Geological evidence shows the Thera volcano erupted numerous times over several hundred thousand years before the Minoan eruption. In a repeating process, the volcano would violently erupt, then eventually collapse into a roughly circular seawater-filled caldera, with numerous small islands forming the circle. The caldera would slowly refill with magma, building a new volcano, which erupted and then collapsed in an ongoing cyclical process. Another famous volcano known to repeat a similar process is Krakatoa in Indonesia.
Immediately prior to the Minoan eruption, the walls of the caldera formed a nearly continuous ring of islands with the only entrance lying between Thera and the tiny island of Aspronisi. This cataclysmic eruption was centered on a small island just north of the existing island of Nea Kameni in the centre of the then-existing caldera. The northern part of the caldera was refilled by the volcanic ash and lava, then collapsed again.
On Santorini, there is a 60 m (197 ft) thick layer of white tephra that overlies the soil clearly delineating the ground level prior to the eruption. This layer has three distinct bands that indicate the different phases of the eruption. Since no bodies have been found at the Akrotiri site, Floyd W. McCoy, Professor of Geology and Oceanography, University of Hawaii, notes that the local population had advance warning of the impending eruption, leaving the island prior to its destruction. However, the thinness of the first ash layer along with the noticeable erosion of that layer by the first winter rains before the next layer was deposited, indicate that the volcano gave the local population only a few months warning.
Recent archaeological research by a team of international scientists in 2006 revealed that the Santorini event was much larger than the original estimate of 39 km3 of Dense-Rock Equivalent (DRE), or total volume of material erupted from the volcano, that was published in 1991. With an estimated DRE in excess of 60 km3, the volume of ejecta was up to four times what was thrown into the stratosphere by Krakatau in 1883 CE, a well-recorded event, placing the Volcanic Explosivity Index of the Thera eruption at approximately 7. The Thera volcanic events and subsequent ashfall probably sterilized the island, as occurred on Krakatau. Only the Mount Tambora volcanic eruption of 1815 CE released more material into the atmosphere during historic times.
The plinian eruption resulted in an estimated 30 kilometers (19 mi) to 35 kilometers (22 mi) high plume which extended into the stratosphere. In addition, the magma underlying the volcano came into contact with the shallow marine embayment, resulting in a violent steam eruption. The event also generated a 35 meters (115 ft) to 150 meters (492 ft) high tsunami that devastated the north coast of Crete, 110 kilometers (68 mi) away. The tsunami impacted coastal towns such as Amnisos, where building walls were knocked out of alignment. On the island of Anafi, 27 kilometers (17 mi) to the east, ash layers 3 meters (10 ft) deep have been found, as well as pumice layers on slopes 250 meters (820 ft) above sea level. Elsewhere in the Mediterranean there are pumice deposits which could have been caused by the Thera eruption. Ash layers in cores drilled from the seabed and from lakes in Turkey, however, show that the heaviest ashfall was towards the east and northeast of Santorini. The ash found on Crete is now known to have been from a precursory phase of the eruption, some weeks or months before the main eruptive phases, and would have had little impact on the island. Santorini ash deposits were at one time claimed to have been found in the Nile delta, but this is now known to be a misidentification.
There were significant climatic changes during the aftermath of the eruption. Based on the observed changes to the bristlecone pines of California; the bog oaks of Ireland, England, and Germany, and the grain crops of China, it is believed that the plume from the eruption added sufficient particulates to the atmosphere to have reduced temperatures worldwide.
from "Minoan Eruption," Wikipedia