On Earth, there are only two different ways to create major volcanoes.
One way is subduction. It occurs when older/heavier rock sinks beneath younger/lighter rock. As the rock descends, it liquefies into magma, climbing up to the surface as a mountain concealing a deadly magma chamber. This is why the Pacific Ring of Fire ranks highly among the most dangerous of all the volcanoes.
The other way is rarer and more stationary. The volcanoes of Hawaii, Iceland, the Galapagos and Yellowstone form from plumes of mantle beneath the surface, making them "hotspot volcanoes". When a plate moves, the plume stays put.
Let's say someone thought up of a mantle plume near or even in a subduction point. Is this geomechanically possible? If yes, then how deadly would a subductive hotspot volcano be?