Volcanic arcs form where subduction is occurring. In the Pacific NW, a line of volcanoes has formed from BC Canada down to Northern California
As one tectonic plate moves under the other, it creates a build up of layered fanned out sedaments called an acreationary prism.
These represent two types of volcanos (upside down). Shield volcanos are low profile and have more mellow eruptions. Strato volcanos are conical and can have powerful eruptions like Mount St.Helens