Mainly summarized from Heiken and McCoy (1984), Bond and Sparks (1976), Druitt and others (1989) and Friedrich (1994). Part of the masters'  and phd theses by Tom Pfeiffer (1999, 2003), geology department Aarhus, DK, under Prof. W. Friedrich.

2.1. The prevolcanic basement

Simplified geologic map of Santorini. After Friedrich (1994).

Non-volcanic rocks are exposed on Santorini at the Profitis Ilias Mountain, Mesa Vouno, the Gavrillos ridge, Pirgos, Monolithos and the inner side of the caldera wall between Cape Plaka and Athinios.
They represent a former non-volcanic island of about 9x6 km extension similar to neighboring Cycladic islands like Anaphe, Ios or Amorgos. The rocks consist of metamorphosed limestone and schist from Triassic to Tertiary time folded during the Alpine folding. The observed metamorphose grade is a blue-schist facies resulting from tectonic deformation by the plate collision in the Oligocene to Miocene. At Athinios a 9.5 million year old Miocene granite intrusion has been found; it is part of the Cycladic Granitic Province and is the source of ore minerals including talcum, chalcopyrite, chrysocolla, magnetite and others.

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continue with:
3. Volcanism
3.1. Eruptive history
3.2. Documented historic activity: The Kameni islands