1.1. Geographic Setting of Santorini

Santorini, one of the Cycladic islands, is a historically active volcano and part of the South Aegean (or Hellenic) volcanic arc in the Aegean Sea, located about 120 km north of Crete.

Santorini, or officially called Thira, consists actually of a group of islands:
- The main island Thera (75,8 km², ca. 7000 inhabitants)
- Therasia (9,3 km², ca. 250 inhabitants)
- Aspronisi (0,1 km², uninhabited)
- Palea Kameni (0,5 km², 1 inhabitant)
- Nea Kameni (3,4 km², uninhabited)

Aerial view of the Santorini island group (courtesy of Birke Schreiber, homepage: Santorini f�r Individualisten). 
The ring-shaped main island Thera enclosing the caldera is well visible. The smaller island in the forground is Therasia. Aspronisi appears as a small spot right of Therasia. Within the caldera, the darker young volcanic islands Palea (on the right) and Nea Kameni (left). The towns of Oia, Imerovigli and Fira on the rim of the caldera walls appear as a white snow-like layer. The cream-colored Minoan pumice layer is best visible at the coastal plain near Oia.

Apart from a small non-volcanic basement represented in the south-eastern part of Thera these islands are composed of volcanic rocks from hundreds of eruptions during the last 2 million years, some of them being large caldera-forming events.
Palea and Nea Kameni formed during several lava eruptions in historic time within the caldera created by collapse of the magma chamber after the Minoan eruption. Nea Kameni is still active with the last eruption in 1950.

1.2. Tectonic setting of Santorini

The South Aegean Volcanic Arc and the tectonic setting of Santorini. After Friedrich (1994).

The caldera and structural setting of the Santorini volcanic field. After Heiken and McCoy (1984).

The volcanic complex of Santorini is the most active part of the South Aegean Volcanic Arc. This volcanic arc is about 500 km long and 20 to 40 km wide and extends from the mainland of Greece through the islands of Aegina, Methana, Poros, Milos, Santorini, Kos, Yali, Nisyros and the Bodrum peninsula in Turkey. It is characterized by earthquakes at depths of 150-170 km that mark the subduction of the African underneath the Eurasian plate, more precisely the Aegean subplate, at a rate of up to 5 cm per year in a northeasterly direction.
The subduction zone dips at angles of 30-40E and the boundary of the two plates is indicated by the Ionian, Pliny and Strabo deep-sea trenches south of Crete. While the Pliny and Strabo trenches are predominantly of strike-slip nature, the Ionian trench is related to thrusting.
The crust of the Aegean is continental with thicknesses in the range of 20-32 km; compared with average crustal thicknesses of the mainland of Greece and Turkey, 40-50 km, a stretching factor of about two due to tectonic extension is implied. The strongest extension seems to have been in the Cretan Trough whereas the Central Aseismic Plateau (CAP) on which the Cyclades are situated forms a relatively stable strongly faulted crust block. The volcanic centers of the Aegean Arc are placed along the southern rim of the CAP. They are aligned on five 60E NE-going seismic lineaments that are interpreted as deep lithosphere rupture zones which permit mantle-derived magma ascent.

The Cyclades are a metamorphic complex area, the Cycladic Massif, that formed in Triassic to Tertiary time and were folded and metamorphosed during the Alpine folding around 60 million years ago. The Cycladic Massif was a coherent landmass until tectonic movement of the plates and the beginning subduction caused its disintegration by subsidence and upheaval of single units following partial flooding in the late Miocene.
Volcanism in the Aegean Arc generally first occurred about 3-4 million years ago with the exception of the island of Kos where Keller and others report ignimbrites from Miocene age about 10-11 million years old.

The volcanic field of Santorini consists of Santorini, the Christiania islands around 20 km to the southwest and the submerged Columbus volcano 7 km to the northeast. It is partly situated on a southwest - northeast trending tectonic horst called the Amorgos Ridge. This fault block is visible in seismic profiles. The northwestern half of the volcanic field lies within the Anydros Graben.
On Santorini itself the major structural features visible on prevolcanic rocks are northeast striking thrust faults like the one underlying Mesa Vouno, passing from Perissa beach through the saddle between Mesa Vouno and Profitis Ilias until Kamari beach.

Most of the tectonic lines seen both on Santorini and on seismic profiles follow the general southwest-northeast trend. The most important one is named Kameni Line. It intersects the caldera and defines most of the known eruption centers. It aligns the Christiania islands, the Akrotiri peninsula and Palea and Nea Kameni. A parallel one, the Columbo line, perhaps identical with the Kameni line, passes through the centers of Megalo Vouno, the maar at Cape Columbo beach and the Columbo volcano. It seems that the rising magma has exploited existing deep-reaching tectonic fault zones.

---

continue with:
2.1. The pre-volcanic basement
3. Volcanism
3.1. Eruptive history
3.2. Documented historic activity: The Kameni islands