A circle of volcanoes marks the boundaries of the Pacific tectonic plate. Scientists call it a Ring of Fire.
A tectonic plate is a massive slab of solid rock. Many, but not all, volcanoes are located near the boundaries of tectonic plates. Some of the world’s largest volcanoes are in or around the Pacific Ocean. The biggest of them all are in the middle of the Pacific Ocean: the Island of Hawaii. Hawaiian volcanoes have their roots at a depth of at least 30 miles.
A hotspot is magma that has burst through the earth's mantle, the layer of the Earth between the outer core and the crust. When the hotspot gets hot enough, the heat escapes through volcanic forces. Over time the Pacific plate has moved to the northwest, creating the Hawaiian Islands. Tourists flock to see eruptions at Kilauea, the most active crater on the Island of Hawaii.
Other hotspots around the world are beneath continents. For example, there is Yellowstone National Park in Wyoming. This hotspot has been in the same place for millions of years.
Geological evidence suggests that a long time ago, the crust (the outermost layer of the earth) of Europe was stretched by a hotspot in the mantle underneath. The North Sea was created farther east, where the Atlantic lies today. The Atlantic finally opened, possibly a result of a very large hotspot.
[Source: The Kingfishers Young People's Book of Planet Earth]