Most volcanic eruptions occur near the boundaries of tectonic plates, but there are some exceptions. In the interior of some tectonic plates, magma has been erupting from a relatively fixed spot below the plate for millions of years. As the plate continuously moves across that spot, a trail of progressively older volcanic deposits is left at the surface. The Hawaiian Islands are a good example of this. The island of Hawai‘i currently sits above the active hotspot, while a chain of older (and no longer active) island volcanoes extend to the northwest, in the direction of plate movement. A few hotspots (like the one in Iceland) have also been found at diverging plate boundaries.
Scientists don’t fully understand how and why hotspots occur, and there is vigorous scientific debate about their origins. A frequently-used hypothesis suggests that hotspots form over exceptionally hot regions in the mantle, which is the hot, flowing layer of the Earth beneath the crust. Mantle rock in those extra-hot regions is more buoyant than the surrounding rocks, so it rises through the mantle and crust to erupt at the surface.
Hotspots and their trails on the earth’s surface do not develop suddenly (within the span of a human lifetime, for example). Scientists are only able to identify hotspots because of their relatively fixed locations beneath the tectonic plates, which produce tracks of surface volcanism spanning millions of years.