Here's why:

* Location: Komagatake is located in the Japanese archipelago, which is known for its volcanic activity due to the subduction zone where the Pacific Plate dives beneath the Eurasian Plate.

* Volcanic Activity: The volcanic activity in the area is characterized by andesitic to dacitic eruptions, which are typical of volcanoes formed through subduction.

* Hotspots: Hotspots are areas of volcanic activity caused by plumes of hot mantle material rising from deep within the Earth. While there are hotspots in the Pacific Ocean, Komagatake's location and volcanic characteristics strongly suggest it's formed through subduction.

Therefore, Komagatake is a volcano formed by the subduction of the Pacific Plate under the Eurasian Plate.