The seafloor between plate boundaries is far from featureless. Volcanic island chains are found far from any plate boundary due to the presence of hotspots (deep-seated and long-lived zones of volcanic activity) in the mantle. Some hotspots, such as the one beneath Iceland, are associated with divergent plate boundaries, which others lie in the middle of oceanic or continental plates. Chains of volcanoes often trail away from mid-ocean hotspots, with the oldest volcanoes, long extinct, now lying far away from the hotspot. These hotspot tracks are aligned along the direction of motion of the overlying plate. They change direction when the plate motion changes and may be interrupted when a new spreading ridge opens up, as it has between India and the Réunion Hotspot.
Some hotspot tracks link to areas where huge amounts of basalt flooded fromt he hotspot onto the surface long ago. The Tristan da Cunha hotspot is linked to flood basalts on both sides of the south Atlantic Ocean.