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 3,600 years ago at the height of the Minoan civilization.Volcanism on Santorini is due to the Hellenic Trench subduction zone southwest of Crete.
The total land area is 90.623 km Santorini is essentially what remains after an enormous volcanic eruption 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. 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.
In recent years, Santorini has been voted one of the world's most beautiful islands (2015 Travelers” Choice Awards Reader's Choice Awards, amongst others).
Santorini remains the home of a small, but flourishing wine industry, based on the indigenous Assyrtiko grape variety.
It is the most active volcanic centre 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 most famous eruption is the Minoan eruption, detailed below.
Eruptive products range from basalt all the way to rhyolite, and the rhyolitic products are associated with the most explosive eruptions.
The ground then slopes outwards and downwards towards the outer perimeter, and the outer beaches are smooth and shallow.
Beach sand colour depends on which geological layer is exposed; there are beaches with sand or pebbles made of solidified lava of various colours: such as the Red Beach, the Black Beach and the White Beach.
The eruption left a large caldera surrounded by volcanic ash deposits hundreds of metres 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.