Jökull


Jökull - 01.12.1985, Side 97

Jökull - 01.12.1985, Side 97
SUMMARY ON THE A GES OF THE TWO RECENT LA VA FLOWS IN EYJAFJÖLL AND THE LATE GLACIAL TERMINAL MORAINES IN SOUTH ICELAND Haukur Jóhannesson lcelandic Museum of Natural History P. O. Box 5320 Reykjavík, Iceland. The Eyjafjöll central volcano is one of many vol- canoes in the Eastern Volcanic Zone (Fig. 1). The vol- canic activity in the Eyjafjöll region is limited to a 5 km wide and 30 km long fissure zone which stretches E-W from the Markarfljót river in the west to the Mýrdals- jökull in the east. The zone corresponds roughly to the fissure swarms of the volcanic systems which characte- rize the axial rift zones of Iceland. Only minor rifting is assumed to take place in the zones accompanying the volcanoes of the Eastern Volcanic Zone. The Eyjafjöll volcano itself is located on the central part of the fissure zone. It reaches 1666 m height and the top of the volcano is occupied by a 2.5—3.0 km wide summit crater or small caldera subsidence. The volcano is still active and the last eruption took place in 1821-1823. The intensity of volcanic activity is, however, very low in comparison to other Icelandic volcanoes. The Eyjafjöll volcano has been active for at least 700.000 years. The oldest exposed formations were erupted during the final stages of the Matuyama paleomagnetic epoch. The vol- cano is built of a pile of lava flows erupted during interglacial periods and hyaloclastites formed by subgla- cial eruptions during glacial periods. Evidence for 9 glacial and 9 interglacial periods has been found. Only two lava flows are of Holocene age. They are in the western part of the Eyjafjöll volcano, named Kambagil and Hamragardar lava flows, respectively (Fig. 2). Kjartansson (1958a, 1970) believed that their age was about 11.000—10.000 years, erupted during the Allerod stage. In this paper the geology of the two flows is reviewed. The Kambagil lava flow was erupted from a crater named Raudahraun (Figs. 2 and 3) on the north side of the mountain range. Kjartansson (1958a) suggested that the flow terminated at about 325 m above the present sealevel. Kjartansson concluded that a glacier termed as Markarfljót glacier flowed along the valley between the Eyjafjöll and Tindfjöll massifs and that the lava flowed down to the glacier and spread out on top of it. The lower part of the flow was then subsequently removed by the glacier. Detailed mapping of the lava flow has revealed that it does not terminate at 325 m height but has reached the lowlands west of the Eyjafjöll moun- tain. But below the 325 m mark the flow is covered by ground and terminal moraines indicating that the Markarfljót glacier has overrun the lower slopes of the Eyjafjöll mountain while the upper slopes were icefree. The lava rests on conglomerate or tillite and no traces of organic material were found thus preventing C-14 dat- ing of the flow. The margin of the former Markarfljót glacier was traced by mapping the glacial striae (Fig. 2). Haraldsson and Palm (1980) carried out seismic studies of the areas south of the research area and suggested that a subsur- face terminal moraine occurred in the outwash plain in Austur-Landeyjar. A comparison of the seismic data and the height of the former Markarfljót glacier on the slopes of the Eyjafjöll mountain with the present Skeidarárjökull indicate that the snout of the Markar- fljót glacier might have been at the terminal moraine of Haraldsson and Palm (1980) (Figs. 7 and 8). A further three minor terminal moraines were observed on the top of the Kambagil lava flow (Fig. 3). The Hamragardar flow was erupted from a nameless crater southwest of the Raudahraun crater (Fig. 2 and 6). The lava flowed along a small valley towards the southwest but the main stream stopped at the cliff edge above the Hamragardar farm and only two rivulets seem to have flowed off the edge. The Hamragardar flow has not been overrun by a glacier and is thus undoubtedly younger than the Kambagil lava. The flow rests also on conglomerate or tillite and no material for C-14 datings is to be found. The major terminal moraines of South Iceland are shown on Fig. 9. They can be divided into three groups, Outer (or Y-), Middle (or M-) and Inner (I-) terminal moraines. C-14 datings of shells from raised terraces in South and West Iceland suggest that the Y-moraine is at least from the Older Dryas stage or about 12.000 years old but may be older. The M-moraines are at least from the Younger Dryas stage or 10.000 years old. The Kambagil flow is thus older than 12.000 years, and the Hamragardar flow is at most 11.000 years old as it has not been overrun by the glacier which formed the Y- moraines but might be younger. JÖKULL 35. ÁR 95
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