterized by a sea-bottom that degrades gently to a maximum depth of 75 m. The central part is a closed and more variable depression, the so-called Middle Adriatic trench, which is 266 m deep.

The Ionian

The Ionian Sea occupies the central part of the southern Mediterranean. Here the maximum depth of the Mediterranean is reached (5,093 m in the Hellenic trench). It is characterised by deep trenches (Hellenic trench, Herodotus trench near the Libyan coast, Malta trench and Pantelleria trench), vast deep abyssal plains to the East, and less deep plains towards the West, as in the area near Sicily and the Sirte plain, near the Libyan coast. The Ionian Sea is characterized by the subduction of the African plate under the Calabrian Arc, making it one of the most geologically active areas in our country. Even though, geologically speaking the Calabrian Arc, belongs geographically to the Apennine range of mountains, it is a small portion of the Alpine range of mountains, like Corsica and Sardinia.

The Aegean Sea

Analogously to the Ionian Sea, the Aegean Sea finds its origin and morphological characteristics in the subduction of the African plate, in this case underneath Greece. This too is an area of intense volcanic and seismic phenomena (the caldera in the Island of Santorini is famous). It is characterized by the presence of over 200 islands and is subdivided into various minor basins, such as the Crete sub-basin, surrounded by a trench that is 2,500 m deep.

The Levantine Basin

It is the easternmost part of the Mediterranean. It is characterised by an average depth of 2,000 m, with a maximum depth of 4,834 m in the Plinius trench and 3,720 m in the Stenone trench in the North. To the South it is characterized by the gigantic cone of the Nile, that brings the continental sediments that cover the abyssal plain of the Levantine Basin.

The main volcanic provinces in Italy are in the Campania region around Vesuvio-Campi Flegrei and the island of Ischia in the Aeolian archipelago, in the area of Mount Etna and in the Sicilian Channel. The three “giants”, Marsili, Vavilov and Magnaghi, are only three of the numerous volcanoes that are present on the sea-bottom in the Tyrrhenian Sea. The volcanic islands of the Mediterranean are only the visible part, on the surface, of volcanoes that are much larger and volcanic elements are also present on the sea bottom surrounding them.

The geological origin of the different active magmatic provinces is not the same, and there are different chemical characteristics of the materials that form them, just like there are different characteristics in the volcanic events. The volcanic activity in the Vesuvian area is tied to the subduction of the Adriatic lithosphere under the Apennine, while that of the Aeolian islands is connected to the subduction of the African plate, under the Calabrian arc. In this case, the magma produced is viscous and rich in silica, and this gives origin to the explosive, spectacular type of vulcanism (as in the case of Stromboli), which however is also dangerous (as in the case of an awakening of Vesuvius). The magmatic activity of Etna, of the volcanoes present in the Sicilian Channel (Linosa and Pantelleria) and the volcanoes in the Tyrrhenean Sea, instead, is due to the fracturing in the earth’s crust, with an uprising of magma that originates from deep below. This magma has a higher basalt content and is characterized by great fluidity: volcanic events are therefore more “peaceful” with lava effusions that are characterised by very fluid flows and minor explosive activity.

The Aeolian Islands

In the Aeolian group of islands, at present the volcanoes that are considered to be active are Stromboli, Vulcano and Lipari. The most ancient island is Filicudi (which was formed 1 million years ago) while the others date back to 10,000 to 400,000 years ago. Alicudi and Filicudi stopped being active tens of thousands of years ago while Salina and Panarea were active up to less than 13,000 years ago. The island of Stromboli consists in a volcano cone that reaches 924 m above sea level. Actually, it is only the peak of a volcano that is much larger, and whose base is on the sea floor 1,500-2,000 m below. The Strombolicchio rock is a part of what remains of the central conduit of the much larger and much more ancient volcano structure.

Etna

Etna’s activity began 600,000 years ago, and it is the largest active volcano in Europe. It formed from the sea bottom of a large gulf. The proof of these initial underwater phases is the finding of particular lava formations, the so-called lava pillows. The progressive accumulation of outflowing material, together with the general rise in the area has led to the surfacing of this volcano. In the future, perhaps, also the large formations of Marsili, Magnaghi, Vavilov and other volcanic structures may surface from the waters of the Tyrrhenean Sea, thus forming a series of islands, analogous to Etna.

Volcanoes in the Sicilian Channel

In the Sicilian Channel, we find some islands of volcanic origin that still show signs of volcanic activity. Pantelleria has a large caldera, the Montagna Grande that rises to 836 m. Its most ancient rocks are 325,000 years old and since at least 3,000 years volcanic activity is quiescent. Linosa is formed from basaltic flows that erupted 1 to 0.5 million years ago, however no volcanic activity has been recorded here since at least 10,000 years. A quite singular geological feature of the Sicilian Channel is the Ferdinandea island, whose geological history is as interesting as the diplomatic case that its birth triggered in 1831.

Submarine eruptions

The word “eruption” immediately brings to mind images of fountains of incandescent lava and columns of ashes and vapour shooting towards the sky. But how does an underwater eruption take place? The pressure of the water limits the explosive force of the activity, while the rapid cooling due to the contact with the cold water determines the morphological characteristics of the flow. The incandescent lava that is produced, feels the influence, in fact, of a very rapid cooling on the surface. In this way a kind of large “drop” of lava is formed, whose surface hardens immediately forming a vitreous “crust”. This is what the geologists call a pillow of lava. The continuous production of lava provokes a break in the crust of the first pillow that opens letting new lava flow out, which in turn forms a second pillow.

Dangerous sea floors?

Great apprehension accompanied the communication of the discovery of large potentially active volcanoes on the Mediterranean Sea bottom. Actually, a submarine eruption, unless it occurs in shallow waters, rarely has any effects on the surface, and usually occurs in a “peaceful”, silent manner. The direct danger for man is relatively low, unless he is passing exactly above the point of the eruption, if this takes place in shallow waters. Direct damages may be made to the submerged anthropic structures such as cables or pipelines, but the possibility that these could have been positioned near a submerged erupting volcano are quite unlikely. Severe damages are incurred by the marine fauna near the eruption, due to the increase in the temperature and due to possible gas emissions that are toxic for the living marine creatures