Jökull


Jökull - 01.12.1979, Page 67

Jökull - 01.12.1979, Page 67
Fig. 7. The Postglacial volcanic systems of the eastern volcanic zone. Eruption sites are shown by circles and dots, divided with respect to rock groups. Particularly active areas are indicated by hatching. From Jakobsson, 1979. a magma reservoir(s) below the centrally situated Krafla caldera, at a depth of about 3 km. When rifting and dilatation takes place, the magma is believed to be injected laterally into the rift system northwards and southwards, accompanied by deflation of the central volcano. It is estimated, that less than 1% of the tholeiite magma (MgO:5.4— 7.4%) fed into the rift system has yet been erupted on surface in the three minor volcanic eruptions since 1975. The lavas are of variable composition and a mixing mechanism of two or more tholeiitic liquids is indicated. From the study of Postglacial rocks in the flank zones, five volcanic systems producing a transitional rock series have been identified, all situated in the eastern volcanic zone (Fig. 7). The large isolated Öraefajökull volcano (Fig. 1) probably also belongs to this group. Only minor tectonic rifting or faulting is associated with the volcanism in the area of the transitional series. No ultrabasic rocks have been erupted in the eastern zone during Postglacial time, but Upper Pleistocene ankaramite intrusions (?) are known to occur in the Eyjafjöll volcano, for example Hvammsmúli (cf. Table 1, no. 5). The Hekla central volcano has been shown to be lying within a swarm of eruption fissures which have produced mild transitional alkali basalts, 5 JÖKULL 29. ÁR 65
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Jökull

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