There is not only one Volcano in Santorini
The Santorini islands are chiefly composed of Pleistocene to recent volcanic rocks. Only the southeastern part of the main island, Thira, is built up of pre- Pleis- tocene, mostly non-volcanic basement rocks. In the lower Pleistonce, about 1 million years ago, subaerial volcanism commenced and continued up to the present time, interrupted by numerous longer and shorter periods of volcanic inactivity. By these volcanic events thick series of lavas and pyroclastic materialwere produced, enlarging the former small non-volcanic island to the size of the later Stronghyle island.
The Pleistocene to recent volcanic activity is ralated toat least seven main eruptive centres, most of them consisting of several volcanoes. The activity of these main eruptive centres, namely the:
Mikro Profitis Ilias volcano,
Megalo Vouno volcanoes,
The post-caldera Kamari volcanoes,
is compiled(with the exception of the Kameni volcanoes) in a volcano-stratigraphic key.
Also, there was a submerged volcano NE of Thira, the Kouloumbo volcano, which sunk after the devastating explosion of 1650 A.D.
The largest volcanic explosions on Santorini took place in the years 197 B.C., 1866, 1925 and the last in 1949-50, without destructive consequences. On the contrary, the non-volcanic earthquake of 1956 caused the death of 49 people and extended to buildings in all of the towns.
The volcanic events were first restricted to the southern part of present Thira, that is, to the Akrotiri peninsula. In that region extrusions of highly viscous dacitic lavas occurred, which built up cupolas or clusters of severals coalescenced endogenour domes, e.g. the Lumaravi-Akrotiri complex or the fault dissected dome of Cape Akrotiri. After a period of volcanic rest, several small volcanoes were formed in the Akrotiri region, producing andesitic to quartz-latiandesitic volcanic rocks.
The caldera walls from Cape Akrotiri up to Cape Tourlos, to the north of Fira town, mainly consist of thick pyroclastic sequences, which overlie the products of the Akrotiri volcanoesand which belong to the great Thira volcanic complex. Its main eruptive centres must have been situated in the region of present-day Nea Kameni. The activity of the Thira volcanoes started with paroxysmal eruptions, which produced a bipartite ignimbrite sheet, up to 50m thick.
At the end of the Middle Pumice Series,(to the north and to the south of the steps down to the harbour of Fira this pumice-fall deposit is strongly welded, forming a conspicuous black sheet of ignimbrite-like appearance) the eruption of the Thira volcanoes, again a volcano-tectonic subsidence occurred, by which a small caldera was formed in the area northwest of the present town of Fira. Strongly marked disconformities in the volcanic sequences of the caldera walls directly south of Mikro Profitis Ilias and north of Cape Fira(gr:apano Fira), respectively, represent the ancient walls of that old caldera, which is known as the Skaros.
Mikro Profitis Ilias volcano
In the north part of the caldera, a new stratovolcano, grew up and reached gradually a height of at least 320 metres. This volcano, had two active craters, are predominantly lavas interbedded with pyroclastics. In mineralogical and petrochemical respects, they are very similar to the volcanic rocks of the neighbouring Megalo Vouno volcanoes.
Megalo Vouno volcanoes
After a period of volcanic inactivity of at least 100,000 years, the Megalo Vouno volcano burst into new life. During its younger eruption phase, a conspicuous black layer was spread over a great part of the island. Naturally, the volcanic rocks of the younger period of Megalo Vouno, overlie those of the older one.
During the younger activity of Megalo Vouno, a new vent was opened by a tremendous eruption. The site of this eruption centre can be localized in the region of the present northern channel between Therasia and northern Thira. The main products of this northern Therasia volcano are represented by a bank of red and black scoria. This significant bank of the red and blank scoria, extended in the upper parts of the volcanic sequences on all three ring-islands, Therasia, Thira and Aspronisi.