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The lavas were erupted at elevations little above sea‐level and the typical magma channels are inferred to have been fissures up to tens of kilometres long. However, in other cases the magma conduits became more highly localized and, rather than producing an elongate fissure volcano, a ‘shield volcano’ of more or less circular geometry provided the lava source. In both cases the highly fluid lavas flowed widely and sub‐horizontally, in some cases for many tens of kilometres. In East Greenland the lavas cover over 65,000 km² with sequences up to 7 km thick (Brooks 2011; Figures 8 and 9, see Plate section). Lavas in the Faeroes, approximately coeval with those of East Greenland, are at least 5.5 km thick (Larsen 1988).

Lavas in the British Isles were far less voluminous and rarely built up successions more than 1.5 km thick. What were considered to be the earliest lavas attributable to the proto‐Icelandic plume are the (Danian) basaltic lavas and trachytic tuffs of the Eigg Lava Formation in the Hebridean ‘Small Isles’ (Pearson et al. 1996). However, the Lower Basalt Formation in Antrim (Northern Ireland) appears to be still older, being dated at 62.6 ± 0.3 Ma (Ganerød et al. 2010). Since the chemical composition of the Hebridean–Irish basalts differs significantly from that of the younger lavas of Faeroes and Greenland it is surmised that they arose from mantle with a composition distinct from those of other parts of the North Atlantic.

A central‐type volcano developed early at Rum (Inner Hebrides), in which high‐temperature (picritic) magmas attained shallow crustal levels. It has been argued that these came from a hot outer sheath of mantle that surrounded the plume itself although geochemical characteristics suggest that it arose from a compositionally distinctive part of the plume (Upton et al. 2002). Rapid erosion of the Rum volcano (Emeleus et al. 1996; Emeleus and Bell 2005) was followed by eruption of the Skye lava field (Hamilton et al. 1998).

In response to loading by the lava successions there was synchronous subsidence of the underlying crust. Thus, it appears that successive eruptions could build up lava sequences kilometres thick while each eruptive site was never far from sea‐level. The intervals between one eruption and the next are inferred to have lasted hundreds to thousands of years. With relatively fast weathering under warm, humid conditions, surface features of the lavas were rarely preserved. However, dendritic drainage patterns of river systems developed on the lava plains and inter‐flow fluviatile sediments are preserved (e.g. in the Hebrides). After each eruption, colonization of the lava surface by plants would have been rapid, probably ferns in the first stages followed by forest growth. Jointing patterns in the lavas provide evidence for chilling against trees; some relic fossils of these remain vertical as with the celebrated McCulloch's Tree in the south west of Mull in the Hebrides.

Figure 9 (a) Flat‐lying basalts of the Geikie Plateau Formation, Gåseland, East Greenland, looking towards 1980 m summit.

Source: Photo by W.S. Watt.

(b) View of the steep east coast of Greenland (the Blosseville coast) between ca. 67° and 69° N, composed of horizontal basalt lavas, west of the seaward flexing.

Source: Photo by B.G.J. Upton.