economy and population. Santorini was ranked the world's top island by many magazines and travel sites, including the Travel+Leisure Magazine, the BBC, as well as the US News. An estimated 2 million tourists visit annually. In recent years, Santorini has been emphasising sustainable development and the promotion of special forms of tourism, the organization of major events such as conferences and sport activities. The island's pumice quarries have been closed since 1986, in order to preserve the caldera. In 2007, the cruise ship MS Sea Diamond ran aground and sank inside the caldera. As of 2019, Santorini is a particular draw for Asian couples who come to Santorini to have pre-wedding photos taken against the backdrop of the island's landscape. Geography Geological setting 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 . The basement rock is primarily composed of metamorphosed limestone and schist, which date from the Alpine Orogeny. These non-volcanic rocks are exposed at Mikro Profititis Ilias, Mesa Vouno, the Gavrillos ridge, Pyrgos, Monolithos, and the inner side of the caldera wall between Cape Plaka and Athinios. The metamorphic grade is 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 Volcanism on Santorini is due to the Hellenic 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 is thinned continental crust. The subduction compels the formation of the Hellenic arc, which includes Santorini and other volcanic centres, such as Methana, Milos, and Kos. The island is the result of repeated sequences of shield volcano construction followed by caldera collapse. The inner coast around the caldera is a sheer precipice of more than drop at its highest, and exhibits the various layers of solidified lava on top of each other, and the main towns perched on the crest. The ground then slopes outwards and downwards towards the outer perimeter, and the outer beaches are smooth and shallow.