1. Introduction

The palaeogeographic evolution of the Aegean during the recent geological era of Upper Cainozoic (the last 23 million years) was marked by important geotectonic changes which mainly took place in the periods of Upper Miocene (from 5 to11 million years) and Lower Pliocene. These changes are related to the movement of great parts of the earth’s crust, called lithosphere plates, in the wider region of the Aegean. The movement of the lithosphere plates provoked intense volcanic activity.

The current situation in the Aegean differs of course considerably from the situation during Eocene-Oligocene, 35-45 million years ago. Then the collision line of the lithosphere plates of Eurasia and Africa was in the area of Cyclades. This collision created high pressure metamorphosis of rocks in the Cyclades (Syros, Naxos, Tinos, Andros etc) and volcanic events in the region of Rodopi. The progressive shift of the volcanism reflects the progressive shift of the collision of the lithosphere plates, which simultaneously was expressed with a turn of the Greek area. This turn, which is certified by palaeomagnetic measurements, turned the Greek peninsula dextrally approximately 38 degrees comparing to the place it was 15-20 million years ago.

2. The Tethys ocean and the Cimmerian continent

The geotectonic evolution of the wider Hellenic - Minor Asia space begins in the much older geological periods of the Mesozoic or even Palaeozoic, 180 to 300 million years ago. The steps of this geotectonic evolution of the Aegean can be summarised as follows. The entire Hellenic space is a creation of an old sea / the ocean of Tethys , which was situated between Eurasia, the integrated continental region of Europe and Asia in the north, and Godwana, the single continent that included Africa, India and Australia in the south. The sea of Tethys functioned as an oceanic space for hundreds of millions of years. In the initial stage, the Palaeotethys separated the two continents. Later, various large continental blocks were separatedfrom the continental region. These continental blocks constituted a continent, known from the mythogeological terminology as “Cimmerian” continent. The Cimmerian continent continued its’ movement, and, during the period 300-180 million years ago, pressed the space of Palaeo-Tethys making a rotation and created behind Tethys the new ocean “Neo-Tethys ”.

Remains of that area of Tethys, as well as remains of that old continental form of the Cimmerian continent, are to be found in the wider area of Greece and Asia Minor. The mountain ranges of Vorra, Vermion, Pieria, Olympus, Ossa, Pelion, Euboea and Cyclades were originally parts of the Cimmerian continent. The substratum of Lesvos along with the block Sacaria of Asia Minor and some other continental blocks also belonged to the Cimmerian continent.

The Palaeo-Tethys ocean closed and was completely destroyed, while the area of Neo-Tethys remained open until the Eocene, circa 45 million years ago. Then the destruction of this ocean was completed by the subduction of the oceanic crust under the Cimmerian continent.

The volcanism which broke out in the area of Rodopi was connected to this collision that took place during Oligocene, 25-30 million years ago.

3. The molassic basin of northern Aegean

Over the same period, a marine basin existed in the north Aegean, where a great volume of molassic sediments was accumulated. The molassic basin of the northern Aegean is extended along the southern margin of Rodopi and includes the region of Axios and Thermaikos gulf in the west, Limnos, the region of Thrace and west Asia Minor. The regions of west Macedonia, east Thessaly and Lesvos up to Asia Minor have an extensive land region, which is constituted mainly by metamorphic rocks that are parts of the Cimmerian continent. In these land areas tropical-subtropical forests developed. This is proved by the presence of a large number of plant fossils. South of this land area another, bigger than that of the north Aegean, marine basin is extended, which also accepts a big volume of molassic sediments. This marine basin is in the region of Grevena, the basin of Thessaly, of Euboea and extends as far as Chios and Asia Minor.

4. Volcanic activity

During Mid-Lower Miocene 13 until 22,5 million years ago the volcanic activity moved to the south and is expressed in the region of the north and central Aegean with extensions to Asia Minor. Samples can be found in the islands of north-east Aegean, Imbros, Limnos, Aghios Eustratios and Lesvos. The volcanic products are less common in Psara, Chios, Antipsara, Skyros and Euboea. The intense volcanic activity is connected to the creation of the Petrified Forests of Lesvos and Limnos. Because of the volcanic activity, large bulks of volcanic material and volcanic ash were drifted by the water and caused extensive mud flows that covered the vegetation of the area.

22,5 million years ago the Aegean looked completed different than today. However, a north marine Aegean basin started to be created, as well as in the region of Cyclades, in the central part, exists a land mass in the centre of which an internal basin was created containing rain and subsaline water. A volcanic zone developed in the east of this wider area.

