Geology of the Aegean

1. Introduction

The Greek landscape, and consequently the area of the Aegean, as regards the age and tectonics, is part of Neoeurope. The rocks that appear are distinguished in three categories: the Palaeozoic or older prealpine rocks (280 millions years), the Triassic Alpine, (230 millions years) that reach even the Miocene, and the Low Miocene postalpine, (23 millions years). In this category belong the molasse sediments as well. Their age is Mid-Eocene (43 millions years) to Miocene.

2. Cyclades

As far as geology is concerned the Aegean presents a complexity regarding the variety of the rocks as well as their age. Categorizing the formations that appear in this area we distinguish the unit of southern Cyclades that includes mainly the islands of Naxos, Paros, Antiparos, Sikinos, Ios, Folegandros, Irakleia. Here prevail thick neritic marbles containing emery. The oldest rocks of the area are pelagic marbles with silex, as well as clastic formations. The geological column is constituted by a big mass of Triassic to Upper Cretaceous (230-65 million years) marbles (with bauxites), in the base are mica and amphibolitic schists with small marble interventions. On the top appears metaflysch.

In northern Cyclades metamorphic rocks of high pressure and low temperature appear in the islands of Syros, Tinos, Andros, N. Euboia, Gyaros, and in smaller appearances in Kea, Kythnos, Serifos, where this metamorphosis has been extinguished by the newer greenschist phase metamorphosis that is accompanied by granites. This unit appears in the area of the Aegean apart from northern Cyclades and Samos.

In some islands of the Cyclades non metamorphic Alpine rocks appear as well.

3. Lesvos-Limnos

In Lesvos five formations occur (Hecht, 1972-1976, Katsikatsos, 1986). The autochthonous series (basement) is the oldest. It appears in the south-eastern part of the island and is composed by metamorphic rocks, mainly schist with lenses and interjections of carbonic horizons. The volcanosedimentary series is obducted on the amenable autochthonous series and is constituted by various types of metamorphic basic igneous and sedimentary rocks (metabasites, marbles, schists). The biggest part of the ophiolitic cover that is obducted on the volcanosedimentary series, is constituted by pyroxenic peridotites, dunites, peridotites and metamorphic basic rocks in the base. The largest part of the island is covered by neogenic volcanic rocks that were created due to the action of several volcanic centres, which were placed in direction southwest - northeast, from the village of Agra towards the village of Sykaminea. Borsi (1972) and Pe-Piper (1978, 1980), using methods K/Ar and Ar39 respectively, determined the age of these volcanic rocks which lies between 21,5 and 16,5 million years. Under the volcanic rocks in western Lesvos oligocenic lacustrine sedimentary rocks are at places. The youngest rocks are Pliocene and Pleistocene, and include lacustrine deposits as well as newer alluvial deposits.

Oligocenic-lower Miocenic volcanic rocks cover also big part of Limnos, which intervene with molasse. Of similar age are the volcanic rocks of Agios Efstratios.

4. Samos-Chios-Psara-Oinousses-Ikaria-Fourni

In the islands of Psara, Chios, Oinousses formations of the Pelagonic zone prevail. They are mainly composed by crystallic limestones and gneiss schist, as well as formations of the Palaeozoic. According to the geological mapping of Besenecker H. (1962-1967), the oldest rocks of Chios are located in the north-west. They are mainly Palaeozoic clastic rocks (grauwackes, schist, flints) as well as limestone. The rest of the island is mainly formed by Mesozoic limestone and dolomites, which cover various chronological periods. Also, a Miocenic volcanic activity of limited extent took place in Chios, as the relatively small appearances of acidic volcanic rocks (rhyolite, alkaline rhyolite) testify, while in the central and southern part of the island miocenic sediments can be found, as well as red clays and silt, green sands and gravel, conglomerates and ferrous sandstones.

In Oinousses mainly low degree of metamorphic rocks (pelitic, psammite) prevail, which are probably older than the palaeozoic rocks of Chios.

The geology of Samos, which is part of the of Attika-Cyclades crystalloschistic unit, is interesting (Papanikolaou D., 1985), as the islands of Ikaria and Fournoi. Samos is structured by a pile of nappes of metamorphic mesozoic rocks, on which sedimentary and volcanic formations of the upper Miocene have been developed. The lithologic structure of Samos is constituted of the Kerketea unit (lower metamorphic sequence), which consists of granite gneiss, heavy bedded and unsorted marbles, while in the upper horizons schist with interjections of quartzite, marbles and cipollin developed . The Ampelos cover (intermediary metamorphic sequence) is constituted by metamorphic formations of high pressure / low temperature, like metabasalt, prasinite, schist and marbles. It is met in the Eastern part of the island and it is constituted by marbles and schist. The Kallithea cover is developed in the western part of the island and it is tectonic obducted on the amenable unit of Kerketea marbles.

The oldest geological layers that have been identified with fossils in the Greek area can be found only in few places of the Aegean area, such as the islands of Chios and Kos. Such layers are limestone 450 million years old.

5. Volcanism

Volcanism in the Aegean is of particular interest and can be divided into four groups. The first group is situated in south Aegean and its age is 5 million years until present time, the second is in the central Aegean and its age is 10,4 million to 6 million years, the third is in the north Aegean and its age is 23,2 to 13,2 million years and the fourth has an age of 33,1 to 23,6 million years.

The submergence of the African lithosphere under the Eurasiatic in one or more levels resulted, apart from the volcanism, also to the calcalcalic plutonism of the Greek region with appearances in the area of Cyclades and in the north Aegean. The volcanic activity is presented in a relatively limited area in a form of an arc, which is extended from Saronikos gulf in the west, up to the island of Nisyros to the east (volcanic arc of the south Aegean). This arc is the visible proof of the subduction of the African plate under the plate of the Aegean.

Along the whole volcanic arc impressive appearances of volcanic rocks can be observed. Most volcanic centres are not active today. The most active volcanic centres that constitute the epicentre of scientific interest are the volcanoes of Santorini, Koloumpo, Gyali and Nisyros.