Question

Why are active volcanoes, such as Mt. St. Helens, usually located in mountain ranges along the boundaries of tectonic plates?

Answer 1

i'll answer this the best and simpliest way i can. active volcanoes reside on mountain ranges  along the boundaries of tectonic plates because tectonic plates move and the two plates in question have the ability for one to go under or slide against each other releasing lava. if i remember this correctly.