One of the most well-known and heavily studied hotspots in the world, the Hawaii plume is responsible for the creation of the Hawaiian – Emperor seamount chain , an over 5,800 kilometres (3,600 mi) long chain of volcanoes, four of which are active , two of which are dormant , and more than 123 of which are extinct , many having since been ground beneath the waves by erosion as seamounts and atolls .