Introduction
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).
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