Figure 6. Summary of the development of Santorini caldera before and during and following the LBA eruption.

(a–c) Pre-eruptive morphology of the volcano¹⁸,¹⁹,²⁰,²¹ and the main phases of the eruption¹,²,³,⁴,⁵. Before the eruption there existed an ancient caldera in the northern half of the volcanic field¹⁸,¹⁹. This caldera was lagoonal, as shown by the presence of fragments of ancient travertine, stromatolites and brackish to marine fauna in the LBA ejecta²⁰,²¹. There was also an andesitic edifice within this caldera⁵. In eruptive phase 1 a Plinian eruption took place, which in phase 2 was joined by the production of syn-plinian pyroclastic surges. In phase 3, eruption of ‘cold' phreatomagmatic pyroclastic flows constructed a large tuff cone that filled the old caldera, cutting it off from the sea. In phase 4, eruption of hot pyroclastic flows took place from multiple subaerial vents, forming at least three ignimbrite fans (NW, E and S), and associated caldera collapse enlarged and deepened the ancient caldera. The main eruptive vents are shown in these figures as red stars (locations well constrained for phases 1 to 3, but speculative for phase 4). Black arrows show schematic emplacement vectors for the pyroclastic flows of phases 3 and 4. (d,e) Post-eruptive opening of the NW and SW straits (based on the present research). At the end of the eruption the caldera was dry and isolated from the sea, probably due to thick accumulations of LBA tuff. The sea first broke through to the NW, where a combination of water erosion and landslip carved out the NW strait and flooded the caldera (blue arrows in d) in less than a couple of days. Submarine landslides (black arrows in e) then opened up the SW straits once the caldera was largely flooded. (f) The present-day volcanic field.