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

The Minoan eruption on Santorini, Greece, was a major Plinian caldera-formi ng event that occurred at ca. 1640 BC. The eruption is generally divided in to four distinct phases: (1)Plinian phase: (2) phreatomagmatic base-surge: (3) ash-flow phase; and (4) non-welded ignimbrite. Many ballistic blocks, m ostly consisting of older lava-fragments with diameters up to several meter s, were ejected during phases 2 and 3. As caldera collapse followed the eru ption, the precise vent location is unknown, but its position during phase I has been inferred from the pumice isopachs of the Plinian deposits. The l ocation of the vent during the following phases has been estimated by measu ring thickness variations, flow directions within the pyroclastic Bows 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-de pendant distribution pattern. Blocks ranging between 0.15 and 1.60 m in dia meter are found throughout almost the entire deposit, but are concentrated in the southeastern parts and within a circle of 14 km diameter, thus sugge sting a maximum range of at least 6-7 km. The center of that circle is rega rded as the most probable ejection region. It is proposed that the subaeria l vent of phase 1 developed into an fracture opening to the SW along: the g eneral tectonic trend. Very large blocks, up to 3 m in diameter (or possibl y 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 the refore appears that at least one other vent opened in the northern half of the caldera, possibly as a new SW-NE-trending fracture. (C) 2001 Elsevier S cience B.V. All rights reserved.