Here's why:

* Subduction Zones: Mount St. Helens is part of the Cascade Range, a chain of volcanoes formed along the Cascadia Subduction Zone. This zone is where the Juan de Fuca Plate (an oceanic plate) is sliding underneath the North American Plate (a continental plate).

* Magma Generation: The subduction process causes the oceanic plate to melt, generating magma. This magma rises to the surface, eventually erupting as volcanoes like Mount St. Helens.

Therefore, the type of plate margin responsible for Mount St. Helens is a convergent plate margin, specifically a subduction zone.