Hotspot (geology)
The problem arises with volcanoes which are not at the edges of plates. In the United States, the Yellowstone Caldera is obviously active. It has geysers caused by constant geothermal energy activity, and the floor of the crater moves up and down as much as 50 meters in a few years. The Pacific Ocean has six or seven chains of islands formed by volcanic activity. Most of these cannot be explained simply as being on the edge of two or three plates.