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 climate without heat or cold extremes. Rockall is made of a type of peralkaline granite that is relatively rich in sodium and potassium. Within this granite are darker bands richer in iron because they contain two iron-sodium silicate minerals called aegirine and riebeckite. The darker bands are a type of granite that geologists have named "rockallite", although use of this term is now discouraged. In 1975, a mineral new to science was discovered in a rock sample from Rockall. The mineral is called bazirite, named after the chemical elements barium and zirconium. Bazirite has the chemical composition BaZrSiO. Rockall forms