... that there is a myth about the volcanic genesis of Thera?

The volcanic genesis of  Thira seems to be evoked in the myth of Euphemus, one of the Argonauts, who, on the instructions of the local god Triton, threw a lump of earth into the sea, north of  Lake Tritonis (in Tunisia). This lump of earth was transformed into the island of Thira.

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God Triton