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 17.15 m (56.27 ft). It is often washed over by large storm waves, particularly in winter. There is a small ledge of , known as Hall's Ledge, from the summit on the rock's western face. It is the only named geographical location on the rock. The nearest point on land from Rockall is 301.3 km, or 162.7 nmi, east at the uninhabited Scottish island of Soay in the St Kilda archipelago. The nearest inhabited area lies 303.2 km, or 163.7 nmi, east at Hirta, the largest island in the St. Kilda group, which is populated intermittently at a single military