Jökull


Jökull - 01.12.1979, Side 20

Jökull - 01.12.1979, Side 20
Fig. 12. Growth of a subglacial, monogenetic volcano. Stage A: A pile of pillow lava forms in deep meltwater lake. Stage B: Slumping on the flanks of the pillow lava pile produces pillow brec- cia. Stage C: Hyaloclastite tuffs are erupted under shallow water conditions. Stage D: A lava cap progrades across its own delta of foreset bedded breccias. Basically from Jones, 1968, with slight modifications. stages. Eruptions from a central crater that would normally produce a lava shield commonly reach a late stage where fluid lava again starts to flow high on the volcanic mound above the influence of ice or meltwater. A lava capping is formed which, how- ever, upon flowing into the surrounding lake crumbles into breccia and forms a delta over which the subaerial lava progrades. The recent Surtsey eruption was similar in character to the latter two stages. Many examples are known where intermediate and acid lava erupted beneath Pleistocene ice sheets. In the case of the more acid members the ensuing volcanic structure formed a steep-sided dome of rhyolite consisting of distinctive rock varieties. Among these are lobes of glassy rhyolite encrusted with a thick layer of pitchstone resemb- ling huge pillows. The lobes are embedded in a granulated glass matrix evidently derived from the crust of the lobes themselves. Rocks belonging to the Upper Pleistocene series can be related to a large extent to volcanic systems that are still active or dormant; the ones regarded as extinct are still recognizable from their topo- graphy and incipient dissection. Altogether some 30 volcanic systems have been defined within the neovolcanic zones of Iceland of which only 6 can be considered extinct (Fjallgardar, Snaefell, Breida- bunga? Esjufjöll, Geysir, Grafningshálsar) since no volcanic activity has occured in them during Post- glacial time. The sequence of glacials and interglacials during the Upper Pleistocene is incompletely known. Very few absolute age data are available and age relationships have to be inferred from indirect evidence. Within the volcanic complexes groups of lava flows alternate with tillite beds and hyalo- clastite rocks giving information about several in- terglacial/glacial cycles. On the Tjörnes Peninsula the lower part of the Upper Pleistocene is represented by a 200 m thick section (Fig. 8) involving two interglacial/glacial cycles. A tillite bed at the base of this section indi- cates that the Matuyama/Brunhes magnetic reversal occured during a cold climate interval. In late Upper Pleistocene time the Tjörnes Peninsula suffered erosion; only occasionally the fringes were covered by volcanic products, among which it is difficult to establish a stratigraphic order. The Snaefellsnes Peninsula may have a longer con- tinuous record of the Upper Pleistocene series but as yet the sections have not been worked out in detail. They include thick piles of subglacial brec- cias, a feature even more characteristic of the Up- per Pleistocene series in southern Iceland. The breccia piles which are commonly several hundred m thick, and only of a local extent, upset to a large degree the stratoid nature which so characterizes the older parts of the volcanic pile in Iceland. As an example a composite stratigraphic column from the Hengill-Grafningshálsar volcanic systems in southwestern Iceland is shown (Fig. 13). It is more 18 JÖKULL 29. ÁR
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