5. The geological history during Mid Miocene

During Mid Miocene, approximately 15 million years ago, almost the entire Greek area, including the Aegean as well, becomes land and constitutes with Asia Minor a single mainland. The sea was withdrawn regionally and expanded from the Ionian sea to the south of Crete and Dodecanese. 10 million years ago, the sea enters from south almost in the entire area of the current Aegean sea. The region of Cyclades that was the continuation of mainland Greece remained land, as well as regions of east Aegean like the islands of Lesvos, Chios, Samos et al., that were connected to Asia Minor. The palaeogeographic conditions however changed rapidly 8 million years ago. The Mediterranean Sea was limited in individual basins. This was due to the isolation from the neighbouring oceans by the closing of the channels of communication. The quantity of evaporated water was bigger than the quantity of water that poured into the Mediterranean from the rivers of the neighbouring land. Thus, a crisis of saltiness in the entire Mediterranean and of course in the Aegean was created. In many regions of the Aegean deposits of evaporites are to be found, such as in the sea area of Thassos, in northern Sporades, in the sea area between Cyclades and Chios, north and east Crete and in the Ionian sea. 6 million years ago the area of the Aegean becomes terrestrial again because of the compression and constitutes a single land with mainland Greece and Asia Minor. The sea at this period had withdrawn to the south and extended from the Ionian islands and the region of Crete up to the Dodecanese. In northern Aegean, the waters of Paratethys were beginning to enter into the Aegean. 3,5 million years ago the tectonic action was expressed with stretching, thus new faults were created or old ones were reactivated so that the sea could enter. 1,8 million years ago, the sea was withdrawn to the south and lakes were created in the north and central Aegean. The tectonics in this period is compressional, for a short time period. During Mid Pleistocene and up to today (the last 700.000 years), the internal Greek area is influenced by a new stretching phase that resulted to the retroaction of old faults and the creation of new ones, for example in Kos, Rhodes, Crete et al. This involves transgression of the sea which takes almost its current form. 21.500 years ago the sea level was 120 m under the current level. This resulted in broad communication between the islands, mainland Greece and Asia Minor. In the region of Cyclades an extensive land was formed. 11.500 years ago the sea level rose and reached -60 m from today’s level, causing the interruption of communication between many regions. Finally 8.000 years ago the sea level reaches almost its’ current level. This resulted in the submerge of many low land regions and the interruption of communication between the islands.

As the geological time passes a shift of the volcanic activity can be observed. During the period of Mid-Upper Miocene, 12 until 5 million years ago, the volcanic activity moves farther to the south, to the south-east Aegean and to south-west Asia Minor. Samples of this activity can be found in the islands of Patmos, Samos, Kos. During this period the exchange of fauna with the region of Asia Minor began. In the same period the intense tectonic activity caused the fragment of the land. Many lakes with subsaline and fresh water were formed over this period, while at the same era we can observe alternations of marine and lacustrine depositions. The volcanic rocks of the Kalogiroi islands constitute the last clue of this volcanic activity and the connection with the next volcanic phase of Pliocene.

During the same period periodical interconnections between the Mediterranean sea and north Aegean must have existed , without however the creation of a barrier that would prevent the migration of mammals from Minor Asia to the south, central and north Aegean. In the base of the old continental Aegean an archipelago was formed where migration bridges of land were created between Minor Asia and Greece. These allowed the arrival of a big number of migrants from the Asiatic steppe, as well as from Africa, in mainland Greece.

6. The geography of the Aegean during Pleistocene

During Pleistocene, the period from 1,8 millions until 10.000 years ago, the palaeogeographic situation of the Aegean did not really differ from the current one. The area around the islands of Lesvos, Chios, Samos, Ikaria, Patmos and Kos constituted a terrestrial part that was connected, during Pleistocene, to continental Asia Minor. The road of migration of mammals was a land bridge or an interrupted land. Later, the elevation of Lesvos at the Mid Pleistocene, about 1 million years ago, allowed new mammals to migrate. There must have been a connection between Lesvos, Limnos and perhaps Imbros as well. This can be concluded, if we examine the resemblance of the mammal fauna between the two islands during Pleistocene. Since the Mid Pleistocene, Oinousses were still connected to continental Minor Asia. Chios was cut off from Minor Asia and became an island very recently. Thus, we pass from continental Aegiis, in the current situation of an Archipelago with hundreds of islands. The intense tectonic activity of Holocaine, the last 10.000 years, is responsible for the final partition of Aegiis.