Here's why:

* Subduction: The Cascade Range is formed by the subduction of the Juan de Fuca Plate (a small oceanic plate) beneath the North American Plate.

* Volcanism: The subduction process creates magma, which rises to the surface, leading to the formation of the volcanoes that define the Cascade Range.

* Earthquakes: The movement and friction between the plates also causes frequent earthquakes.

In summary: The Cascade Mountains are a prime example of a convergent plate boundary, specifically one where oceanic crust is subducting beneath continental crust.