Here's a breakdown:

* The Juan de Fuca Plate (an oceanic plate) is subducting (sliding) beneath the North American Plate.

* As the oceanic plate descends, it melts, and the molten rock rises to the surface, creating volcanoes.

* This process has been ongoing for millions of years, resulting in the formation of the Cascade Volcanic Arc, which includes the Cascade Mountains.

This type of convergent boundary is called ocean-continent convergence.