Mt. St. Helens is in Washington State (about 90 miles from where I am typing this) and is part of the Cascade range. The Cascade Mountains are young in geologic terms, and growing. They were created by a giant subduction zone where the Juan de Fuca plate is subducted (forced under) the North American continental plate, which is forced upward. The resultant heat and magma that is generated is pushed and formed along vents, and Mt. St. Helens was built over time from a long series of these lava producing eruptions. So in other words, it is constructive forces that made the mountain.