what is the subducting plate involved in the formation of the volcanoes of the cascades in north america?

Answer: Pacific plate getting subducted beneath the North American plate

Explanation:

Due to the collision between the Pacific plate and the North American plate, the Pacific plate being denser gets subducted below the less denser North American plate, forming a subduction zone known as the Cascadian subduction zone. The Pacific plate at greater depth undergoes partial melting and the magma eventually rises up towards the surface, forming the volcanoes in the western part of North America, which forms a volcanic arc commonly known as the Cascade arc.

The Cascade arc is comprised of about 20 volcanoes and the magma here are basaltic to rhyolitic in nature and eruptions are both explosive as well as effusive, depending upon the nature of magma and the mixing of water with it.