Magnetic Studies of the Santorini Tephra Deposits

This question appears to be critical in determining the relationship between the Santorini eruption and the disruption of the Minoan civili,sation on Crete in about 1400 B.C.

METHODS

It was, at the time of sampling, assumed that the tephra had been erupted at fairly high temperatures and that the ashes, bombs, etc. would have cooled in their present position. In this case, all of the tephra horizons would have an identical direction of remanence if they had all been erupted and cooled within one or two years of each other, but would have significantly different directions of remanence if the tephra cooling had taken place after intervals of five or more years. This arises because the Earth's magnetic field gradually changes direction and strength, geomagnetic secular variation, and such variations can thus be used to date magnetically archaeological materials for periods when such variations have been monitored, or to determine the relative dates where no absolute record  is yet available (Tarling 1975).

It was found extremely difficult to sample normal tephra using conventional palaeomagnetic techniques and all samples were taken from relatively large particles of tephra, generally greater than 50 cm³, or from walls constructed of tephra blocks. Each sample was carefully oriented, using a sun compass, as the differences in direction associated with 10 - 12 years may have only been a few degrees. The cores were then cut into 2.5 cm high, 2.54 cm diameter cylinders and their magnetisation measured with a Digico magnetometer. One core from each sample was then selected and subjected to incremental thermal demagnetisation (Table 1) in order to remove any magnetisation which had been acquired since deposition.

RESULTS

Although the directions of remanence from cores within individual samples were almost identical, it was clear that the natural remanence of different samples were extremely scattered. The demagnetisation experiments clearly show that this scatter was not due to instability as the individual cores all exhibited stability indices in the stable to extremely stable magnetisation range (Tarling & Symons 1967). Examination of the change in direction of remanence at successive heatings (Figure 2.) did not reveal any evidence for the removal of a systematic component over any temperature range, except possibly between 20 and 25° C, and the observed scatter for individual cores is almost completely attributable to instrumental errors. Detailed examination of the decay of remanence (Figure 3.) indicated that only very small components of magnetisation had been acquired during the last 3,000 years or so.

All remaining specimens were demagnetised at a temperature of 300° C, and all showed little or no change from their original direction. As with the specimens studied in detail, the component of stable remanence was entirely different between different samples and the very small unstable components similarly showed no consistency.

CONCLUSIONS

The high degree of between sample scatter cannot be attributed to instability and it is clear that each sample must have acquired its magnetisation prior to deposition in its present position. These results show, conclusively, that the tephra deposits so far examined were cold when emplaced in their present position and that the Minoan walls had not been subjected to any heating as a result of the deposition of tephra on or around them. Such a conclusion would be consistent with the layers being base surge deposits laid down after the original tephra had cooled to ambient temperatures.

It is hoped to obtain samples of water-lain tephra, preferably laid down between the different layers. Such material should have acquired a remanent alignment during deposition which may still allow the determination of the presence, or absence, of an age-difference between the different layers. It is also hoped that improved techniques, following this preliminary investigation, will allow more detailed sampling of these deposits to be made in the future.

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