Jökull


Jökull - 01.12.1981, Side 26

Jökull - 01.12.1981, Side 26
The zone consists of three volcanic systems or central volcanoes which are arranged en echelon (Jóhannesson 1982a). These volcanic systems are fundamentally different from the volcanic systems of the axial rift zones of Ice- land. The latter are characterized by a ten- sional stress field resulting in open fissures and normal faults (Saemundsson 1978). On the other hand, the volcanic systems of Snaefellsnes do not exhibit such features. The volcanism is re- stricted to a narrow zone, often only a few hundred metres across and no rifting is observed. The Snaefellsjökull central volcano is the westernmost volcanic system of the Snaefells- nes volcanic zone. It is about 30 km long and stretches nearly E-W from Maelifell in Stadarsveit to the tip of the peninsula. The volcanism in the eastern part is restricted to a narrow zone but in the western part it is more widespread especially around Mt. Snaefells- jökull. The activity seems to have been more vigorous in the western part where a 1446 m high stratovolcano, Snaefellsjökull, has deve- loped. The oldest rocks exposed at the base of the volcano are reversely magnetized lava flows, supposed to be just over 700.000 years old. They rest unconformably on an eroded Ter- tiary basement. The cone itself consists of hyaloclastites formed during glacials, and of interglacial lava flows. The postglacial activity seems to have been vigorous. Over 20 individual lava flows have been identified so far and they are all prehistoric. The eruption sites are either close to the top of the mountain or in groups in the lowlands around the volcano. The craters close to the top usually erupt intermediate or acid magmas in contrast to the basaltic magma erupted in the lowlands. (Jóhannesson 1982a). THE ACID TEPHRA LAYERS OF THE SNAEFELLSJÖKULL CENTRAL VOLCANO In August of 1981 a total of 95 soil profiles were studied in the Snaefellsnes peninsula (see Flores 1981). The aim of this study was to trace tlie acid tephra layers of the Snaefellsjökull central volcano and to map their distribution in the Snaefellsnes peninsula. The location of the soil profiles is shown in Fig. 1. The thick- nesses of tephra and soil layers were measured and characteristics, such as the angularity of glass fragments, colour etc., of the tephra were studied, but grain-size analyses were not undertaken. Representative soil profiles are shown in Fig. 2. In this survey the existence of the three acid tephra layers was confirmed. They are in this paper termed Sn-1, Sn-2, and Sn-3, Sn-1 being the youngest and Sn-3 the oldest. A few basaltic tephra layers were en- countered but their distribution seems to be restricted to the neighbourhood of cinder cones, most of which belong to the Snaefells- jökull volcanic system. The thickness of each acid tephra layer varies considerably from one profile to another, even across short distances. In many cases this difference may be due to the ash being piled into drifts by wind; especially if the eruption took place in wintertime and the tephra was deposited on frozen or snow cov- ered ground, resulting in an uneven thickness. In a few places the tephra layers have ob- viously been modified by water (redeposited). Such instances are easily identified because the ash and pumice fragments are more or less rounded. Due to the above-mentioned irregularities and the similarity of the tephra, it was necessary to trace each layer from one area to another by studying soil profiles every 2 — 3 km. In spite of the number of profiles studied, the correlation between some of the sections is difficult. Sn-1 Sn-1 is the youngest acid tephra layer. It is generally at less than 40 cm depth below the present surface. The maximum thickness runs towards the northeast as demonstrated by the isopach map (Fig. 3). The layer is found on the northern side of the Snaefellsnes peninsula but has not been identified on the south side. For 24 JÖKULL 31.ÁR
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