Final answer:
The west coast of North and South America features convergent plate boundaries where subduction occurs, including the transform boundary of the San Andreas Fault and the subduction zone of the Juan de Fuca Plate that led to the formation of the Cascade Range.
Step-by-step explanation:
The continental margin found off the west coast of North and South America is a convergent plate boundary. Starting from the Gulf of California, northward to British Columbia, a range of plate boundaries and tectonic processes shape the margin. Along the west coast of North America, the Pacific Plate grinds northwestward past the North American Plate along the transform boundary of the San Andreas Fault.
Farther north, along the coast of Washington and Oregon, the small Juan de Fuca Plate is subducting beneath the North American Plate; a process which significantly influences the fate of the Juan de Fuca Plate leading to its eventual consumption into the Earth's mantle. The Cascade Range, including Mt. St. Helens, was formed by tectonic processes associated with this subduction zone.
Off the west coast of South America, the Nazca Plate is also subducting beneath the South American Plate, leading to the uplift of the Andes Mountains. This subduction process varies in angle and depth, as evidenced by earthquake data, contributing to the dynamic geologic activity in the region such as the creation of the Andes Mountains and the ongoing seismic activity.