Intense volcanic activity occurs from Pliocene and up to the present age, in the Southern Aegean volcanic arc region. Volcanic activity age inSousaki, Aegina and Poros spans in excess of 2 million years, while Methana, Santorini and

Nisyros are volcanoes formed during the last 1 million years. Milos is host to both old and new volcanoes.

Five million years ago, the land of Aegies, the subaerial Alpine mountain chain that occupied the area of the present-day Aegean Sea, was fragmented and sank below the waters of a beautiful blue sea.

The push of the Anatolian lithospheric block, which slips westwards along the North Anatolia fault, and the uplifting hot currents which are triggered by the northward subducting African lithosphere, stretch and dip large portions of the Aegean continental crust.

In the same age, volcanoes joined forces with land against the sea. The magma generated under the southern Aegean due to the subduction of the African lithosphere found escape paths in the Saronic Gulf where it formed the largest part of Aegina and the Methana peninsula, at the Central Aegean where it formed the Milos and Santorini island complexes and – finally, in the eastern Aegean where Nisyros and its surrounding islets as well as part of western Kos were formed.

1. Small volcanoes grew at Sousaki and Poros, in the Saronikos area; southern Aegina was formed approximately between five and two million years ago. One million years ago, moderate volcanic activity started to build up the Methana peninsula; it was completed in 230 B.C. by the last piece of land that was annexed to Kameni Hora (burnt land).
2. Ananes, Kimolos, Polyegos, Antimilos and Milos are in fact large volcanoes, the creation of which started three and a half million years ago; they were completed through the major eruption of Fyriplaka, Milos, 90,000 years ago. Milos is currently a large pot containing superheated flux. Minor leaks of steam and other volcanic gases can be observed in fumaroles and hot springs, found in abundance on the island. In fact, should an earthquake abruptly break the passage to the geothermal fluids, hydrothermal explosions may occur. These eject hot fluids, mud and rocks located around the passage, thus creating small craters. Several craters of this kind are present in Milos. The last hydrothermal explosion occurred on the island around the 1st or 2nd century A.D., at the Aghia Kyriaki site.
3. In the Santorini area, volcanic activity began approximately 2 million years ago, which built a large volcano at Christiana and small underwater volcanic centers in the southern Thira region emerged later; these are the current hills, all the way from Akrotiri village to Faros (lighthouse). Activity of this volcano was then transferred to the north, in the region between Thirasia and Northern Thira. A large shield volcano developed there, between 500 and 400,000 years ago. In this era, major explosions started to occur in Santorini. A major explosive event occurs at approximately 20,000 year intervals, blasting several billion tons of volcanic tephra. This volcano destruction and creation cycle has repeatedly occurred 12 times in Santorini, in the last 400,000 years. The last of major eruptions occurred approximately in 1614 B.C., an era when a society was thriving on the island, similar to the Minoan civilization in Crete. Within just a few days (4 to 6), a total of 130 billion tons of molten rock were blasted in the air (approximately 60 cubic km of magma), thus creating the current caldera of the island. Following the 1614 B.C. eruption, the volcano continued its activity and formed Palea and Nea Kameni islands. Between 1600 B.C. and 197 B.C., several underwater eruptions and outflows occurred; these formed a major submarine volcanic mountain; the Kameni islands are the peaks of this mountain. The first subaerial activity was recorded in 197 B.C. Since then, 8 more explosions have been recorded (46-47 B.C., 726, 1570-1573, 1707-1711, 1866-1870, 1925-1928, 1939-1941, 1950) and the Kameni islands – the youngest land territory in the Eastern Mediterranean – were subsequently formed. All these eruptions were moderate, not posing significant problems to inhabitants of the island. 1650 saw the occurrence of a major eruption outside the caldera, 8 kilometers northwest of Thira; this produced the Kolumbo submarine volcano. This eruption was the largest during the last millennium in the eastern Mediterranean; it killed around 70 inhabitants on Santorini. The floor of the Kolumbo submarine caldera is currently abundant in hydrothermal activity, where hot fluids at 220 centigrade created ‘chiminies’ like the ones located on the submarine volcanoes of mid-oceanic ridges. These ‘chiminies’ are host to bacterial colonies that survive in this aphotic environment through chemo-synthesis.

4. On the eastern edge of the volcanic arch, volcanic activity began 3.4 million years ago and accumulated new volcanic land in southern Kos as well as small islands to the south of Kos, such as Pergousa and Pahia. A massive eruption 161,000 years ago was the last volcanism activity in Kos. This is the largest explosion ever occurred in the Mediterranean. It effectively ejected hundreds of billions of tons of rock that spread as volcanic tephra all over the region. This currently covers half of Kos island and it is also found in Kalymnos, Tilos and the nearby shores of Asia Minor. After this occurrence, explosions continued further to the south, producing Nisyros, Strongyli and Yali islands. The latest major eruptions on Nisyros 40-35,000 years ago created an impressive caldera at the center of the island. Several hydrothermal explosions, fed by the superheated fluids contained in the Nisyros volcano subsurface, in the last 5 – 6000 years planted smaller and larger craters on the floor of this caldera. More recent ones occurred in 1871-73 and 1887, creating the Alexandros and Small Polyvotis craters. Hot gas blow-offs from hydrothermal craters are most impressive, as the deposit of majestic sulfur crystals is accompanied by the sound of boiling fluids and the acute smell of hydrogen sulphide.