The answer, and I am sure it is going to annoy you, is "nobody knows for sure." Well, at least nobody I have talked with : )
The reasons for this are several:
- The eruption happened around 1470 BC so it was a long time ago,
- The eruption eradicated all of the people in the immediate vicinity, so there was no-one left from the islands above the volcano to ask (of course, I would need a time machine to do this, but you get the point,)
- There were no bodies or other pieces of evidence left to measure from,
- The volcanic eruption caused widespread devastation across the Mediterranean coasts and islands (even destroyed the civilization on Crete, some 75 miles away,)
- This erruption may have caused a famine and starvation in Egypt,
- This eruption, if other eruptions are any indicator, may have disrupted weather patterns across the globe for a while, ruining crops and causing worldwide food shortages,
- We have never had a volcano that big explode again to measure against.
So, I am sorry I cannot be more specific than to say "a lot," but records aren't the greatest from the era and a lot of people didn't know a volcano had even erupted and was causing the damage. Think about it: anyone close enough to see the volcano erupt was dead!
Thera is an island in the Aegean Sea, about 200 kilometers south east of Greece mainland and 110 kilometer north of Crete. Currently this island is known by the name Santorini.
Thera had a well developed civilization called Minoan Civilization. All the people of this civilization living on Thera were killed in a massive volcanic eruption that took place in 1470 B.C. No estimates are available of the people who died in this volcanic eruption. What is certain is that there were no survivors on Thera Island.
The Minoan civilization on Thera Island and the volcanic eruption that destroyed this civilization is considered by many to be the source of origin of the legend of Atlantis.