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The white Minoan pumice and ash
tuff covers most of the island


Until about 20 years ago, it was thought that the shape of the island before the Minoan eruption was circular with a central cone or a system of several overlapping volcanic shields. This model, however, has been considerably modified over the last years (see graphic to the left).

Evidence has been found that the present-day caldera that consists of several basins existed already before the eruption, at least in parts.
This evidence can be summarized as follows:

1. First of all, if the pre-Minoan island was a central cone the volume of the caldera collapse would be around ca. 60 km3 (Druitt and Francaviglia, 1992) which exceeds greatly all dense rock estimations of the total eruption volume, most probably 27-30 km3 DRE (Pyle, 1990). Such a huge discrepancy between the collapse and eruption volume is difficult to explain.

2. The Minoan deposits dip into the caldera at several points, especially in southern Thera and on Therasia, thus indicating that a depression existed before (Pichler and Friedrich, 1980; Heiken and McCoy, 1984; Friedrich, 1994).

3. Stromatolites occur in the deposits and are described by Eriksen and others (1990). Their analysis showed that probably in the northern basin a shallow sea-flooded lagoon existed before the eruption where these stromatolites grew.

4. Druitt and Francaviglia (1990) found deposits of the Minoan pumice plastered in situ at some places on the inner side of the present caldera wall thus proofing that the walls existed before. Further, they observe that other parts of the northern cliffs are relatively eroded and probably not from the Minoan collapse but from the previous Cape Riva collapse.

Friedrich (1994)'s and other recent reconstructions of pre-Minoan Santorini are qualitatively equal and based upon the one from Druitt and Francaviglia (1990). Druitt and Francaviglia (1991) claim that their model is the up-to-date most accurate one.
It shows a caldera that looked quite similar to the present one with a central volcanic island (Pre-Kameni). Its existence is supported by the presence of abundant black, glassy dacite blocks in the 3rd phase products that are similar to the Kameni lavas but absent in other lithologies on Santorini (Druitt and Francaviglia, 1991).

Judging that the total volume of lithics within the deposits is at least 5 km3 (after the published data of Heiken and McCoy (1984) and Pyle (1990)) they estimate that "the volume of the intracaldera volcano (pre-Kameni) must have been at least 3 km3" (Druitt and Francaviglia, 1991, p. 492).
Based on the assumption that the pre-Minoan caldera was shallow, they calculate the corresponding caldera collapse volume to be 22" 1 km3 or 25" 1 km3 if the volume of Pre-Kameni is included. If a Minoan tuff layer of 80-120 m within the present caldera is added the figure rises by 5-8 km3 and the total collapse volume then is about 29-34 km3. If the collapse volume equals the eruption volume, this number corresponds well to the dense-rock estimation of the total erupted magma-volume of 30-33 km3 published by Pyle (1990).