Jökull - 01.12.1979, Síða 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,
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