Tristan da Cunha is not on the Mid-Atlantic Ridge but is a surface expression of a deep-seated hot spot some 400 km east of the ridge (see map right). Hot spots (The Hawaiian Islands are another example) such as these receive magma from deep in the earth's mantle.

The Atlantic Ocean began to form when the African and South American plates were thrust apart about 120 million years ago. Ocean floor spreading continues at the Mid-Atlantic Ridge where relatively shallow rising magma erupts into basalt lava forming low ridges on the ocean floor, pushing Africa north-eastwards and South America north-westwards at a current rate of about 2cm a year.

The present-day Tristan Island Group marks the hotspot activity during the last 18 million years, first forming Nightingale Island (an extinct remnant in the latter stage of erosion), then Inaccessible Island (a younger extinct cone in the middle stage of erosion) and finally Tristan island itself (having the classic conical shape of an active shield volcano).

The Tristan Island Group forms the modern south-west end of a range of otherwise extinct submarine volcanoes, termed seamounts or guyots ( former volcanic islands now planed off by sea erosion) known as the Walvis Ridge. This largely submerged mountain range marks the movement of the African Plate north-east over the stationary hot spot. As the ocean floor spreads, the fixed hot spot leaves a trace-line of volcanoes, each of which becomes extinct as it moves away from the stationary mantle source. Eventually, as the African plate moves further NE, Tristan itself will become extinct and a new active hot spot volcano will build to the west.