Here's why:

* Santorini is part of the Aegean Sea. This sea sits atop the Aegean Plate, a smaller tectonic plate that is part of the Eurasian Plate.

* Subduction Zone: The Santorini volcano is a result of the subduction of the African Plate beneath the Aegean Plate. This process creates a volcanic arc, with Santorini being one of the prominent volcanoes in this arc.

So, while technically the Aegean Plate is part of the Eurasian Plate, it is the smaller, more localized plate responsible for the specific geological activity that formed the Santorini volcano.