3.1. Volcanism on Santorini / eruptive history

Volcanism in the area of Santorini (it was then only a small, non-volcanic island) started about 2 million years ago with the extrusion of dacitic lavas from vents in the area of the present-day Akrotiri peninsula and continued to produce different kinds of lavas and pyroclasts (Friedrich, 1994). However, the most characteristic type of activity over the last 200.000 years has been the cyclic construction of shield volcanoes interrupted by large explosive and destructive events like the Minoan eruption. In detail, the volcanic evolution of Santorini can be divided into six main stages (Druitt and others, 1989):

• Early centers of the Akrotiri peninsula
• Cinder cones of the Akrotiri peninsula
• Peristeria Volcano
• Products of the first eruptive cycle
• Products of the second eruptive cycle
• Kameni shield

-> 3.1.1. Summary table of Volcanic eruption sequence on Santorini

3.1.2. Akrotiri Volcanoes and cinder cones (ca. 2000-500 ka)

The oldest volcanic rocks are found on the Akrotiri peninsula and the Christiania islands. They are composed of dacitic lavas that updomed the sea floor and produced various flows and pyroclastic deposits. They have been strongly altered by hydrothermal activity. The updomed areas still are well visible on the Akrotiri peninsula (Lumaravi and Archangelos hills). From marine fossils embedded in the tuffs, Seidenkrantz and Friedrich (1992) concluded a minimum age for the Akrotiri peninsula of 2 million years.
At the localities of Balos ("Red Beach"), Kokkinopetra, Mavropetra and Mavrocachidi, basaltic to andesitic strombolian scoria cones are exposed that flank the Akrotiri volcanoes. They might be related to the later volcanic centers of northern Thera (Megalo Vouno and Mikro Profitis Ilias). Druitt and Sparks (1996) recently dated them to 344" 25 thousand years.

At this early stage the Akrotiri peninsula probably was a separate island and not yet connected to the non-volcanic island.

3.1.3. Peristeria and Thera Volcanoes (ca. 530-180 ka)

Between 530 and 430 ka a large stratovolcano called Peristeria volcano formed in the north of the present caldera. The cinder cones on the Akrotiri peninsula are probably later products of these shields, too. Today, the remnants comprise most of Megalo Vouno and Mikro Profitis Ilias. At about 350-250 ka another shield volcano grew within the center of the present-day caldera, called Thera volcano. Some of its lava flows are visible at Cape Alai and Cape Alonaki near Fira. The formation of that shield united the former isolated centers and the limestone massif to a compact island.

3.1.4. First eruptive cycle (ca. 360-180 ka)

Druitt and others (1989) recognized two cycles of large explosive eruptions with chemical transition from mafic to silicic lavas. The first one began with the Cape Therma 1 eruption, a thick scoria flow deposit in southern Thera, and includes the deposits of Cape Therma 2, Cape Therma 3. It terminated with the rhyodacitic and very prominent Lower Pumice 1 and 2 (BU₁ and BU₂) eruptions at about 200 and 180 ka. All of these were probably erupted from vents of the Thera Volcano. The first cycle concluded with caldera collapse and created the BU-caldera. Rests of this caldera can be seen at the cliffs below Fira where a discontinuity separates the BU₂ deposits from the overlying layers.

3.1.5. Second eruptive cycle (ca. 180 ka - 1600 BC) and the northern lava shields

The second cycle comprises at least 7 bigger eruptions: Cape Thera, Middle Pumice at 60 ka, Vourvoulos, Upper Scoriae 1 and 2 at circa 40 ka, Cape Riva at 21 ka and the Minoan eruption at ca. 1645 BC. Parallel to or preceding these eruptions different lava shields grew in the northern part of the caldera: Simandiri lavas at ca. 170 ka, the Skaros and the Therasia shields probably between 60 and 20 ka.
Caldera collapse happened at least 3 times during that cycle and destroyed the biggest parts of the existing lava shield complex. The first one occurred after the Middle Pumice, Vourvoulos, Upper Scoriae 1 or 2 or incrementally during all of these eruptions. It created the so-called Skaros caldera that successively was filled by the almost horizontal lava flows of a large stratovolcano which are preserved at Cape Skaros. The Skaros caldera walls are visible at the steep contact of the Skaros lavas against the older cliff walls in north Thera.
The Skaros shield was probably destroyed by collapse during the Upper Scoria 2 or Cape Riva eruption. Finally, another collapse after the Minoan eruption (ca. 1645 BC) deepened and created the present-day caldera.

--> 4.1.1. Models of the pre-Minoan island

3.1.6. The Kameni shield (197 BC - today)

The formation of the post-Minoan Kameni islands, dacitic lava shields, is recorded by human observation and documented by historians.
The ancient writer Strabo is the first to mention volcanic eruptions inside the caldera and described the rising of a new small island during the year 197 BC. After that, at least 8 eruptive phases followed in the years 46/47, 726, 1570-1573, 1707-1711, 1866-1870, 1925-1928, 1938-1941 and 1950 as the most recent eruption.
--> 3.2. Historic Eruptions of the Kameni islands