A map of the South Atlantic showing the position of the Parana and Etendeka flood basalts, Walvis Ridge, Rio Grande Rise and Tristan da Cunha, the current position of the plume. Between 120 and 60 Ma the Tristan da Cunha plume underlay the Mid-Atlantic Ridge and produced two aseismic ridges, the Walvis Ridge on the east side of the spreading cetntre and the Rio Grande Rise on the west side (after White and McKenzie, 1989) Phase 2 of the volcanism produced by the arrival of a new plume head. A

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... 000 to 1,200 km diameter plume head rises beneath continental crust. As it reaches the top of its ascent it begins to flatten and melt, leading to the formation of a flood basalt (phase 1). Melting is due to adiabatic decompression and continues as long as the head continues to rise and flatten. Arrival of the plume head also leads to uplift, which places the lithosphere under tension, as shown by the arrows. The final diameter of the flatten plume head is 2,000 to 2,500 km. Tension introduced by the plume head can lead to run-away extension and the formation of a new ocean basin. If this happen mantle from the hot plume head is drawn into the spreading centre leading to the formation of thickened oceanic crust (phase 2 volcanism). This second phase of volcanism may be more extensive than the first. The plume tail and structure of the head have been omitted to simplify the diagram and the vertical scale is exaggerated (after Campbell, 1998).