Outline the formation of hotspots and explain their relationship to plate movement.

Hotspots, which occur away from plate boundaries, develop when the decay of radioactive elements in the Earth's core heat magma directly above it in the mantle. The heated magma expands and becomes less dense that the surrounding magma, so it rises to the surface in a magma plume. If there are lines of weakness in the crust, the magma can break through and the lava erupts on the surface, where it cools on contact with the ocean. Over time, the volcano builds up and can rise above sea level. The hotspot is stationary, so, when the oceanic plate moves over it (due to convection currents in the mantle), a number of separate volcanoes are formed, creating an island chain. The volcano above the hotspot is active, whereas the older volcanoes which are no longer above the hotspot are dormitory. Where plate movement is slower, a volcano can be above the hotspot and therefore active for long periods of time. By contrast, a faster rate of plate movement means that the volcano is not active for a long period of time, but more volcanic islands are formed, such as the Hawaiian Islands. Marine erosion and subsidence can reduce these hotspot volcanoes to seamounts, which may then form coral atolls over time.