[Fraxinus]

Thera has erupted numerous times over the last 400,000 years, four of which were of such magnitude that the island collapsed and craters were formed.

Does this mean that some of the previous eruptions were even more explosive than the last major eruption?

[SunkenCiv]

The stuff just looked for and found on the sea floor would date to those large prehistoric and very ancient massive eruptions. I think the caldera seen today formed in an eruption over 100,000 years ago. There isn't any evidence for an eruption in historical times (and by historical, I include the Minoan/Mycenaean use of Linear A and Linear B, and cuneiform users in Anatolia) until circa 200 BC.