| Because hot spots (i.e hawaiian) form volcanoes above them. When the plate moves, the volcano moves but the hot spoy stays in the same place and produces a new volcano. Over time, you end up with a long chain of volcanoes. The oldest is furthest away from the hot spot and they get younger as they get closer. Since you have a chain of volcanoes that all formed in the same place, you can follow the plate motion. |