What type of plate boundary is Mount St. Helens?

Mount St. Helens resides within the Cascade Mountain range, which was formed by a subduction plate boundary. However, this particular boundary is a relatively specialized type, in that three different tectonic plates converge at this location.

The Pacific Plate, the North American Plate and the much smaller Juan de Fuca plate all converge at the Cascade Range in an area known as a triple plate junction. This phenomenon causes very complex interactions between the plates, which is part of what experts believe to have contributed directly to the sudden and violent nature of the Mount St. Helens 1980 eruption, which resulted in 123 deaths.