Jökull

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Jökull - 01.12.1979, Qupperneq 66

Jökull - 01.12.1979, Qupperneq 66
the Postglacial and they form the active volcanic zones which intersect Iceland from southwest to northeast (Fig. 1). The Postglacial extrusives form distinct petrological provinces. About 18 volcanic systems have produced rocks belonging to the tholeiitic series, and these systems form what can be termed the axial volcanic rift zone of Iceland (Fig. 1). Active tectonic fissures and normal faulting with formation of shallow grabens is characteristic of these systems. The tholeiitic systems vary appreciably with regard to size, the range of rock compositions extruded and the rate of extrusion. On the Reykjanes Peninsula (Fig. 6) three types of basaltic lavas are recognized in the comparatively small systems, i.e. picrite basalts (oceanites, cf. Table 1, no. 1) forming small lava shields, olivine tholeiites forming large lava shields and tholeiitic lavas extruded from volcanic fissures (cf. Table 1, no. 2). There is thus a distinct correlation between the morphology and the chemistry of the lavas. Petrochemically, two trends are indentifiable, being a lava shield oceanite — olivine tholeiite trend and a fissure tholeiite trend. Studies on the western Reykjanes Peninsula suggest that observed regular temporal variations may reflect cyclic volcanic activity such that each cycle' begins with the formation of oceanites. Fractional crystallization is recognized as being important in the development of the rocks, but as is Fig. 6. The Postglacial volcanic systems of the Reykjanes-Langjökull zone and northernmost Reykjanes Ridge. Sugges- ted location of the plate boundary is shown. Also shown is the location of high-temperature thermal areas and known picritic lava shields. From Jakobs- son et al. 1978. so often found when a detailed study is made, other processes must also be sought to explain the compositional variation; for example variations in degree of partial melting of the source rocks and volatile transfer or other gravitational processes working in the magma at shallow level. Intermediate and acid rocks have not been found outcropping on the Reykjanes Peninsula except in the Hengill system. In the two tholeiitic fissure swarms of the eastern volcanic zone (Fig. 7) the basalts are tholeiities of a narrow compositional range. Most of the Veidivötn basalt lavas carry abundant macrophenocrysts of plagioclase (An 88), probably cumulated in the magma at shallow depth by a floating mechanism. Although the Lakagígar swarm is for the most part covered by the Vatnajökull glacier, chemical and petrographic analyses indicate that the subglacial Grímsvötn caldera also belongs to this volcanic system. Several very voluminous basalt lavas have been extruded from these two tholeiitic swarms, such as the prehistoric Thjórsá B lava (13.5 km3) and the well-known Lakagígar lava of 1783—1784 (13 km3). In the northern zone, the tholeiitic Krafla fissure swarm has been studied in detail from various points of vjew, mainly in connection with the present rifting and eruption episode, which has been in progress since December 1975. It has been suggested from seismic and ground deformation evidence, that magma is being fed continously into 64 JÖKULL 29. ÁR
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