Here's why:

* Convergent Boundaries: These occur when two tectonic plates collide. In the case of Mount Rainier, the Juan de Fuca Plate (oceanic) is subducting (diving) beneath the North American Plate (continental).

* Subduction: The denser oceanic plate sinks below the continental plate, melting rock in the mantle and creating magma. This magma rises to the surface, erupting as volcanoes like Mount Rainier.

Mount Rainier is part of the Cascade Volcanic Arc, a chain of volcanoes formed due to the ongoing subduction along this boundary.