Voluminous volcanism took place west of Greenland and northwest of the British Isles in the Palaeocene, 6–7 Ma before the start of sea-floor spreading in the northern North Atlantic. Very hot (picritic) magmas were produced simultaneously in such widely separated areas that they could not all have been above the central stem of the same mantle plume. A single, large plume head with temperatures of c. 1500°C over a diameter of c. 2000 km can explain the observations if surface volcanic activity took place only where the lithosphere was thin. The presence of picrites does not indicate proximity to the plume stem.

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