Volcanoes can occur either at (a) _____ such as occur in Washington and Oregon (Mt. St. Helens, Mt. Hood) or at mid-plate (b) _____ such as at Hawai'i?
1) (a) convergent boundaries, (b) hotspots
2) (a) divergent boundaries, (b) subduction zones
3) (a) transform boundaries, (b) rift valleys
4) (a) collision boundaries, (b) volcanic arcs