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This PDF file includes: Methods: Collection and Processing SOM Text Figs. DR1 to DR7 Tables DR1 to DR3 References Methods: Core Collection and Processing The cores were collected using an underwater pneumatic hammer attached by airhoses to a tending compressor at the surface and operated by divers below (Fig. DR1). Once drilled, the cores were capped and removed using airlift bags. After collection, each core was photographed, described, and sampled at 1 cm intervals (unless sediment character required larger intervals), and subsampled for granulometry, micropaleontological analysis, and dating (Figs. DR2-DR6, Table DR1 and DR2). Chronological ages were based on (depending on availability in core) ceramics, OSL, and C14 (Tables DR1 and DR2). Granulometry was completed using Laser particle Analyzers (on a Beckman laser Coulter counter and Malvern Multisizer). Values from Malvern Multisizer varied from Beckman by a maximum of +/-1%. Micropaleontological collection, analysis, and statistics were based on the methods of Fishbein and Patterson (1).

The well‐known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large‐scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.

Tsunami were generated during the Late Bronze Age (LBA) eruption of the island of Thera, in the southern Aegean Sea, by both caldera collapse, and by the entry of pyroclastic surges/flows and lahars/debris flows into the sea. Tsunami generated by caldera collapse propagated to the west producing deep-sea sedimentary deposits in the eastern Mediterranean Sea known as homogenites; open-ocean wave heights of about 1.9-17 m are estimated. Tsunami generated by the entry of pyroclastic flows/surges and lahars/debris flows into the sea propagated in all directions around the island; wave heights along coastal areas were about 7 -12 m as estimated from newly identified tsunami deposits on eastern Thera as well as from pumice deposits found at archaeological sites on northern and eastern Crete.