The Tristan da Cunha group is one of a number of oceanic islands lying close to the Mid-Atlantic Ridge: new oceanic crust is being produced along this ridge, and as crust is formed, the plates on either side of the ridge are being pushed apart. This Sea Floor Spreading occurs at 5 - 9 cm per year, and there is some evidence that the rate is regular. Calculations show that nearby Nightingale Island is now 43,000,000 cm from the Mid-Atlantic Ridge, which means that it would have been above the ridge some 18 million years ago. As Tristan and Inaccessible are both substantially younger than this, a "Hot Spot" must exist below the islands, as they are still active.
The Geology of Inaccessible Island is difficult to study, owing to the large depth of peat bog which covers the plateau in the centre of the island. Nevertheless, the numerous cliffs afford excellent opportunities to study outcrops. The cliffs and shorelines are the most dynamic parts of the island, due to the nature of the sea and weather. Weathering is high and there is much evidence of slumping.
The cliff section at Blenden Hall appears to have slipped some 70 metres, probably attributable to an earthquake on the island. Much of the shoreline is a product of cliff slumping and the throwing up of material by the sea during storms.
The island is purely volcanic, and appears to have formed around 5 million years ago. Tristan da Cunha is relatively young at 1 million years, and Nightingale Island the oldest in the archipelago at 18 million years old. The main componentss of the island are basalt lava flows interbedded with pyroclastic material. These flows dip gently eastwards throughout the islands, indicating that the volcanic centre was to the west of the present island. Sub-surface soundings show a large shallow area at this point, supporting the theory.
The western and south-western parts of the island contain intruded masses of trachyte: these structures were injected into the active volcano and subsequently exposed by weathering. Dykes and sills of the same material can be seen in the south-western part of the island.
Parasitic cones and vents produced pyroclastic materials (e.g. ashes and agglomerates) which can be found arounf the edges of the island: at Blenden Hall the cliffs have a yellowish appearance due to the weathered ashes. A number of cinder cones are present on the eastern part of the plateau.
