Vent development during the Minoan eruption (1640 BC) of Santorini, Greece, as suggested by ballistic blocks

Abstract

The Minoan eruption on Santorini, Greece, was a major Plinian caldera-forming event that occurred at ca. 1640 BC. The eruption is generally divided into four distinct phases: (1) Plinian phase; (2) phreatomagmatic base-surge; (3) ash-flow phase; and (4) non-welded ignimbrite. Many ballistic blocks, mostly consisting of older lava-fragments with diameters up to several meters, were ejected during phases 2 and 3. As caldera collapse followed the eruption, the precise vent location is unknown, but its position during phase 1 has been inferred from the pumice isopachs of the Plinian deposits. The location of the vent during the following phases has been estimated by measuring thickness variations, flow directions within the pyroclastic flows and impact directions of ballistic blocks. In this study, the vent location has been estimated by analysis of the size distribution of the ballistic blocks. Phase 2 blocks show no clear size-dependant distribution pattern. Blocks ranging between 0.15 and 1.60 m in diameter are found throughout almost the entire deposit, but are concentrated in the southeastern parts and within a circle of 14 km diameter, thus suggesting a maximum range of at least 6–7 km. The center of that circle is regarded as the most probable ejection region. It is proposed that the subaerial vent of phase 1 developed into an fracture opening to the SW along the general tectonic trend. Very large blocks, up to 3 m in diameter (or possibly more) were also ejected during phase 3, with the largest ones being found only in the northern half of the island group near the caldera rim. It therefore appears that at least one other vent opened in the northern half of the caldera, possibly as a new SW–NE-trending fracture.