Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 25

Palaeocene–Eocene basaltic ashes occur widely across north‐west and central Europe (Knox and Morton 1988; Egger et al. 2000). It has been concluded that the ashes were the products of extremely energetic fountaining of magma close to the time of ocean‐opening (Fitton and Larsen 2001; Larsen et al. 2003). The continental crust thinned and subsided, eventually sinking beneath sea‐level as it became ever more laden with lavas and mafic intrusions. At this critical stage, close to parting and formation of the embryonic ocean, the rising magmas interacted with shallow sea waters, producing cataclysmic steam‐driven ash eruptions.

Figure 11 Sub‐aerial (picritic) lavas on the Svartenhuk peninsula, West Greenland (71° 30′N). These lie within the flexed zone, dipping westwards towards the Baffin Bay spreading centre. Such ‘seaward‐dipping reflectors’ are generally sunk below sea‐level but in this instance they are well seen sub‐aerially.

Source: Photo by T.C.R. Pulvertaft.

Ashes from these eruptions travelled hundreds of kilometres eastwards and are reported from as far away as Austria (Egger et al. 2000). Those falling into the shallow seas that covered much of western Europe were generally diluted by normal terrestrial sedimentation and are thus hard to recognize. However, in western Denmark in a shallow marine basin in which diatomite sediment was being very slowly accumulated, some two hundred ash layers contrast strikingly with the intervening white sediments (Figure 12, see Plate section). The alternation of the ‘ash’ beds and white clay reflects the repose periods between eruptions (Fitton and Larsen 2001; Larsen et al. 2003).

Whilst the older ash layers are inferred to have come from sub‐aerial volcanoes on the thinning continental lithosphere, the climactic (‘Stage 4’) ashes are attributed to a time when the locus of the proto‐Iceland plume had shifted away from the Greenland continent into the sea‐covered opening rift. Interaction of incandescent magmas and sea water caused the change from relatively quietly effusive to violently explosive eruptions.

Figure 12 Dark ash layers contrasting with white diatomite sediments on the island of Fur, Limfjord, Jutland. Originally deposited sub‐horizontally, the sequence was later severely deformed by Pleistocene ice sheets.

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