Santorini (Greek: Σαντορίνη,officially Thira (Greek: Θήρα,is an island located in the southern Aegean Sea, about 200 km (120 mi) southeast from Greece's mainland. It is the largest island of a small, circular archipelago which bears the same name and is the remnant of a volcanic caldera. It forms the southernmost member of the Cyclades group of islands, with an area of approximately 73 km2 (28 sq mi) and a 2001 census population of 13,670. The municipality of Santorini comprises the inhabited islands of Santorini and Therasia and the uninhabited islands of Nea Kameni, Palaia Kameni, Aspronisi, and Christiana. The total land area is 90.623 km2 (34.990 sq mi). Santorini is part of the Santorini peripheral unit.
Santorini is essentially what remains after an enormous volcanic explosion that destroyed the earliest settlements, on a formerly single island, and created the current geological caldera. A giant central, rectangular lagoon, which measures about 12 by 7 km (7.5 by 4.3 mi), is surrounded by 300 m (980 ft) high, steep cliffs on three sides. The main island slopes downward to the Aegean Sea. On the fourth side, the lagoon is separated from the sea by another much smaller island called Therasia; the lagoon is connected to the sea in two places, in the northwest and southwest. The water in the center of the lagoon is nearly 400 m (1,300 ft) deep, thus making it a safe harbor for all kinds of shipping. The island's harbors all lie in the lagoon, and there are no ports on the perimeter of the island; the capital, Fira, clings to the top of the cliff looking down on the lagoon. The volcanic rocks present from the prior eruptions feature olivine and have a notably small presence of hornblende.
It is the most active volcanic center in the South Aegean Volcanic Arc, though what remains today is chiefly a water-filled caldera. The volcanic arc is approximately 500 km (310 mi) long and 20 to 40 km (12 to 25 mi) wide. The region first became volcanically active around 3–4 million years ago, though volcanism on Thera began around 2 million years ago with the extrusion of dacitic lavas from vents around the Akrotiri.
The island is the site of one of the largest volcanic eruptions in recorded history: the Minoan eruption (sometimes called the Thera eruption), which occurred some 3600 years ago at the height of the Minoan civilization. The eruption left a large caldera surrounded by volcanic ash deposits hundreds of feet deep and may have led indirectly to the collapse of the Minoan civilization on the island of Crete, 110 km (68 mi) to the south, through a gigantic tsunami. This theory is not, however, supported by chronology, in that the collapse of the Minoan civilization did not occur at the date of the tsunami, but some 90 years later. Another popular theory holds that the Thera eruption is the source of the legend of Atlantis.
The Cyclades are part of a metamorphic complex that is known as the Cycladic Massif. The complex formed during the Miocene and was folded and metamorphosed during the Alpine Orogeny around 60 million years ago. Thera is built upon a small, non-volcanic basement that represents the former non-volcanic island, which was approximately 9 by 6 km (5.6 by 3.7 mi). The basement rock primarily comprises metamorphosed limestone and schist, which date from the Alpine Orogeny. These non-volcanic rocks are exposed at the Profitis Ilias Mountains, Mesa Vouno, the Gavrillos ridge, Pyrgos, Monolithos, and the inner side of the caldera wall between Cape Plaka and Athinios.
The metamorphic grade comprises a blueschist facies, which results from tectonic deformation by the subduction of the African Plate beneath the Eurasian Plate. Subduction occurred between the Oligocene and the Miocene, and the metamorphic grade represents the southernmost extent of the Cycladic blueschist belt.
Volcanism on Santorini is due to the Hellenic Trench subduction zone southwest of Crete. The oceanic crust of the northern margin of the African Plate is being subducted under Greece and the Aegean Sea. which comprises thinned continental crust. The subduction compels the formation of the Hellenic arc, which includes Santorini and other volcanic centers, such as Methana, Milos, and Kos.