Benue Trough the Central African Shear Zone and for the volcanic island of St. Helena in the Atlantic ocean. Three periods of magmatic activity (volcanic action) have been identified, 147–106 Ma, 97–81 Ma and 68–49 Ma. The first is prominent in the north of the trough, and contemporary with magmatism in Brazil, probably occurring during a period of crustal extension before the Atlantic started to open. The second is found only in the south of the trough, and may belong to a period when the extension of the Atlantic had slowed down, ending with a period of compression. The third and last
Rockall Ledge, each secured by four 80-tonne rock-anchor bolts. Establishing that the rock is part of the United Kingdom and its development as a light beacon facilitated the incorporation of the island into the District of Harris in the County of Inverness in the Island of Rockall Act 1972 and reinforced the UK Government's position with regard to seabed rights in the area. In 1978, eight members of the Dangerous Sports Club, including David Kirke, one of its founders, held a cocktail party on the island. Former SAS member and survival expert (the Irish born) Tom McClean lived on the island