Náttúrufræðingurinn

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Náttúrufræðingurinn - 1992, Side 49

Náttúrufræðingurinn - 1992, Side 49
tral volcanoes form a caldera. Nearly all Icelandic central volcanoes liave devel- oped such phenomena. One of the excep- tions has been the Hekla volcanic system. Recently, a theory has been presented which suggests a caldera formation in the Hekla volcanic system during the Holo- cene. The theory is based on the following arguments: 1. The subglacial hyaloclastite forma- tions surround the Hekla volcano but are nearly absent from the mountain itself. Volcanic activity is always most intense in the centre of the volcanic systems, and there the accumulation of volcanics is greatest. In all comparable volcanoes in Iceland, hyaloclastites form the highest peaks - Hekla volcano is the only excep- tion. 2. The hyaloclaste ridges southwest and northeast of the Hekla volcano become higher as approaching the volcano, but are abruptly curtailed a short distance away from it. 3. In two places close to Hekla are blocks tilting towards the volcano and these seem to be bounded by arched frac- tures. 4. Where the Hekla fissure intersects the assumed caldera rim, there are pecu- liar xenolith occurrences. This may be in- terpreted as anomalous volcanic activity. 5. Outside the assumed caldera rim in the vicinity of Hekla, no lava flows have been found dating from 4500 years ago until about 500-600 AD. Thus there seems to be a time gap of 3000 years. Tephro- chronology, however, does not indicate any change in the volcanic activity during this period. The most obvious explanation of this time gap is that in the course of the great eruption which produced the H-4 tephra, a caldera collapse took place. If this theory is valid, it took about 3000 years to fill the caldera to the rim with la- va flows. 6. In the area between Hekla and the Vatnafjöll are some eruptive fissures. They are discontinuous and an 8-km gap occurs where they cross the assumed Hekla caldera. 7. Within the assumed caldera, only in- termediate magma (andesite) is erupted, but outside it mostly basaltic magma. It is likely that the curtailed hyaloclas- tite ridges, southwest and northeast of Hekla, have originally continued into the Hekla mountain itself. In the course of a caldera collapse, the parts of the ridges closest to the volcano have subsided. The fact that basaltic magma is not erupted within the caldera, may be explained by a magma chamber at depth beneath the vol- cano, containing intermediate and acidic magma acting as a density trap to the bas- altic magma. The assumed caldera is about 10 km in diameter (Fig. 4) but may consist of more than one smaller calderas, in similar man- ner as the Askja volcano in Northern Ice- land. In historical times, there have been 17 eruptions in the Hekla proper and 5 erup- tions, mainly basaltic, elswhere in the vol- canic system (Table 2). 191
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