A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centers known as hotspots and probably also to have caused flood basalts. It is a secondary way that Earth loses heat, much less important in this regard than is heat loss at plate margins. Some scientists think that plate tectonics cools the mantle, and mantle plumes cool the core. Two of the most well known locations that fit the mantle plume theory are Hawaii and Iceland as both have volcanic activity.